Radiant barrier with spray foam insulation
I am exploring the use of LP Tech Shield or foil-faced radiant barrier in an open-web truss roof. The question arises with the use of spray foam. I typically spray enough foam (.5 lb open-cell) to achieve R-38 in our roofs. All manufacturers of radiant barrier sheathing say not to spray against their product — it will render it useless.
A foil manufacturer suggested the foil on the inside of the truss, leaving an air gap between the foam and the foil. We don’t ventilate our attics — they are always conditioned spaces. Any ideas? Or can anyone explain why I wouldn’t spray against the foil? Even if the heat transfers into the foam, how far will it actually penetrate? Is the air gap intended for moisture concerns?
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Replies
What is your purpose for spending the extra money on foil-faced roof decking? If it is, in fact, to block solar heat gain then there needs to be an air space on at least one side of the radiant surface or it has no effect.
Foil faced sheathing is for an attic that is insulated on the flat or a vented cathedral with a gap between foil and insulation. You can apply a radiant barrier on the underside of the roof trusses to reduce radiant transmission to HVAC ducts and the interior ceiling, but that won't stop the radiant flux where it counts - at the outer, sun-heated surface.
You can either:
1) forget the radiant barrier and continue as you've been doing with high R-values
2) add an under-truss radiant foil barrier, with air gap as recommended, for a minor improvement
3) use foil-faced sheathing and vent the space between it and the insulation, which would be the best option
Rob,
A vented roof assembly, with an airspace between the roof deck and the insulation is thought to increase the life expectancy of asphalt shingle roofs. This is the thinking in MN at least and most building inspectors are quite adamant about venting. I too, believe in venting the roof at the eave and ridge with a provision to keep this ventilation air from degrading the R-value of the ceiling insulation. I am in total agreement with Robert on this detail (along with the Earth being flat) and have had zero callbacks using this method.
Robert, Seems like there would be a tremendous heat buildup with the foil-faced sheathing but as I recall, most of the roofs I saw in Houston and Dallas were typical asphalt or fiberglass base shingles. Would not a vented roof accomplish the same cooling effect as the foil-face and make the roof surface cooler? The problem with the Texas built homes is all of the HVAC in the attic and the need for efficiency reasons to stay inside the thermal envelope via a cathedralized roof. This really limits design options.
Doug,
Foil faced sheathing (+ airspace)makes the roof warmer and the surface below cooler (when the sun is shining.)
Doug, I believe this is a myth.
There are other solutions besides cathedralized roof/attics.
Doug when I say the surface below I mean the surface below the air space.
I'm going to qualify that statement. It may be the best of the three options for reducing summer heat gain, but I'm not an advocate of adhering anything to a wood substrate (such as sheathing) which restricts moisture diffusion as this can cause structural damage in the event of a roof leak by preventing the sheathing (particularly OSB) from drying.
A vented roof always dries better than an unvented roof, and a roof covered with felt underlayment better than a roof with self-adhering membrane (bituthene).
You're right. Using a radiant barrier under a dark colored roof would be over-heating Peter to cool Paul (so to speak). Light-colored and tile or metal roofs are the only sensible option in a hot, sunny clime (other than living roofs or roof ponds).
John,
I agree there are other options to the cathedralized roof, one being a main floor mechanical room with open web floor trusses on the 2nd floor.
A savvy architect should be able to design a roof system for a single level house that would keep the ductwork in the conditioned space but allow for a typical venting and insulation system. A vaulted parallel chord truss with a flat chord as the thermal boundary and a false flat chord below that to run ductwork.
Doug,
something tells me that you like trusses
;-)
Floor and roof trusses offer flexibility, open design and are cost effective. I use a fair amount of I-joists for roof systems as well.
Trusses make remodeling problematic, are far more vulnerable to fire (we firefighters won't do an interior attack in a truss-built building because of probability of quick structural collapse), and both trusses and I-joists are more susceptible to moisture damage (plate corrosion or web rot) which can cause structural collapse (particularly in spray-foamed hot roofs which hide and hold leaks).
Trusses make remodeling worth doing, I have turned many a frog into a prince with floor and roof trusses and the occasional lvl beam. Bad builders have moisture problems, good builders design and build durable structures.
Good builders are gone once the warranty expires. The best-built house can develop a roof leak from normal wear and tear, lack of maintenance or acts of God. Good design leaves a structure that is resilient enough to withstand the normal vicissitudes of life in the real world, including the potential for fire.