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Here is a purely technical question that I’ve been wondering about. Does anyone have any data or formulas to compare infra-red transmission through the different insulation materials that are available? My attic floor is dense packed with cellulose which seems to do a good job at keeping the second floor cool, but my wife would like to use our boiling attic as a studio.
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Fleetwood.
Joseph Fusco
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*The attic is accessible through a staircase (which is also dense-packed under the threads), it is un-vented, with no VDR other than lead paint and plaster. The cellulose was blown in 5 years ago under the attic floor boards. The floor joists are old 2x6's so I've maybe got r-18 in there. The house is in Northern New Jersey. Oh, the shingles are white and are nailed over T&G 1x10's or 12's, I can't remember. There are 2 windows at the gable ends .
*Fleetwood.
Joseph FuscoView Image
*I appreciate what Joe is doing for you, but I think it will give you more info than you originally asked.To answer your question: Cellulose is a radiant barrier, foam is a radiant barrier, fiberglass is not a radiant barrier. This is simply stated, but true.-Rob
*1. The cellulose was blown in soon after we moved in, but we haven't had any ice damming since. The house is 2 stories, with a gable roof. The basement is unfinished and dry, at least of visible water. The attic stays cold in the winter and it's windows work reasonably well. The rest of the house has had 50% of the windows replaced as rooms are re-done. The cellulose was blown in during the summer and we immediately noticed the difference in the second floor bedrooms. Asfar as the energy costs during the winter, I don't have a comparison due to the short time we were in the hose before the installation.I've seen no evidence of moisture buildup in
*Ok, I get that, but to what degree is that true. I've read on this board about the flashlight through fiberglass test, but does this also apply to the non-visible spectrum? Not wanting to be Claude Raines, Fleetw
*Rob,
Joseph Fusco
*Fleetwood & Joe,I'll agree with Rob on this one. I've did two attic redo's last summer, both attics were miserably hot in the summer ans unusable. Both were unfinished, within the building envelope, with FG batts in between the rafters. One house just had poly over the FG, the other had poly and drywall. Both houses had vented roofs, properly size, full-length soffit channel to ridge vent. Channels were clear (clear vent paths). Both houses also had gable-end windows, one also had three dormer windows on the front. Opening the windows produced a breeze, but the radiant transfer overwhelmed the heat removal via the breeze. In both cases, the FG was removed, drywall applied and cells blown in. Trust me, after the cells the heat reduction was dramatic. Neither homeowner wanted foil-faced RFBI which I wanted to do. Within a few days the entire house, especially the second floor, had cooled down. The second floor was much cooler, both reported lower elec usage later on due to less air-conditioning. I've followed up with them as I'm planning on doing my own attic this year. I'll end up using foil RBFI as part of the package, though. I'll most likely be posting a Q in the upcoming months for additional info.Regards, Mongo
*This might be a good place to ask a question:Other than fire resistance how is dense pack celulose you all refer to different than the sawdust insulation that was common in the NW and SubArctic around the turn of the century? Better job? Just latest interation? Looking for knowledge.
*I remember Fred L talking about a similar job he did. I think he used 1" or 2" foam, DP cellulose and 1/2" drywall over the whole thing. I tried to find the post in the archives, but I think it was lost in the crash.
*Good memory on your part...I think I remember the same, he filled the rafter cavities with DP cells, no vent channel. I think he used two layers of 1" foam, gapped 3/8ths of an inch and gunned foam in the gaps, then the drywall over that. Don't positively recall furring strips, though they may have been in there somewhere.Was that the post where he "smoked" the room afterwards and "the smoke just hung there...?"
*I think your memory blows mine away on this one. I think I'll do the same in my attic, except I'm worried about decreasing the size of the room too much. A roof with a higher pitch would have been nice.
*While it isn't the question I answered, I have a hunch that a RRB (radiant reflective barrier) is the possible conclusion to this discussion. Be careful on this.Reflective foil of any type or configuration attached to bottom side (facing down) of rafters got panned in an obscure study that found it actually prevented conducted heat from escaping in late afternoon. Apparently RRB's reflect back noon time radiant heat before conductive heat actually enters the house. As the house mass heats throughout day, the conductive heat is trapped in late PM and throughout the night by RRB's. I can dig out reference if you need it.Why not build down the the rafters with plywood gussets and 2x2's as nailers, fill the cavities with DP cell and sheet the ceiling like any DP wall cavity (no VDR other than the paint).My hunch is that your whole house will be cooler in summer and warmer in winter if you fill the rafter cavities and rely on them instead of the attic floor. tedd
*Oh my, Fred, you have desecrated the holy temple, wherein fiberglass and cellulose are forever in mortal combat.Very different, search about.
*I remember that one because he said he blew a puff of something tetrasomething and someone asked what the heck that was. I have a puffer now but the air here moves so fast you don't get to see much....Maybe i can dredge it up.Have to agree about the IR being a big factor. In our attic crawl space there is a definitely rotisserie effect. In some areas the fg batts had fallen down and you could feel the radiation coming from that very spot like a hearth. I doubt fg is transparent to IR (maybe it's just the kraft paper!), but the consensus here seems to be that cellulose does a much better job blocking it.oh here: the house as a coffin from the archives (I was searching in another window).What are the negatives of foil polyiso v. all cellulose? Never quite understood this, except that if the foil gets dirty it is worthless.
*Sawdust insulated walls I've seen settle. Also - just as dense pack has slightly less insulation, I'd assume that sawdust might be even less because it would be more dense. Also, animals and insects seem not to like cellulose - something about the newsprint ink I seem to recall someone saying - and I believe they do like sawdust.
*The varmits really don't like the boric acid added to the cellulose. Though I imagine the inks are not good for them. Another reason for soy ink. :)
*All I know is that before I tore out the fg, I had squirrels living there - and now they don't come back. I believe FredL told me it was the ink.
*Oh. What were the squirrels eating before?
*How does polyurethane (the spray-in foam) score as a radiant heat (what I would called IR) barrier?
*they were just living there - bringing food in
*Re reflective barriers & dust, an oddball link:"Ecoguard" reflective chips http://www.savenrg.com/fibrglas.htmand an explanation of heat transfer in REALLY BIG TYPEhttp://www.savenrg.com/1rfactor.htm
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Joseph Fusco View Image
*Would adding some loose cellsi on top ofa foot or so (R40) of f/g batts in an attic space create a useful radiant barrier. If so, how much loose fill cells would be necessary?Removing otherwise "good" f/g and discarding is not a viable option. . . labour, dumping fees, unnecessary bulk in a landfill etc.curiously-pm
*Fleetwood. Let me see if I can explain minus the technical jargon.When radiation passes through a series of materials it is partly reflected, partly transmitted and partly absorbed. Absorption takes place when the radiation passes through a material. AS it passes through it becomes progressively absorbed and the radiation becomes weaker and weaker. If the material is a poor absorber, the radiation will pass through it with little loss in intensity. But in a highly absorbent material the radiation is totally absorbed in just a short distance into the material.It is as though all the radiation were absorbed at the surface of the material.Low density fiberglass is transparent and permits heat to move through it faster than through cellulose insulation, which is more opaque than fiberglass.The proof of this is seen in their R-values: R-2.7/inch for fiberglas vs. R-3.7/inch for cellulose. Fiberglass's low desnity and low R-value show that it does not block radiant heat as well as cellulose fibers.Keep in mind that the effective R-value of any insulation is temperature dependent: the R-value of both cellulose and fiberglass drops as the temperatrure increases. On the other hand the R-value increases as the tempersature drops.The Florida Solar Energy Center did some research on fioberglass insulation to determine what effect the changing R-value had on a house's total cooling load.they found that during the hottest time of an August day in Orlando, Florida there was a 10% increase the total cooling load. Unfortunately the analysis was not performed on a cellulose insulated attic. Based on the data it seems that the effect on cellulose would be half that of fiberglass. By the bye. Radiant heat cannot be flushed out of an attic. GeneL.
*andrewAn RRB is an IR barrier/reflecter. Its the IR that people call "radiant heat" and converts to conductive heat when absorbed by a solid. The question you are asking doesn't actually make sense. The real question should be, I believe, is: how much IR is reflected off the insulation verses how much is absorbed (as opposed to transmitted since once its absorbed its no longer IR). Not much is reflected off of insulation unless it has a RRB attached to it. With or without an RRB, if the insulation is snug fit without any adjacent air space it will not reflect any IR. Without adjacent air space of atleast 3/4", an RRB of any material will not work. Insultation is supposed to lower or prevent conductivity and when IR hits it, the IR turns into conductive heat and is resisted the same way indoor heat is resisted from leaving the building. This is why cell makes so much sense to me since glass particles (FG insulation) are highly conductive and paper is not. IR hitting FG will be converted and conducted through the wall more efficiently than cellulose. Poly is transparent so it passes IR unless its up tight to adjacent surfaces - then it can only conduct heat and not transmit nor reflect IR If you check out Reflectix Inc on the web, they will send you an article on the IR absorption and reflectivity of a list of building materials along with samples of various BBB's. "Reflectivity" , "emissivity" and "absorptivity" are the key words for a web search. Don't combine RRB and VDR in a single membrane.
*Thanks for the reply, Gene. So basically the R-value gives a good idea of the IR blocking ability of the insulation ( with the corrections for temp. you mentioned). How do you feel about the radiant barrier made of metal chips? Is this just hype? Not anonymously, Todd 'Fleetwood' McEvoy
*My point exactly. Insulation cannot pass IR or its not insulation. IR is not in the visible light spectrum. You don't see IR when you shine a light on insulation and see some diffused light on the other side. If IR can pass, then heat passes since IR is heat radiation. But if IR strikes a non emitting solid, it transforms to conductive heat. It can't possibly continue as IR until the heat reaches the other side of the solid and emits off that other side as it converts to IR. That's what radiant in-floor heating is all about.And there are RRB's that are combined with VDR for an "all in one" solution. I don't like 'em but they are available.Using RRB's like foil chips on the attic floor is a dust catcher so they don't work after a while. If you want to keep heat out of a building then stop it at the outer most surface -- a roof material and surface that reflects IR. If you think there is a lot of attic condensation problems now in heating climates where RRB's are hardly ever used, wait until they are put on attic floors. The benefits of returning indoor heat back down through the attic ceiling may be there; but, the problem of returning outdoor IR back up to the roof won't be a pleasant thing to see. And this problem is even greater with under-rafter RRB's that actually work the opposite to what thery are claimed to do - they hold in IR that has migrated through the building mass and reflect it back into the attic. RRB's won't stop conducted heat transfer.
*I think I'm finally coming to some understanding of what is going on...If I understand correctly, it's not quite right to say that the R-value indicates the IR blocking ability of the insulation. The insulation (like any other material): *REFLECTS some radiant energy (visible, IR, UV, whatever). In the case of cellulose, not very much. In the case of aluminized plastic film, a lot. *ABSORBS some radiant energy, and then CONDUCTS the heat created when the radiation is absorbed, through the thickness of the insulation. If the material is a fluid (like air), it can CONVECT the heat through its thickness as well. Cellulose both absorbs more radiant energy than fiberglass, and conducts the heat thus formed more slowly. The R-value indicates the resistance to conductivity, but not the degree of absorption of the radiation in the first place. If properly installed without voids, convection within the insulation should not be an issue, but if there are significant vids, e.g. around electrical or HVAC components, it could be. BTW, heat is generated when other frequencies besides IR are absorbed. (I think we just associate it with IR because we can only detect IR by the heat it generates, since we can't see it; or maybe our skin absorbs more IR than other frequencies and so is heated more by it.) *TRANSMITS whatever radiant energy is neither reflected nor absorbed. This energy becomes heat only when it is absorbed by something, be it the air inside the building envelope, or your skin. The radiation, as Gene says, can not be "flushed" out of the attic, although the heated air conceivably can. *EMITS radiant energy by virtue of having a non-zero temperature. The hotter the material gets, the more it radiates. Low E windows emit less radiant energy than untreated glass of the same temperature. I've never seen this concept discussed in relation to attic insulation, however.Still doesn't answer the original question about the relative reflectance of various building materials including different types of insulation, but I think another poster made reference to Reflectix, Inc. for that information.It seems to me that a material of high reflectance would be most useful outside the building envelope if the goal is to decrease the temperature inside the envelope. Furthermore, if the highly reflecive material is inside the attic and stays reflective (i.e. no dust -- yeah, right), it should increase the temperature of the roof. A, theoretical at least, problem with regards to shingle life in hot climes and ice dams in cold ones. So a white roof seems in theory like the best way to keep the sun out. Furthermore, a thick layer of a highly radiation absorbent and highly resistive-to-conduction (R-value) material (like cellulose?) should slow the heat gain sufficiently for the cool night to come before the house gets appreciably warmer or strains the AC system. In really hot climates, where the nights are consistently hot as well, the RRB concept makes more sense to me. You'll notice, all the testimonials for that chipped aluminized stuff are from Arizona.Should multiple reflective layers work better than just one? I haven't tried to work that one out, yet. They might, actually. It might matter what the actual numbers are concerning % of radiation transmitted and % reflected (and % absorbed).Sorry for the length of the post.
*Yes, multiple reflective surfaces with intermediate air spaces work better than one. Air space must be on the reflecting side of the mterial.
*Insultaion is a radiant barrier but not a reflective barrier. Anything that intercepts the radiant heat is a radiant barrier.Porosity/permeability would determine whether the RRB was also a VDR or not. You know this rule better than me: the rule of 1 in VDR's. Since southern homes will have VDR's on the inside of the wall cavity, using a RRB/VDR combination on the outside of the wall cavity would be a disaster -- so they are made to be porous. The samples I have are peppered with pin holes. The chips version of RRB's are also intentionally designed not to be a VDR. Certain house wraps are theoretically, reflective and porous. Of course, as you have shown me in other conversations, the interior VDR in a cooling climate is not the best. So VDR/RRB combo products on outside region of wall cavity would probably be a good solution. This would work in the top of rafter bays in heating climates provided no VDR was in attic floor -- but neither RRB's nor VDR's are a good ideas in attic floors.
*tedd,
Joseph FuscoView Image
*Dear Todd,No, never. IR opacity is one element of thermal resistance. But insulation is rated at 75 degrees. So products such as fg which transmit lots of IR can get higher ratings that do not indicate their practical effect.Tedd, Every type of insulation transmits some IR. Even in cels, IR heat transfer occurs in the air spaces between the particles. It performs better under high heat loads because those spaces are relatively small. In low density insulation, the interstitial spaces are large and IR really does go right through those products.I recall seeing a show about space shuttle tiles. A small cube of the same ceramic was heated to several thousand degrees. Then it cooled a bit, they showed someone holding it by the corners while the center was still cherry red. That material was very transparent to radiation, but had incredible resistance to conduction.Joe,“It's only when a mass has absorbed enough "energy" and becomes "hotter" then the surrounding area that it begins to radiate "energy" in the low frequency, Infrared.”Nope. Human comfort has a large component of IR. Even walls that are 20 degrees colder than our bodies emit IR that keeps us comfortable. I think you’re confusing conduction with radiation. Surely only warm objects conduct heat to colder objects. That property does not apply to radiation. Everything radiates no matter what the surrounding temperature is.I do agree however, that there is a lot of misinformation about radiation floating around. Thanks for hammering that point home.Regards, Fred
*FredYou have to give me a reference on this. I can see (no pun) that IR would pass through FG but I am baffled as to how it can pass through Cell. How is this tested to ddistinguish between a direct pass through verses IR radiated off of conduction-heated insulation.In the post that started this thread, the issue was the IR passing through the roof into the attic -- I think that was the issue, its been so long now and I am too lazy to scroll up. Even if IR can pass through cell, it isn't going to pass through the roof and sheathing. The heat gain in attics it seems to me, is largely conducted heat passing through the building mass from IR hitting it on the outside. When this conducted heat arrives at the insualtion it continues to conduct, I doubt any is radiating across the insulation. But I am always willing to learn.I still maintain that if insulation passes IR its not insualtion -- or at leaset its poor insulation. If what you say is true then RRB's on the outside of the insulation would save cooling and heating costs in northern climates. Why are RRB's loosing out in favor of using insulation in cooling climates ?Come to think of it, I do get a sunburn on cold days but I don't get a sunburn through any clothing like wool mits. Isn't the shuttle ceramic's a disapater/emitter rather than an insualtor ? I don't see how the ceramic passes IR but rather I believe it absorbs it, converts to conducted heat and radiates/emits it. I don't see the connection to insulation. Please explain.Tedd
* Fred,
Joseph Fusco View Image
*OK guys, I'll add my two cents. I haven't a clue how to format the text (mainly because I don't care).Fred - "Even walls that are 20 degrees colder than our bodies emit IR that keeps us comfortable."Joe - Only if the room the wall is in is colder then the wall.Rob - Joe used an absolute ("only if") that simply wouldn't hold true. Say this room had an air temperature of 80 degrees, and the opposite wall was 40 degrees colder than our body. Both walls are black. For this to be steady state there would be a tremendous amount of heat being radiated out of the -20 degree wall, through the space, and being absorbed by the -40 degree wall. The air temp almost makes no difference because radiant heat transfer is so much more efficient than convection. In this case you can bet that the -20 degree wall would feel more comfortable. Of course this probably only could happen in a lab. In a house I would agree that a wall that is 20 degrees colder than our body probably wouldn't radiate much heat to us. The -20 wall would radiate tremendous energy WRT the -40 wall. Our body would radiate less energy to the -20 wall than the -40 wall, I would consider that more comfortable. Surface temperature is dependent on surrounding conditions.Fred - "I think you’re confusing conduction with radiation. Surely only warm objects conduct heat to colder objects."Joe - Surely you jest! Colder conductive objects conduct heat from warmer ones.Rob - Sound like a wash - both agree that energy transfers from hot to cold (assuming similar surface colors). Or - paraphrasing my heat transfer book - ther is only heat and negative heat. Cold is expressed in terms of not hot.BTW - Joe I am impressed by the trap you set earlier with regards to infrared vs the visible spectrum. I hadn't logged on in a while, and in fact I probably would have skimmed it anyway.I was recently discussing this thread with a co worker (another college weenie engineer)and the problem is that surface temperature is dependent on surrounding conditions. To fully know what is happening in these situations you need to know the radiant emission and surface temp for each surface in the attic. The air temp would be useful for reference. Oddly enough, the cooler surface could be the one radiating more energy. Take the underside of open sheathing vs densepacked bays. The sheathing in the open is probably going to be cooler because it is shedding it's heat. The sheathing in the densepacked bay would get hotter because it can't radiate heat downward. Old timers tell me that this will melt my shingles together and void the warranty - but that's another dead horse 8-).-Rob
*While we are on the subject, I have to commend Fred L. about a year ago I posted about installing a whole-house fan in my at the time uninsulated, though heavily shaded roof. He said it was radiant heat, but I disagreed because it didn't feel directional. I now knoe why. The tops of the walls were not insulated either! The blown-in cels had settled and left all but the first foot of the walls uninsulated. ( This is my anecdotal data for not liking cels, but I've changed since then). The new roof has 2.7" rigid iso board over the old deck, and the bays are not yet packed with cels. The upstairs will now stay about 15 degrees cooler than last year (in the past two hottest days of summer). And I still have tarpaper over the ridge vent opening! Boy I bet it will really cool off when I get that venting going huh? - NOT! The radiant heat is now very apparent from the uninsulated north facing gable wall. Very directional and noticable. What amazes me is that the roof has large portions that the sun never touches, yet it heated enough to make it very uncomfortable. BTW - the old roof had 5 1/2" batts in the collar ties (1 1/2 story hip roof) that didn't do a thing for blocking the radiant heat.The breeze seems to stop at night here lately, so we have used two 24" fans in the upstairs winows to blow the warm, newly stagnated air out. In about an hour the upstairs is quite cool. Last year these two fans running non-stop provided no relief.While we are talking thermal comfort. I did something else this year that has helped dramatically. I trimmed all the maples back such that they are no closer than 5 feet from the house. It seemed last year that the trees would hold noticably hot air under them until late at night, now the hot air seems to draft up and out lots faster.-Rob
*Rob,
Joseph FuscoView Image
*Rob,
Joseph FuscoView Image
*Shirley, you are WRONG.Fred is correct.conductiontransfer of HEAT or ELECTRICITY through a substance, resulting from a difference in temperature between different parts of the substance or from a difference in electric POTENTIAL. Heat may be conducted when the motions of energetic (hotter) molecules are passed on to nearby, less energetic (cooler) molecules, but a more effective method is the migration of energetic free electrons. Conduction of electricity consists of the flow of CHARGES. Metals are thus good conductors of both heat and electricity because they have a high free-electron density. Cold objects do not actively do ANYTHING to get heat/energy transfer from hot objects. The hot object's heat is FORCED UPON the colder object. This is basic high school science.
*Shirley, you are WRONG.Fred is correct.conductiontransfer of HEAT or ELECTRICITY through a substance, resulting from a difference in temperature between different parts of the substance or from a difference in electric POTENTIAL. Heat may be conducted when the motions of energetic (hotter) molecules are passed on to nearby, less energetic (cooler) molecules, but a more effective method is the migration of energetic free electrons. Conduction of electricity consists of the flow of CHARGES. Metals are thus good conductors of both heat and electricity because they have a high free-electron density. Cold objects do not actively do ANYTHING to get heat/energy transfer from hot objects. The hot object's heat is FORCED UPON the colder object. This is basic high school science.
*Franz/FredL,
Joseph FuscoView Image
*Joe - You left one very important component out of your "wall" hypotheses, the outside energy source. If there isn't one, you can forget about these wall radiating anything because there isn't anything to absorb. It also seems to me that my "only if" was far less complicated then your multi-conditional scenario.Rob - That is why I said "steady state." Obviously for the example to work, there must be a heat source on the -20 wall, and a refrigeration source (substantially larger) on the -40 wall.Joe - As for the second critique;If you don't recognize the fundamental difference in the language of the two statements, then who am I to argue with you.Rob - I read it twice and still didn't make sense.Joe - I guess by the lack of a critique on the third that you agree with it.Rob - Yeah, It's OK. But I guess one could ask if a better terminology would be radiation and "negative" radiation. Being that thermo likes the heat and not heat terms.Joe - As for your statement;"To fully know what is happening in these situations you need to know the radiant emission and surface temp for each surface in the attic. The air temp would be useful for reference. Oddly enough, the cooler surface could be the one radiating more energy. Take the underside of open sheathing vs densepacked bays. The sheathing in the open is probably going to be cooler because it is shedding it's heat."This could be a most erroneous implication. If you've ever touched a tile floor in someone's home it feels cool. If you take the temperature of the tile it's the same a the ambient room temperature. The reason it feels cool is because it "absorbs" heat very quickly from your hand. Ever touch a "hot" pot? Bet you got burned! That's because your finger absorbed energy form a "radiant" energy source. In the sole case of living beings, things that feel cold, "absorb energy from you", things that feel hot "radiate energy to you." This is also a very good indication of what the object is doing in it's environment, if it wasn't, living things would be in a world of trouble!Rob - I belive this statement is the erroneous one.The tile is colder because of a physical property called "specific heat." The tile is actually cooler because there is not enough available energy to make the tile the same temperature as the air. A hot pot is indeed a radiant energy source, however if I got burned when touching it it was purely from conduction. If I held my hand near it and got burned it was probably from radiation. Convection, in the hot pot scenario, is right out the window when it comes to burning my finger.My last statement was specifically addressing the amount of energy being transferred via radiation from the sheathing. Do you think the surface temperature of the steel pot would be hotter or cooler if a block of dry ice were placed next to it? It would be cooler (generally) because it is transferring more heat (higher delta T despite lower surface temp?) Would the steel be hotter if an insulating jacket were placed around it and it could not radiat or convect? Assume there is a material in the pot that has no boiling point, otherwise the temp in the steel wouldn't change noticably.I'm surprised you disagree with the insulated vs. uninsulated sheathing argument. Seems to me that the interior surface of the sheathing in three adjacent bays in the same roof with the same exposure to the sun would have different temperatures. I would bet that the coolest interior surface temperature would be the open bay, followed by the fiberglass bay, followed by the DP cells (or foam) bay. I doubt convection would play much of a role in the open bay. The one that would feel the hottest would also be the open bay, followed by fg, followed by cels.I have no attic, there is 1 crawl space above the collar ties (1 1/2 story hip/gable). There is thermal startification no doubt. Only this year from peak to ceiling the delta T is about 20 degrees and last year it was about 100 degrees. This year there are lower temps overall as well.-Rob
* Rob,
Joseph Fusco View Image
*Joe - "The only thing I disagree with is the mechanism that heats most attics. I beleive it's conduction and convection. "Which I guess brings us back to the original question of this thread: how much radiant energy is reflected/absorbed/transmitted by various building materials, assuming the souce is the sun? For example, asphalt (let's leave aside the question of the color of the granules for a minute -- maybe just do black shingles or white), OSB, solid wood, fiberglass, cels. I should think that would be an empirical question that could be measured and answered (in terms of fractional amount per linear unit of thickness). That might form a more solid basis for an opinion about the relative importance of radiant heat to the temperature of your average attic. Or at least the relative importance of radiant heat transmitted into the attic on the temperature of the attic. I can't imagine the radiation absorbed by the roofing materials and conducted through them is unimportant.Just a thought.
*Joe - Not alot of time today, but I'll talk a little."If you can derive some implication of an outside energy source here, you win. Your quoted "steady state" means nothing. The language leaves something to be desired also."The heat source here wouldn't matter - what I am saying is that this is an experimental setup. The term steady state means alot here. It means that some process has reached equilibrium. In this case the wall radiating radiates enough heat to match that being absorbed by the other. I personally don't care what the heat/refrigeration source is. Steady state experiments also make it rel easy to gather data.I agree conduction helps heat attics. Heat conducts through the roof to the interior of the space. I disagree that convection does much. For convection to work effectively the lower surface must be the hotter of the two. For heat to transfer from the roof to the attic floor it has to get radiated. Thermal stratification is the best indication that convection isn't working so well. The heat is gathering at the roof because light convection currents carried it there. To complete the cycle of convection we need something at the top to be shedding the heat so that it's density may increase and the air would drop down to get heated up and rise again. In the case of the attic the whole upper surface is hot. Without radiant heat transfer the floor wouldn't increase in temperature a bit. I will agree that the roof is shedding some heat by radiating it back into space, but this would be minimal compared to the amount being radiated to it from the sun.-Rob
*You need to review English 101 for Dummies and learn how to spell and construct sentences.Then maybe it will be easier for you to communicate on a much higher level than you have been. You mix fact and fantasy to produce a slurry of drivel. This is a simple concept. Concentrate and try to stay on track. Heat energy moves from the hotter thingy to the colder thingy. End of subject.
*
Joseph Fusco View Image
* Rob,
Joseph Fusco View Image
*"I believe you need to familiarize yourself with the "The Kinetic Theory of Gases." You can search the archives for the discussion on "Pressure Plans" to see what I posted in concern with this topic. I'm just lazy and don't feel like re-typing it. I will add that the expansion of any gases is a cooling process. It doesn't need "something" to transfer energy too."I will quote from "Introduction to Fluid Mechanics, Third Edition." In Appendix A "Physical properties of air." At atmoshpheric pressure, and sea level, at 80 degrees, air has a density of .0735 lb/ft^3; At 120 degrees air has a density of .0684 lb/ft^3. I have assumed you cannot pressurize your attic. We can see that the air at a higher temperature has a lower density which means increased volume per pound. All you need to do is explain how taking heat out of the air raised it's temperature to 120 degrees.I forget if it is the 0th law or the 1st law of thermodynamics, but one of the two says "energy is neither created or destroyed" for the air to change density energy was indeed transferred.By "Kinetic Gas Theory" I assume you are referring to PV=nRT. This law explains how it is possible to expand a gas and lower it's temperature by increasing it's volume. The factors n and R are constant, so lets look at PV=T. From the perspective of volume the equation becomes V=T/P. So to increase volume (expand a given mass of a gas and lower it's density) you either increase it's temperature, or lower it's pressure. Since we are talking atmospheric pressure, you would need to pull a vacuum on your attic and lower it's absolute pressure to about 12 psia (not likely) or increase it's air temperature. "Three days ago when this thread begin I when into my attic space to measure the temperature. It's about 5' from floor to ridge. There is R-38 of FG insulation on the floor and none in the rafter bays. The attic is vented. The outside conditions where clear, 89 degrees and 75% humidity. The space below the attic was conditioned at 74 degrees. The time was 2pm an the temperature in the attic was 114 degrees at the ridge and 90 degrees at the floor, your assumption was correct, no radiation. The unique thing was the humidity was only 28% at the ridge."Do you have a means of measuring the surface temperature of the floor? The air temperature was 90 degrees as it came in accross the floor and something had to heat it to get it to rise to the peak at 114 degrees. By the way 89 degree air at 75% humidity would be expected to be 28% humidity at 114 degrees, This is why it is called "relative humidity." A given weight of air will still contain the same weight of water at 89 or 114 degrees. The difference is that the 114 degree air could hold three or so times more water than the 89 degree air. So it is "relatively" drier.-Rob
*Hey Fools-co,You've been spending way too much time in the attic heat. The unique thing was the humidity was only 28% at the ridge. Duh! Ever look at a RELATIVE humidity table? I see Rob's already pointed that out, but I figured that dense-packed skull of yours will need the over emphasis.Franz
*Rob;
Joseph FuscoView Image
*Von Bozo;
Joseph FuscoView Image
*Thank you Rebeccah! I was sure i wasn't the only one feeling lost.Heat energy moves 3 ways: convection, conduction, and radiation.Consider a Thermos: it resists each of these transfer mechanisms in different ways. The glass vessel is double-walled, with the only physical connection at the top -- this limits conduction. The space between the vessel walls is a partial vacuum, limiting convection. Finally, the walls of the vessels are silvered, reflecting radiation away. So hot contents stay hot, and cold ones stay cold.Consider a roof: ideally, you would place a perfect reflector behind and not touching the roof deck, then remove all the air between the two. Do the same towards the interior space. Not very practical. All of the various insulation technologies are attempts towards this ideal, and they usually focus on the dominant heat transfer mechanism of conduction. Incidentally, the problem with allowing the reflective layer to touch the wall or roof deck is that conduction will overwhelm radiation in significance and render the reflector "effective" but utterly irrelevant.I consulted a rocket scientist (well, aerospace engineer) on this one (my best friend). Notice how they put that silver blanket on satellites to reflect solar radiation -- most of their heat problems concern power dissipation from the electronics.
*b Density Rules boys...you're all just taking up space!Fractally ;~j
*Jack,
Joseph FuscoView Image
*It's a choice to obey....hmmm.Cutting to the chase, near the stream (,')j
*JackieNever cut into a chase. . . you might expose a chimney. . . where there's smoke there's potential for a buzz, I mean fireHow's tricks. . . didja ever catch up with that crack female plumber???-pm
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Here is a purely technical question that I've been wondering about. Does anyone have any data or formulas to compare infra-red transmission through the different insulation materials that are available? My attic floor is dense packed with cellulose which seems to do a good job at keeping the second floor cool, but my wife would like to use our boiling attic as a studio.