I keep seeing these adds on TV where the guy is spraying silver colored paint/coating on the underside of the roof deck. Also I have seen in the lumber yard foil faced decking. Please explain to me how the reflective surface on the underside reflects the suns rays/heat to the outside. True, it has been many years since collage physics, but this just doesn’t compute in my way of thinking.
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It doesn't "reflect" the heat.
That is a misnomer.
Surfaces that have a high IR reflectivety also have a now emissivity.
A surface that is above absolute zero will emit IR radiation. IIRC it is proportional to the 4th power of the abs temp.
But the amount emited is also a function of the emisitivty of the material.
So the for a typical roof mcuh of the energy from the sun is absorbed by the shingles (in most cases low reflectivity/high emisivity) that heat is conducted through the sheathing bottom of the sheathing.
Then from the bottom of the sheathing has a high emisivity then ti will radiate that heat energy to the attic floor.
However if the bottom has a radiant barrier it will be low emisivity and not radiate as much to the floor. And since the sheathing can't give off the energy that way the sheathing and shingles will be a couple of degrees higher and the shingles will radiate more energy off into space.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Thanks for your reply Bill.
I hate to be so dense, but I will not get any less dense if I don't ask.
What does IR stand for, also IIRC.
Thanks again.
Infrared (below the visible wavelength; sensed as radiant heat)
IIRC (Iffin' I's recallin' carectly)
Do you know if the coating really makes much of a difference? Seems a bit of a stretch to me. But I dunno.
I don't believe it has any real effect. Perhaps measureable under controlled conditions, but not really useful.
Forrest
The aluminum stuff is about 93-97% "effective". The best of the paints is about 75% and some as little as 30%.Now that said, that is of the radiant heat that falls on the roof.The sheathing is still hot which will conduct to the air near by and they convective transfer to the rest of the attic.And of course you might be trying to "cool" the space with 99* outside air.Florida Solar Institue and another orgaization (Southface, Southwall ???) has done a lot of work in that area.They claim that it worth it in the deep south..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I might consider it in new construction, if it only added a couple hundred bucks to the cost of the house.
You are far enough north that benfit from summer gains might be less that the losses from winter gains.Because of the cooler temps I don't think that radiant energy out of the attic floor is signifcant.But I am in kansas city and I would use the sheathing with barrier on it for a new build. That is if I did not have a foamed roof and make the "attic" conditioned space or vaulted ceiling..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
You haven't been in my finished attic on a 90 deg day. It's insulated very well, but its hotter than a $10 pistol up there. Your average yearly cooling degree days are 1288, mine is 866 so you would benefit more than me. I've never seen any real data on the barrier. Problem is people make numbers prove what they want to. That's why I am highly skeptical.http://www.nrcc.cornell.edu/ccd/nrmccd.html