We are in the middle of remodeling our home, and are having difficulties sourcing the following material on short notice:
1/2" plywood roof sheathing with radiant barrier applied
Are there any sources in the SF bay area (we are in Palo Alto) that might have this in stock or available in a couple of days? Note that this a plywood product is required -> several places have an OSB product available, but the OSB does not meet the structural requirements of the design.
All, spray on paints are, by definition of several sources, are "not" radiant barriers. They are actually Interior Radiation Control Coatings (IRCCs). A radiant barrier has to have a minimum of 10% emissivity and 90% or more reflectivity (California Title 24 code is even more stringent). There are several manufacturers of the reflective paints, all with different reflectivities. A foil is a "known quantitiy" by virture that the inherent properties of aluminum foil have a 3% emissivity and 97% reflectivity. Paints are almost always applied by a contractor just because most people do not want to rent a spray applicator , deal with the mixing of the paint, etc., etc. Whereas, foils can be applied by different methodologies. Another consideration about paints is that if the decking ever has to be replaced due to damage, the paint goes away as well.
Regarding the San Francisco situation, concur that if you can't find the OSB or plywood decking with the foil already adhered to it, there are foils and other foil products that can used as underlayment.
I have a list of links to several unbiased, government and university technical sources on my website:
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If anybody needs further info or specific information, don't hesitate to contact me.
One of the major problems with any radiant system is that the dust that forms on its surface will degrade it reflectivity. One of the biggest problems with radiant barriers. Oak Ridge has a good deal of info on this, one doc I see referenced from your website "Radiation Control Fact Sheet" found on the Oak Ridge website. It discusses how these barriers defrade over time. Even so, people who live in the sun belt, Arizona and the like, would benefit greatly from adding a radiant control barrier to their homes.
Okay, I've been an energy nut for a while, and have known about radiant barrier decking for some years. But this is one I don't get.
The radiant barrier is essentially a layer of aluminum foil on one side of the plywood. The aluminum foil reflects heat. In the attic, the barrier is on the bottom side of the plywood, so would reflect heat back into the attic, which is generally where I am trying to get the heat away from. Isn't this counterproductive????
If someone were to put the foil on the top surface, it would be useless because the shingles would be in contact with the foil, killing it's effectiveness immediately.
What'd I miss?
(On the other hand, if I have a foil surface on the TOP of the decking, but under a curved tile roof, there would be a space for the foil to be effective, by reflecting the heat back up to the tiles. The dust problem remains.)
The key idea is that if an object is in equilibrium with its surroundings, then the emissivity e = 1 - reflectivity r. So a highly reflective surface has low emissivity. This allows the radiant barrier plywood roof deck to heat up from the sun without radiating as much heat into the attic as normal plywood. A side effect of this is that the radiant barrier plywood will be hotter than normal plywood would be under the same conditions.
If I understand you, a shiny hot surface will be a poor emitter of heat, even if it is much hotter than its surrounding air? And a flat black pipe will be a much better heater/emitter than will a shiny pipe? I've got a lot of general background stuff, but I never picked up on that one. It's intuitively obvious that a flat black surface would absorb heat, but no so obvious that it would also be a better emitter. I got some readin' to do...
How much a surface emits (radiates) is independent of the temperature of the surrounding air, it just depends on the surface temperature. At a given surface temperature, a more reflective material will emit energy at a lower rate than a black surface would.
The best way to comprehend the physics of a radiant barrier is to think about the emissivity and not the reflectivity. By definition, a radiant barrier must have a minimum of 10% or less emissivity and be positioned so it is exposed to an air space, i.e., your attic. With that said, emissivity and reflectivity are closely related in that when you add them together, they must equal "1" (as Wayne accurately states).
But because most people more readily understand "reflecitivity" and not "emissivity," companies typically promote a material's reflectivity. So when you position your roof decking with the foil already adhered to it facing downward toward the attic, think that the foil is "emitting" only 3% of the radiant heat from the other side. It doesn't matter how the radiant heat on the other side impacted the foil, i.e., radiation, conduction, convection, the foil "still only emits 3% of that heat." As is with IRCC paints, they typically promote a paint reflects 65%-75% of the radiant heat, but...they also emit 25%-35% of the heat from the roof decking.
Dennis, you are correct but studies, especially by FSEC show that dust collection and the resulting radiant barrier degradation is when a foil is placed on top of attic floor insulation. System deterioration is nil or minimal when the foil is placed on the bottom of the rafters, between the rafters or next to the roof decking.
Craig, unless there is shade on the roof "all day long," one should consider radiant barrier on all areas of the roof, including attached garages. The entire goal of the a radiant barrier system (including ventilation) is to reduce the temperature of the attic so there is not as much heat the attic insulation has to "resist" ("R"-value) and thereby migrate into the living spaces of which requires the HVAC unit to run longer. All roof areas will contribute to heating the attic even if they face the East and North. Granted, the roof decking surface temperature may be approximately 10 degrees F lower on a North or East facing roof (compared to the South and West roof lines), but when the surface temperature is 120+ degreees, those roof areas will still be cooking the attic.
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