I just read the article on Radiant Barrier Sheeting and it got my attention quickly and also begged several questions.
I am preping to start building a new house within a few weeks here in south Louisiana. Summer heat is high – 90 to 100+ is not unusual.We just got off of 3 nights in the low twenties. Temperature swings are pretty good.
I was thinking of using foil/bubble/foil sheeting under the OSB sheeting on the roof. You have to let it droop down to get the 3/4″ air space on the exterior side. This supposedly keeps the heat from getting into the attic in the first place during the summer and keeps heat in the attic during the winter.
Does anyone here know how the foil sheeted OSB compares to the foil/bubble/foil sheeting?
TIA
Replies
You don't need to let it drop down.
One surface has have free air space. That can be the bottom.
Materials that are high reflective are also low emmisive.
That means that it won't radaint the energy from the hot roof down into the house.
I have not seen any specs, but I would expect that that the radiant sheathing would be as effective as the separate bubble foil and easier to install.
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I became interested in radiant type insulation in about 1985. My understanding is that without the air space (droop) you get much more conductive/convective heat transfer. While it is true that the higher the reflectivity the lower the emissivety the air space added to the thermal performance in every test results I saw.
The most graphic test I saw for this was to hold a sheet of reflective Mylar against a large coffee urn, it will get hot immediately, but if you hold it 3/4 inches from the side of the pot you can keep your hand on it all day long.
Just what I remember from what I read and saw. I misstated the experiment when I wrote the above reply. The correct way ( and what I saw and took part in ) was to hold the film to the side of the coffee pot and hold your hand a small distance from the pot and your hand will get hot.
Now having said that , I am no expert, I looked into this type of insulation and remember that it would be most effective in high solar gain areas ( applied under the roof), or in high heating degree areas (under the floor)
I do remember that the Sate of Florida did extensive testing and found that in Florida under roof applications dramatically reduced the air conditioning (cooling) loads.
Edited 12/11/2006 12:26 am ET by dovetail97128
But we aren't concerned about how hot the material gets. But rather how how much heat it radiates off..
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
But we aren't concerned about how hot the material gets. But rather how how much heat it radiates off.
You're a genius! And where do you think the radiated energy comes from other than thermal energy?
Take a perfectly emissive material to absolute zero and tell me how much it radiates.
How are you going to keep it at absolute zero?.
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
How are you going to keep it at absolute zero?
Exactly. It won't emit anything, it will absorb and get warmer.
For a given emitter (aluminum foil) and absorber (attic floor), the radiant transfer is directly proportional to the temperature difference between the two. The hotter the foil, the more radiant transfer. If the foil is lower temp than the attic floor, the transfer goes the other way.
Don't worry. You'll get it eventually. Give it time.
:)
DG
But you never indicated how you are going to keep that foil cooler.And after the top service of the foil collects dust then it would become a nice absorber and it will heat up..
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill,
The amount of energy radiated from any given surface is directly proportional to it's temperature in degrees K..
The amount of conductive energy absorbed by any given surface is directly proportional to the difference in temperature between the surface and the contacting material.
Any given material will absorb less heat from any given source if there is an air space between them.
Re 1) above, It is best to keep the roof as cool as possible, this means ventilation.
Re 2) above, there will be less heat radiated from the reflective sheet into the attic if the radient surface can be kept as cool as possible. This means ventilation.
Re 3) above, a dusty, or even flat black, surface will absorb less heat if air gapped than if in direct contact. This also means ventilation.
All three above means that an air gapped radient barrior will radiate less heat into an attic than one in contact with the roof no matter how dusty the top of the barrior gets.
SamT
Now if I could just remember that I am a businessman with a hammer and not a craftsman with a business....."anonymous". . .segundo <!----><!---->
What is the specific heat of the ventalation air?How much air do you have to move to keep the radiant barrier cool?You only have so many BTU's/hr from the sun. You can either radiate it back out or remove it by venting or you can allow the attic to heat up.If you could get rid of the heat by moving air then you don't need the radiant barrier. Existing vented attic would be cool.And if it worked that ventalating air would keep the radiant barrier on the sheathing cool.Remember the discussion was not about having or not having a radiant barrier. It is using a roof sheathing made with a radiant barrier on one side or using a separate radaint barrier streched across the bottom of the joist (or as the OP orginally mentioned acrossed the top, but baggy so that there would be a partial gap)..
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
How much air do you have to move to keep the radiant barrier cool?
There's 'cool' and there's cooler.
If it's cooler with ventilation than without there will be less heat radiated into the attic. See 82680.21
You only have so many BTU's/hr from the sun. You can either radiate it back out or remove it by venting or you can allow the attic to heat up.
If you could get rid of the heat by moving air then you don't need the radiant barrier. Existing vented attic would be cool.
If you don't get rid of the heat, the system (roof + barrior) will reach an equilibirum where the heat radiated up and down and convected off the roof will equal the heat gained from the sun. This would be where the barrior is radiating the most heat into the attic. Adding ventilation to remove heat from the system reduces the heat available to radiate downwards.
About the dust, sure it reduces effectiveness, but that doesn'rt mean it reduces it to zero. Assume for the sake of argument that we replace that shiney foil with a black body sheet. Perfect absorption, perfect radience, no reflectivity. Ignoring the convective and conductive heat gain of the sheet, the perfect black body sheet will reduce the heat radiated into the attic to exactly 50% of what the roof is radiating downwards.SamT
Now if I could just remember that I am a businessman with a hammer and not a craftsman with a business....."anonymous". . .segundo <!----><!---->
The so-called "black body" radiation of heat from a perfect emitter (emissivity= 1.0) is proportional to the fourth power of absolute temperature. Two surfaces facing each other will radiate heat toward each other, and net heat will be transferred from hot to cold.
If dust accumulates on a foil surface (having high reflectivity, low emissivity, low absortivity), the dust, having higher absorbtivity and thus lower emissivity and reflectivity, will heat up and transfer heat by direct conduction to the foil material, and the other side of the foil material will radiate more heat at whatever it faces (the attic floor). As time passes and more dust accumulates on the foil surface facing the underside of the roof, the net effectiveness of the foil will be reduced. Just how long and to what extent depends on the amount and nature of dust in the air. Coal dust would be terrible, while a light-colored dust would have less effect.
Bill ,
Thanks for the post.
I misstated the experiment in my post so I will go back and add the corrected version.
Bill, In S. LA you definitly need an air space on the hot side.
To Adrian,
The reflective value is the same for the foils, bubblewrap and coated foam board. There is a greater 'R' value for the bubblewraps and foam boards.
More later-- watching the Saints game
Dovetail is mostly correct. If you have a space on both sides it prevents the foil from being heated by conduction, so it will have less to radiate (and convect) in the first place.
If you understand what convection is, then no, the air space does not prevent convection. That's why the foil around the coffee pot still gets warm (but not hot).
Rather than installing the foil over the rafters with droop, put it flat on the bottom side of the rafters. You will have less material used and less radiating surface, which further decreases radiated energy.
The bubble foil has close to zero R value. It's aluminum.
You need an air space on ONE side.I don't know which the "hot" side would be in your case except the roof.If you put a film on the bottom of the rafters and depend on the "top side" of that film reflecting heat them you are right it will work. But only for period time. Then the sheet will become coated with dust and it will stop being a reflector.But if the bottom of the sheet has low emissivity then it won't radiate out the heat into the attic. But you can accomplish the same thing in one step using a roof sheathing with a low emissivity coating (aka AL foil) on the bottom (attic side).See this thread.http://forums.taunton.com/tp-breaktime/messages?msg=58390.7Unforuntaly Both South Face and Florida Solar Institue have moved things around so none of the links are good and I don't have time to search them..
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Boats,Talk to me some more - after the Saints beat Dallas :) - You are sounding somewhat like I have been reading and hearing.
Adrian,
How bout dem Saints? They're so good when they're on their game.
I agree with Dgbldr, when he says that is is more efficient to put the foil on the bottom side of the rafters rather then putting a sag in it below the sheathing.
BillH also made a good point about the reduced reflectivity due to dust and dirt build up.
Doing the droop method you spoke of- condensation and moisture would be a big concern to me. You would need to ensure both top and bottom communication to vent any moisture. If the foil was on the bottom side of the rafters then you could vent from the soffit to the ridge with much more ease.
The perforated foil is being used by a lot of builders in the area in place of house wrap. Others are just concentrating on the south side of the house with the foil products.
Typical S. LA construction has an unconditioned attic with AC ducting routed in the space. Any effort made toward reducing that attic heat is money well spent in my book.
'R' values for the the bubblewrap and foil are minimal as someone stated earlier. An alternate method is to use foil faced rigid foam which has 'R' values up to R-7 per in" for the ISO products.
I've seen the foil coated osb but have never used it or know anybody who has.
One of the problems I had in researching the foil products, is that the so called experts were also the salesmen for the product.
What sold me was a walk through one house that was blacked in and used reflective foil that was 20 degrees cooler then the identical house down the block with out foil.
Boats brings up a point I have been concerned about - the "so-called" experts are also the salesmen.My cousin built a house with the foil/bubble/foil and his attic is incrincredibly cool during the summer. That is why I am / was planning to use it. I like the idea of stapling it to the underside of the rafters. Then use a good ridge vent and perforated soffit to vent off the heat. I wonder how much better it would be to use the foil backed OSB plus the f/b/f?It has bugged me for years why builders do not do something about that huge heat box driving heat down into the hose then battle it with AC. I think I will need to use some insulation between the joyces in the ceiling to help keep heat in the house during the winter. I'm getting down to the last few weeks where I have to decide exactly what to use and it is confusing to know which salesman (expert) to believe.
Adrian, if permissible by local codes, you also may want to investigate using a sealed attic. In certain warm climates with minimal winter freezing it makes sense. Talk to a local expert that can help you decide if it makes sense for your specific circumstances.
Sealed attics can work very well with radiant barriers.
Sorry all, just can't stay out of this any longer.
RADIATION- The process in which energy in the form of rays of light or heat is transferred from body to body without heating the intermediate air acting as the transfer medium.
CONDUCTION-A process of heat transfer whereby heat moves throught a material or between two materials that are in contact with each other.
CONVECTION-Transfer of heat by movement of liquid or a gas.
The above deffinitions are taken from Wirsbo Complete Design Assistance Manual 4th edition.
Air is considered a gas.
Radiation can be reflected in the same manner as light is. Althought dust may collect on the surface it will still reflect radiant energy, to think otherwise would mean that every piece of foil backed insulation now barried in walls has stopped functioning.
Again I'm sorry If I've burst someones bubble.
Since we are on bubbles, fbf does have some R value
Say did you here the one about blowing bubbles when you where a kid.
"Althought dust may collect on the surface it will still reflect radiant energy, to think otherwise would mean that every piece of foil backed insulation now barried in walls has stopped functioning."In most cases they never worked in the first place.Never had the air space needed for them to work as a radiant barrier..
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
http://www.ornl.gov/sci/roofs+walls/radiant/rb_02.html"Most of the field tests have been done with clean radiant barriers. Laboratory measurements have shown that dust on the surface of aluminum foil increases the emissivity and decreases the reflectivity. This means that dust or other particles on the exposed surface of a radiant barrier will reduce its effectiveness. Radiant barriers installed in locations that collect dust or other surface contaminants will have a decreasing benefit to the homeowner over time.The attic floor application is most susceptible to accumulation of dust, while downward facing reflective surfaces used with many roof applications are not likely to become dusty. When radiant barriers are newly installed, some testing shows that the attic floor application will work better than the roof applications. As dust accumulates on the attic floor application, its effectiveness will gradually decrease. After a long enough period of time, a dusty attic floor application will lose much of its effectiveness. Predictive modeling results, based on testing, suggest that a dusty attic floor application will lose about half of its effectiveness after about one to ten years.".
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Perhaps, in the fullness of time an adequate layer of hot dust would accumulate and become an insulator. Most folks have to deal with what is available in there price range here in real world of today.
But cost has nothing to do with this.Installing the radiant barrier sheathing should not cost any more than sheathing plus barrier. And no extra labor..
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Andy Engle had a blurb in the last FHB, said the 7/16" RB sheathing is a bout $5 more than regular OSB...I'm pricing a job that has it spec'd, guess what, it is $5 buck more...and I'm also thinking we'll have to be little more careful handling and installing it...that costs more, too<G> Oh God said to Abraham, "Kill me a son"
Abe says, "Man, you must be puttin' me on"
God say, "No." Abe say, "What?"
God say, "You can do what you want Abe, but
The next time you see me comin' you better run"
Well Abe says, "Where do you want this killin' done?"
God says, "Out on Highway 61."
But what would it cost you to install regular sheathing AND a separate radiant barrier stabled to the joists.One that you either have to put up first and not damage it with when you put on the sheathing.Or install it from the underside after you install the sheathing.Note this is not an IF. He is wants to use a radiant barrier.The question is which one (or both)..
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
The other downside with the stuff is that you can't cut a piece for a hip, and just flip the scrap upside down to start the next run.
I'm glad the pricing was right. That's nearly always the hardest thing to nail down.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)
"Get off your dead #### and on your dying feet." Mom
"Everything not forbidden is compulsory." T.H. White, The Book of Merlin
ORNL proved that dust does, in fact, affect fbf's usefulness as a radiant barrier TO SOME DEGREE. Their test was a dusty attic floor, and they lost about 30% of its effectiveness.That may or may not be more extreme than other applications, but it can and will affect it TO SOME DEGREE.That is why I rarely, if ever, recommend radiant barriers and always prefer to fill what has to be the "air space" with these products with something with real R-value.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Heh.. well look what happens when you type up a post and wait awhile to come back and hit submit. Many others said it better already!-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Here is very good article on radiant barriers.
http://www.ornl.gov/sci/roofs+walls/radiant/rb_01.html
The installation section show the different combinations of places to install the barrier.
But from what I saw in the charts there was more variation from the the type of test than the location of th barrier.
Thus I will standby my comments aobut just using the radiant barrier on the sheathing.
Except as I said in the other thread, think about making it conditioned attic.
Here are some more information.
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/92/920704.html
http://tinyurl.com/yyw4qo
I think that this is on target with your orginal question.
http://www.greenbuilder.com/sourcebook/RadiantBarrier.html
"Multiple layers.
One layer of a typical radiant barrier material will block 95% of radiant heat gain. A second layer for the purpose of blocking additional radiant gain blocks less than 5%.
There is an added R-value due to dead air spaces between the layers."
IE installing a sheating with a radiant barrier along with a 2nd barrier on the joist will reduce the heat load. But the reduction is minor.
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
For anyone that is interested the Florida Solar Energy Center is the soruce of much research on both radiant barriers and cooling loads in roofs/attics.http://www.fsec.ucf.edu/en/You can get some information by clicking on the different links.If you use the search using appropriate terms you can find the different research papers.And they have published some of their stuff at JLC. I remember one article where showed that increase mass insulation in the attic increased the peak cooling load. Bascially it kept absorbing heat during the day and releasing in the late afternoon when the air temp was also the highest.Some of their research also covers the affects of different type of roofing materials.
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Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
What article on Radiant Barrier Sheeting did you read? Was it a recent article in FHB magazine? Or was it a thread in this forum? Or somewhere else? Thanks.