Posted By mlevendo on 01/27/2009 1:05 PM
I got conflicting info about warranties of shingles on a nonvetilated attic. The spray foam insulator installer and my project manager both suggested to use the vent shutes. Didn't add that much cost and I now know for sure I will have no issues about the warranty if a claim would need to be filed.
Building codes are not uniform across the US. In some areas code still requires venting between the insulation and the roof decking even with spray foams, others allow application directly to the underside of the roof decking. Some areas restrict the thickness of foams applied directly to the roof decking.
IIRC in Seattle it's 5.5" max thickness(?), venting chutes required, but neither is the case where I now live in MA. But in some locations near me, unvented roof structures are required to have some large fraction (40%?) of the R value to be spray foam if the rest is going to be fiber-insulation (of any type)- you're not allowed to build with fiber-insulation in direct contact with the roof decking. (Presumably based on local climate-specific issues about keeping the dew-point layer inside the plastic to avoid condensation in the fiber??) The codes seem to be in a state of flux...
Going with 2lb foam on the underside of the roof deck presents a couple of issues. It's a class-I vapor barrier, and depending on the layup of the exterior of the roof deck you may end up with a vapor trap, and eventually mold/rot compromised roof decking. And because it's completely waterproof, it makes roof leaks difficult to detect early, and difficult to locate once detected. Half-pound foam will spread the leak a bit , but it'll drip down pretty much directly below the leak. (I'm personally wrestling with what to do in a cathedral ceilinged attic space in my own house that has 2x6" full-dimension rafters. If I go with 2lb foam for max R value I may then be prevented from putting a layer of XPS, ISO, or composite board over the roof decking, achieving a full thermal break when I re-do the roof in a few years. I may go the half-pound route, with R3 fiber-faced ISO on the inside for a thermal break.)
The whole shingle warrantee voiding thing is bogus. Shingle color, climate zone, and orientation of the roof pitch makes a much larger difference on peak shingle temps than vented vs. unvented. Think about it- venting under the roof decking has about R1 of insulating value between the poorly convection-cooled underside of the decking and the shingles anyway. At least 95%+ of the cooling of the shingles is from re-radiation of the heat and contact/convection with the freely moving open air, not whether some miniscule volume of low-flow air is moving under that R1 of decking & felt. Building Science Corp has studied the issues fairly carefully- a condensed version of what to do can be found here:
http://www.buildingscienceconsulting.com/resources/4-Understanding_Attic_Ventilation.pdf
http://www.buildingscienceconsulting.com/designsthatwork/hothumid/appendix2.pdf
Basically, hot-roofs work, vented roofs don't. It's far more difficult to perfectly air-seal between the attic floor & the living space than it is to make a perfect pressure & insulation boundary by foam-insulating the roof at the rafters all the way down to the top of the walls.