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I’ve read some discussions on attic ventilation, but it seems a lot of people are “back east”. I live in Houston, Texas. I saw a comment about seeing an attic temp of 130 when the outside temp was 90 – is this the temp difference I should see in Houston in a properly vented attic ?
I have ridge vents plus soffit vents. I have read that the amount of ridge vent area should equal the amount of soffit vent area – correct ? If I have 650 sq in of ridge vent, then I should have the same amount of soffit vent area ?
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yah
*Dan. Ideally thr ridge net free venting area (NFVA)and the soffit vent NFVA should be balanced...matched. However, ans absolute match is not necessary. Any unbalance sh oudl be in the soffit NFVA. It is permissible, and desireable to ahve 10 to 15% more soffit NFVA than ridge vent NFVA. The dominant heat transfer mechanism in an attic is radiation. Venting an attic will not "flush" out the radiation. In a "correctly" constructed attic: the ceiling below the attic...attic floor- is air tight, there is no ductwork in the attic space, and vented to code :1/300( for every 300 sq. ft. of attic floor area(ceiling area in BOCA code) there is 1 sq. ft. of NFVA venting. In such an attic the air change per hour(ACH) is 3 to 6 ACH. This results in about a 3% reduction in heat transfer from the attic to the living space below. Not very impressive is it? Install leaky ductwork in the attic and you get a 25% increase in heat transfer to the rooms belwo. The answer is not with ventilation, nor is it with attic fans.The answer is insulation and radiant barriers.. in the South. Search the FHB archieves for more information on the subject. Hope this helps. GeneL
*Gene, I am finally "getting it" about radiation (though I think I always knew it in the back of my mind). But to keep the radiant heat out of the attic, the ideal location of the barrier would be on top of the shingles (not very practical). If I put the barrier somewhere "inside" the attic -- bottom of rafter, top of ceiling joists -- isn't some of the "reflected" radiant heat going to be trapped, or is it going to be re-radiated back to the roof?
*Gene,Thanks for the info... it helped a lot. I'm actually putting in radiant barrier in my attic now - still working on it though - been really hot of late. According to your info, I need several more soffit vents in the main attic - although from what you are saying, it won't help that much- but it's cheap and I guess every little bit helps. The previous owner blew in some cellulose over the master bedroom & bath, which had fg bats in between the ceiling joists (2x6's looks like). Is this something I should think about in the main attic ? Any idea what R value just the fg bats in the main attic gives me... and what it should be ?I'm also thinking of putting on some 3M window tinting on our upstairs west facing windows to reduce heat gain some more - but was put off some by the price - $5.50/sq ft - works out to about $100 per window---is it worth it ?Again thanks
*You mentioned window tinting....The look may not be appropriate for your house, but have you ever considered inexpensive rollup shades hung from the outer edge of the soffits ? I'm a firm believer in stopping the sun before it hits the window.We have several 6' wide ones on one side of our house to block the afternoon sunlight from hitting the wall.Wind can be a problem but we usually roll them up when it rains or it gets gusty.If they get damaged the cost of replacing them will not break the bank.We use white ones, but I've seen folks using rattan colored ones.By the way, I lived in Humble for two years and know how hot it can get. And also remember the out-of-nowhere sudden downpours that lasted 5 minutes and afterwards sitting at an intersection with my muffler gurgling in the water....
*Alan,One reason I was thinking of the window tinting was that we have a homeowner's association that evidently passes judgement on exterior changes to the homes in our subdivision. We had thought about sunscreens, but the rep who came out lived in our area and said he didn't think the association would let us put screens on just 2 windows. From what he was saying, sounds like they wouldn't go for roll-up shades either !Actually, I really don't like the view out the window WITH sunscreens on - was in a house once with those on, and the view out was not real clear. I guess I think that tinting is appropriate - didn't anticipate real dark tinting anyway. Those downpours are just part of the joy of livin' in the bayou, I guess.
*Crusty and Dan. Chapter 17--Insulation, in my book Complete Building Construction, 4th Edition published by Macmillan Publishing Company, contains a 20 page article on Radiant Barriers.This shoulkd answer most of your questions.Cellulose is preferable to fiberglass because it is less transparent to radiation than is fiberglass. Fiberglass readily allows radiation to pass into it. Cellulose which is more opaque reduces the amount of radiation penetrating and confines it to the first few millimeters where it tuns to heat--sensible heat.Where in the soffit the soffit vents are located is important. The soffit vents must be located next to the fascia board--and preferably 8-inches away from the outer wall. This location minimises rain and snow penetration. GeneL.
*Gene,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Joe, I don't think radiant is the dominant "heat transfer" mechanism in the attic, but I do think it is the dominant "heat gain" mechanism.I believe the two are different. Heat gain is what causes the attic to get hot (mostly radiant, from the sun); heat transfer is how that heat gets into the living spaces (radiant from the shingles/sheathing, conduction & convection through the insulation package).Any "hot body" will radiate heat toward a colder body. We usually think of radiant as being an extremely hot body (the sun, a cherry red burner, a wood stove) but it doesn't have to be. Once the shingles/sheathing get hot enough, they do indeed radiate heat DIRECTLY to the living space, without the need for conduction or convection.Rob, you may be right about not needing to vent the "reflected radiant heat" that I referred to. When you place a reflective sunshade near the windshield of your car, the radiant heat (from the sun) still comes through the windshield, but the car is MUCH cooler than with no sunshield. Right? This sort of proves your point.I was putting up some foil-faced foam today and there was a note that said that a 3/4" air space on the reflective side of the foam has an R value of 2.77 per ASHRAE. I'm sure this is in reference to leaving an air space between the foam and the brick veneer. So it seems that a small air space is beneficial in some applications, but in a ceiling, it appears we might be better off just omitting the air space and adding that much more insulation. The "opaqueness" of the cellulose, relative to the fiberglass, is just about a no-brainer.Given the obvious superiority of the cellulose available today (vs. what was available 15-20 years ago), why are we still using fiberglass?????
*Crusty,
View Image © 1999-2000"The city is not a concrete jungle, it is a human zoo." Charles Wadsworth
*Why Crusty - thank you. Several years ago on this forum I stated nearly the same as you, basically "The "opaqueness" of the cellulose, relative to the fiberglass, is just about a no-brainer." and was told that this was foolishness, that radiant heat had nothing to do with visible light passing through fibreglass. Maybe things have changed.Dare I even suggest that if you put enough radiant heat on one side of a steel plate that the dominant heat transfer from the other side is by radiance?
*Joe, I guess it just comes down to how hot does a "hot body" have to be before it becomes radiant? (I ought to be able to find that somewhere.)Rob Susz has quoted a military study several times (which he was apparently involved in) that showed that human comfort is more a function of surrounding surface temperatures, not the surrounding air temperature. If my skin temperature is only 98F and the walls of my house are only 70-100F, then by Joe's definition, neither radiates significant amounts of heat.So why is an air temperature of 72F quite comfortable in the summer, but in the winter I feel cold (assuming I'm wearing the same clothes)?? It's because in the winter, the walls are cooler than in the summer (though probably by only 10-20 degrees) and I radiate heat more quickly to a colder wall. So when dealing with human comfort, it appears that a hot body doesn't have to be all that hot, only hotter than the surrounding surfaces. That's because our bodies are so wonderfully constructed that a skin temperature change of only a degree or so greatly affects our perception of hot and cold.After reading what I just wrote, Joe is probably going to agree, but point out that my skin doesn't "see" the shingles/decking, but only the interior walls, so my explanation doesn't prove anything. He may be right. Somebody help me out here.
*Crusty,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Joe, I think I learned about the human comfort thing (radiation that is) in college, perhaps in my Solar class. But maybe I just dreamed it up -- I'll find my old text book and see.Where did you learn about it?
*Crusty,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Joe, I will again go on record as saying I disagree with you.Convection (and stack effect) will take the heat up and away from the living space. In a vented roof it will take heat up and out the ridge vent. Hot air rises and last I remember the living space was under the attic.Conduction takes place through roof rafters and ceiling joists. Long way for heat to travel.I would also like to ask why if a roof is vented, do you still sweat your butt off up there? I thought the venting takes all the heat out.-RobP.S. - was not involved in the military study, light will pass through fiberglass batts, Go ahead and tell me I'm still wrong.
*Rob,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Radiation makes an attic hot in the summer...It's the same as sitting too close to a roaring fire in the fireplace Joe....near the stream,ajLove you to(o)
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View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Scary for you I guess....near the stream,aj
*T!,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Yeah Joe, I also have a long description in the archives too, but I can tell you it was at least two years ago, I just went back through the archives and never got to it.But due to the civil nature of the debate I will reply.Convection is the air movement resulting from the heating of the air, which causes it to become less dense, and it subsequently rises relative to cold air.Stack effect is the stratification of this air - say in a closed space like an atrium or stairwell. Examples:Convection is used to our benefit in houses and barns whereby there is a cupola or roof ventilator that allows the heated air to rise up and escape, thus drawing in cooler air. It is also used in swamp coolers out west where the evaporative effect of water is used to cool air that subsequently flows downward, while the heat released due to phase change and evaporation flows upward.Sratification is used to our benefit in things like off peak cooling water applications, where, at night, cooling water is drawn off the top of a large tank, run through a tower and returned to the bottom of the large storage tank. During the day the cooling water for the building or process is drawn off the bottom and sparyed back on the top of the tank. The two never meet, the hot on the top and the cool on the bottom.-Rob
*heating by radiation...Joe Joe...warmly being radiantly warmed by my buddy the flame artist...the weasel...ajtraining my attack lobsters to do battle under fire!
*Rob,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*That's it?
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
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*!
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*"Radiation, or more correctly thermal radiation, is electromagnetic radiation emitted by a body by virtue of its temperature and at the expense of its internal energy. Thus thermal radiation is of the same nature as visible light, x-rays, and radio waves, the difference between them being in their wavelengths and the source of generation. All heated solids and liquids as well as some gases emit thermal radiation. The transfer of energy by conduction requires the presence of a material medium, while radiation does not. In fact, radiation transfer occurs most efficiently in a vacuum. The calculation of thermal radiation is based on the Stefan-Boltzmann law, which relates energy flux emitted by an ideal radiator (or blackbody) to the fourth power of the absolute temperature.""Conduction is the transfer of heat from one part of a body at a higher temperature to another part of the same body at a lower temperature, or from one body at a higher temperature to another body at a lower temperature in physical contact with it. At the macroscopic level the heat flux (i.e., the heat transfer rate per unit area normal to the direction of heat flow) is proportional to the temperature gradient (delta-T): q = -k dT/dx where the proportionality constant k is a transport property known as the thermal conductivity and is a characteristic of the material.""Convection relates to the transfer of heat from a bounding surface to a fluid in motion. If the fluid motion is induced by a pump, blower, a fan, or some similar device, the process is called forced convection. If the fluid motion occurs as a result of the density difference produced by the heat transfer itself, the process is called free or natural convection. Newton's law of cooling is given as: q = h (Tw - Tf) where Tw is the surface temperature, Tf is the characteristic fluid temperature, and h is the heat transfer coefficient. In the case of natural convection, h varies as some weak power of delta-T (ie, Tw-Tf) such as 1/4 or 1/3 (i.e., delta-T raised to the 1/4 or 1/3 power).""In practice, heat transfer frequently occurs by two mechanisms in parallel. A typical example is heat conducted through a plate is removed from the plate surface by a combination of convection and radiation.""The McGraw-Hill Handbook of Essential Engineering Information and Data," Ejup N. Ganic & Tyler G. Hicks, 1991.Joe...... Gene, Rob, & I are not making this stuff up.
*What happens when your attic heats up (cooling is a different deal) is this: the shingles are heated only by radiation from the sun, unless the ambient air temperature, or a "hot wind" happens to exceed the shingle temperature (not likely in the summer). The heat passes through the shingle by conduction (internal), and then into the decking by conduction (two solids in contact with each other). The heat then passes into the rafters by conduction.The air immediately adjacent (i.e., in contact with) the decking and rafters is heated by conduction, whereupon it starts to move (rise) resulting in a natural convection air current, which then actually serves to cool the decking/rafters, IF it continues to flow adjacent to them. If a power or wind-driven ventilator is in use, the decking/rafters are cooled by forced convection (IF the air flows adjacent to them) and natural convection is negligible.In an unvented attic, the air is essentially still and the natural convection currents are minimal. As the air heats up, some heat is transferred to the ceiling/walls by conduction. But air is a very poor conductor, so the dominant heat transfer is direct radiation from the hot decking/rafters.If the attic is vented (natural or forced) the hot air rises and flows out through the ridge vents or fans, lowering the mean air temperature. So less conduction takes place than with an unvented attic. The dominant heat transfer mechanism is still radiation from the decking/rafters.Rob, putting a radiant barrier, such as foil-faced foam board, up against the decking is ill-advised. The heat that comes through the shingles/decking by conduction will be blocked from being radiated to the living space, but where is it going to go?? It results in a higher temperature in the roof deck, and eventually finds its way through the foam board via conduction (two solids in intimate contact), at which point the foam board becomes a radiator to the living space. Radiation is dependent ONLY on the temperature of a body and how well the hot body is "seen" by another body.However, leaving a 3/4" space between the foil-faced foam board prevents solid-solid conduction and allows the heat coming through the decking to be carried away by natural convection. The foam board will still heat up, but it will not get as hot, and will thus not radiate as much heat to the living space.You cannot make radiant heat - from ANY source - disappear. The key is to confine it or capture it in such a way that it can be removed by economical means. To get effective cooling of the attic via natural convection (soffit/ridge vent combination) the air flow must be laminar, that is even and uninterrupted and parallel to the surfaces being cooled. This is best done if the air flow is "contained" in a closed channel. The way I'm achieving this is to put foil-faced foam on the bottom of the rafters, from the soffit all the way to the ridge vents. There are some "gaps" to allow hot air within the attic space to enter the convective channels (rafter bays) but not many. I will test this next summer with a thermocouple array that will take and record temperatures in a number of strategic locations.
*CrustyI'm starting to like you. Actually, that's not true I've always liked you.Two questions/statements:1 - I don't remember ever advocating the application of foil faced foam against the roof deck, but no matter. Heat will build up in the roof deck as you say, but what does it matter if the rest of the cavity is filled with cellulose, or more rigid foam, or apray foam? There would be no airspace on the underside either, so heat flow is purely by conduction through the mass of insulation. Even if the underside of the rafters were sheeted with foil faced foam, the surface temp would be minimal due to the mass of insulation above it blocking the conductive flow of heat.2 - I still say you are nuts for throwing away the whole rafter bay for use as a vent channel.3 - bonus question - Would a closed, sealed chamber that was heated and insulated (isothermal) have convection that eventually led to stratification? What about an unvented roof?-Rob
*Rob, the only place I'm "throwing away the rafter bays" is where they are not a part of the bonus room ceiling. I'm attaching it to face of the rafters because it's much faster than putting in between them. And I'm also blocking the radiation from the rafters, which have an R value of only 9, as you pointed out. I'm doing this on most of the south side, starting out on the front porch which is a shed roof and ties into the main roof about a third of the way up.In the bonus room "slope" I've furred out 2" and will leave only a 3/4" vent channel. Behind this I'll use wet-blown cellulose, or if that won't work (i.e., stick to the slope), I'll blow in cells. If it's not cool enough next summer, I'll add more foam on the slope, add furring and then another layer of drywall (would I need to "ventilate" the original layer?).The knee walls will get their own foil-faced foam on the outside, and because of the way the trusses were made, then get a full 7" of cellulose. In the west walls, I'm putting 1/2" foil faced between the studs, against the sheathing and will then blow cellulose.If I were only going to be in this house for 5 years, then yes, I'd be certifiably "nuts." But I'll be here the rest of my life. My wife determines where to set the thermostats based on the last electric bill. If I want to stay cool upstairs, I've got to keep the heat from ever getting in, or I'll be "toast" (literally).
*what radiance we all receive from you Joey...Can I call you Joey?....Now I have question...Ever hear of a Napolian complex?....So just how tall are ya Joey?.....As you always say....Just answer the question Bozo....near the stream laughing with my cute little attack lobsters....ready to get you...aj
*Tony,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Rob,I agree that using the whole rafter bay for a vent channel is a waste. The real question in my mind is can you quantify the benefit of a minimal air channel? Will it remove more heat by breaking the conduction transfer from the decking to the insulation than you would block with the equivalnt space full of insulation? This question would seem to be most relevant in a cathedral situation where the amount of total insulation you can have is limited.OF course this is all ignoring the moisture-romaval and shingle-life issues.Crusty, in your case, I think the most effective way to keep the entire space coolest would be to fill the whole damn thing with cells. Fill the whole space behind the kneewalls, the entire area above the flat ceiling and up into the peak, and the entire sloped section. You can get an impressive conductive/radiant barrier by doing so. It's sure to outweigh the cost of not insulating the entire area in order to achieve an air channel. Or if you really want that air channel, minimize it, an then fill everything underneath it down to the flat ceiling with cells. The kneewalls are a sacrifice of storage space, so I can see not filling them up if that is an issue.Steve
*Nope...You're right...Can't be the Napolean thing....have to go back to the bully thing and the "Italian manly way" of talking to your disrespected fellow civilian....am I closer...bozo, near the stream playing with fire...aj(the egitt and the weasel)...hey, I bet that would make a great kids book...Joe...can't call ya Joey if ya are a big lug I guess...I'll let ya have the last word here as this is going in circles yes?...let me have your last bit of radiance my fellow New Yorker...
*Joe. Good hearing from you. I've been off the "äir" since Sunday. Just got back on this morning.As for radiation being the main heat transfer mechanism in an attic, what can I say other than we disagree. All we can agree on on this subject is that attics get hot.CRUSTY. You might be interested in Joe Lstiburek's paper, "Measurement of Attic Temperatures and Cooling Energy Use In Vented ans Sealed Attics in Los Vegas, Nevasa. You might get a copy form his web page http://www.buildingscience.com. GeneL.
*Ton-ahhhh. . .,
View Image © 1999-2000"You clinking, clanking, clattering collection of collagenase junk!" The Wizard
*Crusty,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Joe, my intent was to share some objective information about radiation, convection, and conduction (vs. what you or I "think" those terms mean). Your made a statement earlier in this thread which leads me to believe that you don't really understand radiation all that well. My impression is you seem to think the only significant radiation on the planet is from the sun. What don't you understand about a.j.'s stove reference?Also Rob was throwing around the term "convection" when he actually meant "natural" convection (vs. forced convection which is a very different animal).Did you even bother to read the information I posted? If you had, you would be able to conclude rather easily that there is minimal heating due to convection and conduction in an attic, that most of the heat is indeed radiation, directly from the hot decking/rafters.
*Crusty,
View Image © 1999-2000"But, some people without brains do an awful lot of talking. . ." The ScareCrow
*Joe, I think that I can answer that question. Now bear in mind that I took the day off, so this is Andy talking, not the web goombah, if I can make that seperation. This stuff is fascinating, and I find it hard to stay away.The plate glass would reflect some of the radiant heat from the stove back the way it came, and assuming that the glass is cooler than the stove, it would also absorb some of the heat. I think the ratio of reflction to absorbtion would depend on the color of the glass. The immediate noticeable effect would be that a person standing on the far side of the glass would feel cooler. However, I think that as the glass absorbed more of the heat radiating from the stove, it would eventually become warm, perhaps warm enough that it would begin to radiate heat that was sensable to our stationary person. I imagine that our subject would notice a cooling effect, even after the glass heated up, because the reflected radiation would find another path from the area. If, however, no such path existed (impossible in reality, I think), then the glass would eventually heat up to the point where it radiated exactly as much heat as was sensed prior to its placement.Now, back at you, what do you think the effect of placing the glass in front of the stove would be?Andy
*Ooh, ooh, wait, I thought of another factor. Some of that heat would pass directly through the glass, depending on the reflectivity of the glass and the wavelength of the energy. Andy
*Andy you ignorant slut. My college physics professors used to play tricks like this with me. ASSUME NOTHING. Joe never said the stove was lit to begin with, and he also never said there wasn't any other heat sources. Especially coming from that cute little ski bunny next to me but I digress... And besides, what're you playing hooky for? Get to work! No wonder we only get an issue every other month!
*You only get an issue every other month? That's odd.I did assume the stove was lit, and that there is a higher delta T on the stove-side of the glass than on the other side. I assumed no snow bunnies.Ignorant slut indeed.
*Well 8 issues a year, may as well be 1 every other. I do pity you not having snow bunnies.
*Andy,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Are you saying joe that the plate of glass is similar to the fiberglass insulation layer in the attic?....I'll tell ya one thing joe...I am not touching that glass but if it was say a six inch thick wall of wood ala cellulose insulation, I would...Wood better than glass...yes joe?Near the stove with the glass doors open as is the case in the attic till ya get to the insulation....aj the enigma...
*Ton-ahh,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Good to see the entertainment is mutual...Are we just running out of material for this wack and yak?near the potentially frozen stream,aj
*
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*This is what I am saying....Comfort is the goal....And radiant heat is so comfortable due to the radiant heating devices warming the occupants and the air cooling them. That's why I like woodstoves, gas stoves, radiators and radiant floor systems the best...Warm feet from a warm floor and cooler air is so nice...And the heat is right where wanted, not up at the ceiling unless the radiant panels are at the ceiling...In that case the occupants are definitely heated directly through radiation...To check this out, just go stand under one at HD or at a hockey rink.Hot water baseboard is my second choice as it scorches the air a bit less than forced hot air by warming the walls a bit as a by product of warming the air along the exterior walls....Forced hot air is my last choice as it heats the air to heat the occupants which makes the air stuffy and warm to breath along with dry...And the walls will run cooler in this type of heat making the occupants need a higher air temperature to feel warm. Warmer air & cooler walls not desired...near the streamajJoe...nice post and broad use of the patented Fusco no-nonsense style...Your radiation explanations are acceptable to me and seem to agree with most of what I have tried to say...Yikes!@
*Tony,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Yeah Joe...What do ya think of my comfort ideas posted above?...Oh and the name isn't Tony but I like Tony the Tiger and tigger too!near my radiant buddy by my stream,aj
*
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Joe...i Comfort is great, but radiation doesn't heat air. . .Well it does heat air but not very much compared to highly radiant absorbant items like humans and roofs...So blue is not a tony either...You must think we are Italian cause we can shadow box with the stallian of the board...Well, I'm Polish, Danish, English and nearly Mayflower American mutt...near the stream sticking with me moniker...aj
*Crusty...Rob...Steve...Last words are being solicated and then I think this thread will be declared...burnt toast!@ near the stream,aj
*Joe - The radiant from the roof doesn't heat the air directly. It first heats everything it "sees" like the ceiling and rafters below. This is where the conduction you discuss starts - the rfaters and gyp get hotter and conduct to the living space. They also heat the air and start natural convection.This is how radiant heats air in an attic. I would also like to point out that if there were no air, or cold air in the attic (power ventilated), the heat would still raidate to the ceiling below.I singlehandedly volunteer to do the glass test.I propose to use an open fire, 1/4" uncoated tempered plate glass, and just for fun an R-11, 19 and 38 batt with and without the facing!If the terms above are no good for the test, then I'll just burn the fire and and my scrap wood and sit looking stupid like I usually do. I usually preach building science to the dog, because he will listen.-Rob
*Rob,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Joe, in an earlier post you warned me about "Most problems with this subject come for using one's own intuitiveness to draw conclusions." Are you not doing exactly this.Thinking about your plate glass challenge. Let's do it this way. Instead of sitting in a room with a piece of plate glass between you and the stove, I'll sit in my car on a sunny day with the windshield between me and the sun. Are you saying I wouldn't feel the sun coming through the glass because it's reflected? Other than the temperature of the radiator (sun vs. stove) what's the difference in these two examples. None that I can see. I will feel the suns warmth.The outside air temp this morning was about 40 degrees, but with the engine off and no heater on, the inside of the car was considerably warmer, so it couldn't have been convection or conduction from the outside. What I understand you to be saying, is the sun merely heats the glass and then the glass heats the inside of the car by conduction/convection. But if I place a reflective sunscreen 6" from the glass, the inside of the car remains cold. So what happens to the heat that was in the glass. If it was conduction/convection it should still enter the cars interior. Where does it go??Let me give you a challenge. In your attic, tack a piece of 1/4" hardboard (or 1/2" plywood) to the ceiling rafters. On a nice hot sunny day, measure the temperature of the roof decking (underside), the air space at a point halfway between the deck and the "test" panel, and then the test panel (you'll need a contact thermometer for this). I am confident that the roof deck will be the hottest point, and the test panel will be hotter than the air space. Try it. Go ahead, I dare you.
*Joe, I agree with everything you said.Can you tell me what I staed that is at odds with physics?-Rob
*Crusty,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Rob,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Joe, you know you're not convinced. You've either become bored with this discussion, or you're wearing down because Rob, a.j., and I just won't quit. But you are not convinced.Radiation doesn't heat the air in attics -- you are right about that. The air is heated by conduction/convection from the roof decking and the rafters, as you've stated. But the living space (knee walls, ceilings, etc.) ARE heated MOSTLY by radiation, and some by convection/conduction (as you say).I inferred from your stove/glass statements that the glass was supposed to block the heat radiating from the stove. If that is true, I just wondered why a car window doesn't block the radiation from the sun.I'll take my own challenge and report back on the results.
*Crusty,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Joe,I will expand on your statements below, that I agree with 100% I have numbered them for reference1) Bodies/things that are "hotter" will transfer their energy to bodies/things that are "colder".Yes - Through conduction, convection, or radiation.2) Radiation does not "heat up" air directly.Right – Radiation heats objects that are at a lower tempearture these objects absorb this energy, become warmer and start convection (except in a vacuum)3) Air is a poor conductor of energy.I agree – due to low specific heat and density4) 99.999% of all energy transfer to a gas is through conduction and then convection.I agree5) Temperature is the measure of the average speed of the molecules in a gas volume.Yes, and due to higher speed, there are more collisions per unit of time, which forces the molecules further apart, thus becoming LESS dense, or lighter relative to a colder quantity of the same gas (coefficient of thermal expansion)6) An object that is 100° F is placed in a room at 80° F. That object will cause the average temperature in the room to go up.Yes, it will conduct heat to the air around it and cause convection, and radiate heat to the surroundings thus causing convection, assuming the surfaces and walls were at 80 degrees to start with7) An object that is 80° F is placed in a room at 100° F. That object will cause the average temperature in the room to go down.Yes, by removing heat through a reverse of the convection/conduction/and radiation described in number 5 above, assuming there is no other source of heat, and the surfaces and air were both at 100 degrees to start.8) Assuming a constant energy input to the room, the object and the room air temperature will eventually move to a state of equilibrium.Yes, with one exception, depending on the room conditions and the heat source, there may be stratification of the air, thus causing a temperature gradient from top to bottom in the air due to number 4 above.I think there is enough here to describe the dynamics of attic heat gain.The sun heats the roof through the radiantion component of number 1The roof gets hot on the top and transferrs the heat through to the bottom of the roof through conduction component of number 1The roof now transferrs heat in all three ways (number 1):Conduction to the air below, and through the framingConvection of the air currentsRadiation of energy to the ceiling below – the ceiling is colder with respect to the roof undersideDue to number 2 above the air is not heated by radiationDue to number 6 and number 4, the framing and ceiling below now heat up the airDue to number 5 the hot air gets less dense and risesDue to number 3 the air cannot remove all of the heatDue to number 8, the ceiling gets hotter and hotter as it approaches equilibrium at an elevated temperatureDue to number 1 above the heat gets into the living spaceHave I missed any of the players?Now for the math part:I’ll refer to Crusty’s earlier post, number 30 in this mess.Radiation is a function of the amount of energy transferred is a function of the emissivity of the surface and the temperature of the surface TO THE FOURTH POWER.This is actually more related to the emissivity of the receiving surface, if there were no surface to receive the radiation, none would be radiated. We can simplify the math by saying both surfaces are the same emissivity and use the temperature difference for comparison.Conduction, based on R value but basically a linear mathematical relationship, more temperature = proportionally more energy. This is a 1:1 relationship. 1 more degree = one more unit of energyConvection is based on the temp difference to the ¼ or 1/3 power. What this means is you raise the number to the upper value in the equation(in this case 1, no raise) then take the lower number as the root – in this case we would take the cube root, or quadratic root (just like square root, that goofy long hand division symbol, but with a 3 or 4 to the left instead of a 2)So lets say we had a temperature difference of 20 degrees (100 minus 80) Math is in Excel languageRadiation = 20^4 = 160,000 heat units Conduction = 20*1 = 20 heat unitsConvection = POWER(20,1/4) = 2.115 heat unitsConvection = POWER(20,1/3) = 2.715 heat unitsAlso note that convection due to ceiling temperature cannot happen until after it absorbs radiant energy, because #5 explains why convection transfers no heat to the ceiling.So when we say radiant is the predominant mode of heat transfer in an attic, we mean by a factor of 59,000 times. Relative to convection. And by a factor of 8000 vs. conduction.Even if we argue coefficients of emissivity of these surfaces, it cannot be off by 100%I realize this is overly simplified, but more detail would make it more boring.Based on number3 and number 5 I don’t see how you think convection plays a part in heat gain in the living space.Number 8 and number 3 combine to cuase the equilibrium temperature to increase continually until the heat source, in this case the sun, is removed.-Rob
*Joe, I'm a little bored with this too, and as soon as you explain how the sun/car windshield is different from the stove/plate glass, then I'll leave it alone. You seem to be avoiding dealing with my example. Why?. You stated: "I never said the glass blocks the "heat" radiating from the stove. I said it blocks the air molecules that bring the "heat" to you."Well, the textbook I quoted clearly states that radiation does NOT require air molecules, in fact, radiation works best through a vacuum.Your example of feeling the warmth of the sun through a window, and the window is not hot, proves the point. Not only will the sun's radiation not be reflected by the glass, it will pass right through the argon fill (no convection), right through the glass (no conduction), and directly to your body. Radiation is in no way dependent on air molecules to transfer energy, which becomes heat when, as Rob points out, it strikes a receiver.And you won't accept my challenge (about the "test panel" in the attic being hotter than the air surrounding it) because you fear it's true, and what would that do to what you "know" to be true?
*Crusty,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Rob,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Hey...This is the best we all have discussed this topic since the sun first began to radiate...Joe...You are correct in all your physics as far as I can remember...(College was twenty years ago!)...Crusty you are right....Rob...you are right...I think we are all in agreement actually...Read the posts again and you will see what I am seeing...Joe is just not interested in heat until delivered...Crusty, Rob and I are thinking the delivery is what's most important in the attic situation...Well if we call a truce, I think we might all realize that our combined thoughts are somewhere near the reality of the actual hppenings in an attic...I am leaning this way for two more non scientific thought out reasons...A car will get nice and toasty warm on a sunny winter day...But also a fire is much warmer with the glass doors open even though my house is not heated as well cause so much air is now sucked up the chimney...Lets figure this out together guys...near the stream,aj
*A little side observation from the peanut gallery:My woodstove has a glass door and cast-iron sides. If I stand in front of the glass door with the fire roaring, my legs get very hot. If I step around to the side, but no further away, where my legs can't "see" the fire, it's much cooler on my bare flesh.Is glass or iron a better conductor? I always figured that I'm getting hot in front of the glass via the added elemant of radiant transfer.Am I off base here?Steve
*Steve,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Here's my $.02,Radiant heat in an attic is present, but only in a very tiny amount. To use Zerby's stove observation- glass is more transparent to radiant heat than cast iron. Shingles and plywood are less transparent to radiant heat than glass. Replace shingles and plywood with glass, and I believe the attic temperature would soar right off the chart. In summary: Glass-very transparent to radiant heat.Shingles and plywood(or cast iron)-very opaque to radiant heat.John
*JRS....The sun sends lots of visible spectrum...the shingles absorb it and re-radiate it in the infa-red spectrum...The glass home would not do the job as well being the inside of the attic is not made of such absorbing/converting materials....02 cents of mine...near the stream,aj
*Hi Joe,It's an airtight stove. The glass door is a single door with a gasket all around it (in good shape). There are air intake vents in the lower corners of the front face of the stove, but I can put my hand right in front of those and feel no difference in heat versus any other cast-iron part of the stove, and definately not as hot as in front of the glass. It is truly a line-of-sight from the fire phenomenon. No doubt it would be hotter yet if the glass weren't there.Steve
*Hi Steve,
View Image © 1999-2000"By the time a man realizes that maybe his father was right, he usually has a son who thinks he's wrong." Charles Wadsworth
*What I'm saying is that radiant heat heats objects, not air. The objects, in turn, heat the air. When the sun is cut off by a solid (not transparent) object, the radiant component is effectively stopped. The best example I can think of is an old fashioned window blind. Just a simple piece of rolled up paper can stop a window's ability to heat up a room by cutting off the sunlight. It has little mass, and therefore, little ability to transfer the sun's heat by conduction. A roof composed of shingles, plywood, and rafters, also stops the sun's ability to heat the attic by radiant energy. But because it has substantial mass, as it heats up, it is able to heat the attic by conduction.This is why I believe the radiant component of heat transfer, as it relates to attic air temperatures, is negligible. For what it's worth.John
*Joe and JRS...I was in my attic a few times today to move my tennis storage items in...The attic is trussed...the roof was warm at 10am...the room was cool but warmer than outside...At noon the room was warmer but there was intense heat hitting my head near the roof...The air was warmer there...so I ducted lower...the air was definitely cooler lower but the heat still hit me...Then I touched the answer to this all...A part of the truss...a 2x4 was at the middle of the area...the side facing the roof radiant heat was hot and the side away from the radiator was cool, as cool as the surrounding air...Radiation boys...is at work no doubt...near the stream,aj
*
View Image © 1999-2000
*Joe, certainly I understand what I read. You're splitting hairs. Obviously the glass in the stove is not heated by radiation, but by conduction of course. It's not the same thing as the sun/window -- you know that. "when it's 2° outside and the sun has been shining for 6 hr's when I open my car door it should be about 90° inside. . ." is an assinine statement. If there were no conduction of heat AWAY from the car by conduction/convection (and yes, re-radiation to the surroundings) that statement would be true. Joe it is a FACT that the interior of a closed car in bright sunlight will be warmer than the surrounding air, and this can ONLY be due to radiation.JRS, you seem to be assuming that there is no radiation other than from the sun's. Not true. As discussed earlier, the "minimal" radiation you refer to is what makes you feel hot or cold when surrounded by exactly the same temperature air, but with different surrounding surface temperatures. (Why is a 72 F (air temp) room pleasant in the summer, but feels cold in the winter?)Put the board in the attic where it can "see" the hot decking. If it doesn't achieve a temperature higher than the surrounding air, I'll eat my shorts!I'm done with this discussion. Joe, you will never be a believer because you don't want to -- you've spent too much time and energy trying to convince us otherwise. You'll keep splitting hairs and avoid doing any "simple" research to prove your theory one way or the other.
*Crusty...I did the experiment....What do ya think of my post?near the stream,aj
*hah, hah, ha... you guys are having such a good time with those brick-bats...punch and judy.....the problem is the controls arn't there to prove the thesis..the attic is too complex and there are too many ways to slide out from under any conclusion...FIRST.. is it a heating situation... 0 deg outside.. 70 dg on the other side of the attic floor /ceiling structure ?or is it summer.. 100 deg outside and 70 deg on the other side of teh floor/ceiling ?.. ok.. next ?am i the only one in the world who constantly types (((teh)) instead of ((the)) ?some of this discussion is esoteric.. and some has practical application...lets concentrate on practical application...1)the radiation of the sun thru the roof structure..cooling application.. soffit vents air current to ridge vent.. removes some of the heat from teh back surface of the sheathing.. thus limiting the upper temperature of teh roofing material.. thus extending the roof material life span..2) it ( the ridge vent/soffit vent) doesn't do a negligeable amount of removing heat from teh attic as a a total system.. but the net change is a plus for soffit /ridge vent...removing heat from back of sheathing b and removing any excess moisture that may be present FROM WHATEVER SOURCE..3) radiant barrier techniques.. most of the test results i've seen show the optimum effectiveness of radiant barriers with a 3/4 inch max space between the radiating surface and teh barrier....some of this has to do with the tendency of larger spaces to promote convection currents..crusty .. interesting that you are using your radiant barrier to channel the soffit flow direct to the ridge.. it will have a greater cooling effect on the back of the sheathing... but it will not remove any moisture entering the attic space from other sources..4) if there is no radiant barrier.. then the sheathing bottom surface is a radiator and it's next visible black body is the truss members or the attic floor / ceiling system5) if this system is sufficiently insulated .. the radiant gain is defeated.. and teh entire discussion as a practical matter... becomes moot..6) in a cathedral ceiling application (which this is not.. remember... it's an attic).. in that situation..it is dealer's choice as to the strategy to pursue..pack 'em solid with cells.. or use a soffit vent---airvent chute to the ridge vent (Shingle Vent II) pack the joist bay with insulation and some rigid foam accross the bottom of the rafters so you have a good thermal break at the rafters..have any of you ever used the radiant cove heat..?we've used it for twenty years as the back up heat for our solar heating systems...they're electric heaters located about 6 inches below the ceiling line .the electric rod element heats teh cast-aluminum radiating surface which is aimed at the occupied areas of the room.. usually located over cold surfaces like windows...they look like a valence... they heat black bodies.. very responsive.. very comfortable...and very expensive if the sun hasn't been out for a couple three days and the solar is depleted...the bottom surface of the roof sheathing is a very good radiant heater..next question.. you prefer brick-bats .. or louisville sluggers as your weapons of choice ?ok.. class dismissed.. some day when you guys are as old and wise as i.. you'll thank me....b but hey, whadda i no ?Pogo
*I was wondering if the rapid falloff in the intensity of heat that you feel from a fire through the glass doors vs. the lack of falloff of intensity of the sun's heat through a window is related to the "inverse square" rule that I know about concerning visible light.From my days as a photographer I know that the intensity of the light falling on a subject is inversely proportional to the square of the distance from the source of light. Thus, if a subject moves twice as far away from the light source, the intensity of the light drops by a factor of four. If your subject moves six times further away from the light source, the intensity of the light falling on the subject drops by a factor of 36 and so on. So when the light is very close to the subject and you move the subject closer or further by a foot, the corresponding change in exposure required is quite large. Conversly, because the sun is so far away from the subject, any change in distance the subject makes has no measurable affect on the exposure. Heat Energy being merely a different frequency of the same waves that make up the visible spectrum, would behave the same way, no?Steve
*some of it has to do with the wave-length of the sun's light vs. the wave -length of the fires flame...put a light meter on both...
*Steve...You are dead on right...I also see this as a huge difference between the sun as the radiant source and an Earth bound radiating source.Near the stream and the attic roof that radiates,ajMike...your knowledge is most welcome...but not the bats! hahaha
*Crusty,
View Image © 1999-2000"But, some people without brains do an awful lot of talking. . ." The ScareCrow
*Radiant heat is part of the equation, here's more reasons why I think that part is negligible.10-12 years ago radiant barriers made a big splash. They were promoted as the panacea to high air conditioning costs. Lower your summer cooling costs up to 40%!.....they claimed.Well guess what. Didn't happen. Angry consumers paid good money and failed to see results translating into lower cooling bills. After a few years many of the companies promoting these radiant barriers are nowhere to be found.If they were so effective, their use would be promoted by the local power companies to help alleviate peak summer loads. Instead, they promote other power saving strategies such as turning up the thermostat a few degrees, and rescheduling non-essential power use to off peak hours.I think the market place shows us how effective they are.John
*Joe, I guess you've found the key to happiness.... obviously you think as little as possible.Mike, your points are all valid (as usual). I am not totally enclosing the rafter bays; there are openings for exactly the reasons you state (moisture & heat removal), and I'm only using the foam on the south-facing roof. My thinking (sorry Joe, I'm doing it again) is to accelerate the air flow by creating hotter convective currents. This flow will be laminar for the most part, and will create a suction at the openings, drawing air into the channels and out the ridge vents. Think of it (sorry Joe) as a "convective pump" of sorts. Yes, an attic is complex, and my challenge assumes a summer day which makes it about moot for the time being. But the "test panel" will still be hotter than the surrounding air, on a hot summer day with full sunlight -- this can only be due to radiation. Yes, a.j., this can be directly observed, but I was hoping to get Mr. Fusco to try something that he couldn't otherwise explain away (I'm sure he has an explanation as to why your simple observation is wrong).John, radiant barriers cannot keep the heat from entering the attic space. Once it enters, even if deflected/reflected by a barrier, it's still there. If not removed by an effective venting system, it will eventually find it's way into the living space via conduction/convection.
*Crusty, If radiant barriers can't keep heat from entering the attic space, then you are agreeing with me that radiant energy is a non-factor in the issue of attic heat. Then why in the world would you install a radiant barrier?John
*I thought that the radiant barriers (wouldn't these be better labeled infared reflectors?) did work as long as they remained reflective but that as dust and grime accumulated over 12-18 months they became ineffective. Seems that this has been tested and shown to be the case.Interesting discussion. So far I'm sticking with the published material - like in Home Energy magazine and much elsewhere by Lstiburek - that radiant is the major means of transfer of heat form outside to the interior through the attic.
*John,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Has this radiant topic toasted all the particapants?near the stream,aj
*John, the key point (to me) is "removed by an effective venting system" -- I am combining the radiant barrier with what I believe will be an effective venting system. Even in the sloped ceiling rafter bays I will leave a 3/4" air space to allow air to vent to the ridge. The rest of the bay (furred out) will be packed with cellulose. If this doesn't do the trick, rigid foam will be used on the inside as well.Joe, I apologize for being so hard-nosed about this whole thing. You are certainly entitled to your opinion. My statements are only opinion because they are based on my understanding (technical and intuitive) of the mechanisms involved, and are presently unsupported by any empirical data. Stating my opinions as "fact" is rather arrogant.I have also personalized this discussion, which I should not have done. You are obviously thoughtful and intelligent and I should give your opinions the same respect that I would wish for mine, and I haven't done that. Sorry.
*The sun is radiating well today....near the stream,aj
*Pita or anyone....Why did the picture disappear?near the stream,aj
*You need to put it on the server first. Currently the picture is coming from "file:///c:/windows/TEMP/FrontPageTempDir/wpe3.gif".Try posting it as an attachment, then use the attachment URL as the image source...
*Trying Pita....
*Still trying...
*You've got to be kidding. . . . . .
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*I insulated the vaulted ceilings, kneewalls, and endwalls of our small cape with kraft faced R-13 FG and a 4-mil poly VB. I was limited to R-13 because of the 2x6 rafters. I have a shingle-over ridge vent and soffit vents at the frieze and I installed polystyrene raft-r-mates to create a continuous air space. Recently I poked around and discovered an alarming amount of condensated between FG bats and the raft-r-mates. I didn't want to expose the FG to the outside air but it seams like the raft-r-mates are blocking moisture reaching the vent space. Should I pull off the poly barrier and let the moisture disapate into the attic? What about the rest of the attic that has already been drywalled?condensation in PA
*Is the condensation on the insulation side of the raftr-mates? With a poly VB on the inner wall, where do you think the moisture is coming from? The poly barrier is blocking the moisture from the inside, right?
*You should start a new thread and re-ask the question. Its a good case study for this board.
*Paul. do the polystyren e raft-r-mates extend continuously from the soffit vents to the ridge vents? GeneL.
*I believe that your 4 mil poly has failed and/or you have not properly sealed the ceiling fixtures etc. and is allowing moist air to escape and being trapped by the polystyrene.Gabe
*the moisture is on the insulation and on the insulation side of the raftr-mates; the sheathing on the other side is dry. the rafter mates are continuous from soffit vent to ridge. my thoughts of the moisture source are:1. from humid air trapped behind the FG and the poly when it was installed2. from the sidewall stud cavity which extends down to the kitchen below (insulated with cellulose-no VB)
*Gabe. And the escaping moisture is condensing on the raft-r-mate becuase it is now the first condensing surface. n'est-ce pas'? GeneL
*Absolutely Gene, it has taken on that title.Gabe
*
I've read some discussions on attic ventilation, but it seems a lot of people are "back east". I live in Houston, Texas. I saw a comment about seeing an attic temp of 130 when the outside temp was 90 - is this the temp difference I should see in Houston in a properly vented attic ?
I have ridge vents plus soffit vents. I have read that the amount of ridge vent area should equal the amount of soffit vent area - correct ? If I have 650 sq in of ridge vent, then I should have the same amount of soffit vent area ?