Just saw the new Sherwin Williams Low E paint in their magazine. Anyone tried it? Seem to work?
http://www.sherwin-williams.com/pro/sherwin_williams_paint/sherwin_williams_paints/interior_paint/pdfs/SW_EBarrier.pdf
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Greetings K,
This post, in response to your question, will bump the thread through the 'recent discussion' listing again.
Perhaps it will catch someones attention that can help you with advice.
Cheers
Do you have this post set up on a function key or someting? {:-)
Never seen anything like this from a company I'd ever heard of before.
Wonder what would happen if you painted the sheathing before doing the roof?
Guess I'll stop by the S&W store in town so they can say "HUH?"
Joe H
Thought about doing the same thing, but I already threw their magazine away.
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If you mean to paint the exterior sheathing before applying the roofing material, this would have no effect.Radiant barriers only block radiant heat. When sunlight hits the shingles they absorb this radiant energy and it then moves through the shingles and sheathing via conduction. Radiant barriers cannot block conductive energy. If there were a 1/2" air space (or more) between the shingles and painted surface then it would have some effect as the conductive energy would then become radiant energy.
"If you mean to paint the exterior sheathing before applying the roofing material, this would have no effect.Radiant barriers only block radiant heat. When sunlight hits the shingles they absorb this radiant energy and it then moves through the shingles and sheathing via conduction. Radiant barriers cannot block conductive energy. If there were a 1/2" air space (or more) between the shingles and painted surface then it would have some effect as the conductive energy would then become radiant energy."You are right, but you are wrong.The problem is terminology.It is actually emmisivity that is at work when the radiant barrier is on the "inside of the roof". A material with low emisivity can't radiate the energy into the building.Florida Solar Institue has a bunch of papers on this.If you search under my name and radiant for the last 6 months you should get the links.BTW, only thing that i could find at there site on low e paints whas a comment that it was a type of radiant barrier. But no data on how well they work.
Bill,Thanks for the reply. I will read those papers and reserve further comment until then.But, are you saying that a radiant barrier would be as effective attached/applied directly to the underside of roof sheathing as it would be with a 1/2" (or more) air space? It seems to me that the air space would allow the conductive heat to be transferred back into radiant energy. This in relation to reflect solor energy only, not blocking radiant heat loss from the inside.Or does this fall under actual emmisivity? You lost me on that one.
The radiant energy hits the "back side" (whether it is the back side of the radiant barrier material or shingles attached to the barrier).That heats up the material. Then on the "inside" that heat can be give off by conduction if it is in contact with other material, by convection (by that is not at work her as the barrir is at the high point) or by the material re-radiating it into the house.How effect that material is in re-radiating is called emissivity.If it is low then it won't radiate it into the house.I think, that in general material that are good radiant reflectors are also have low emissivitiy.
Okay, now that's interesting.So if the roof sheating were made of lava stone, or something else with high emissivity, the painted on radiant barrier would have little effect? Again, this example only has to do with blocking solar energy.Out of curiousity, what kind of interior surface temps do we see on the underside of roof sheathing in areas with a lot of direct sunlight? I'm not talking attic temperatures, I mean the surface temp of the plywood, OSB, or whatever is used for the sheathing. Let's assume typical asphalt shingles.
Yes and yes.
You can do a simple test if you can get some of it from them. Maybe they can use to market. Grab scrap of sheathing. Paint half of one side. Put heat lamp on, measure temp on other side. We were getting about a 30 degree difference between the painted and non.
And before someone says something about the heat and the shingles . . . think about foam. Not an issue. You can direct apply icynene or similar direct to the underside of the roof deck with no impact on shingle life.
Thanks RW, appreciate the info.
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Is this test supposed to duplicate the equivalent of painting the exterior side? In that case you should try the test again with shingles installed. With the shingles installed flush against the painted surface the heat will be able to move conductively right through the paint.In fact, the only reason to paint roof sheating with these products would be to block radiant heat loss from the inside. It will not have an effect against blocking exterior radiant energy without an air space in front of it.
nope. I dunno why you'd paint the outside. The website even shows fairly clearly painting the underside of the roof deck.
That's just to block radiant heat loss from the attic. It would not do a whole lot to block radiant heat from the roof.
Did you bother to read the PDF?
HOW DOES E-BARRIER SAVE YOUR CUSTOMERS MONEY?
Whether there’s radiant heat coming into a house or going out, your customers are paying more than they should. When E-Barrier is applied to the underside of attic roof decking or to previously coated commercial decking, it creates a barrier that reflects radiant energy, helping to prevent it from entering or leaving the building. E-Barrier reduces heating and cooling costs by as much as 15% as certified by an independent testing laboratory*.
Just to the left of this bit of info is a cute little drawing of sunshine bouncing off the roof.
Joe H
I don't know much about the specific SW product, but in generaly radiant blocking paints have faired well in testing. The knock against them, and most other radiant barriers, is that they lose their effectiveness over time as dust or any other film builds on their surface.
This product seems to be geared towards applications where recoating would be difficult. As such, I'm not sure how much effect it would have ten years down the road.