Using SIPs for my upcoming roof project
I have a 4’x 12″pitch 50 square ranch roof. Open interior rafters and existing T&G decking above rafters. I will be installing a metal roof over and am interested in a R50 SIP as insulation above the T&G board decking. I need to know the best way to seal the exterior (top) of the T&G boards, and then how to seal both the underside and top of the SIP Panels. I also need your suggestions on venting continuous soffit to ridge plenum. Perhaps a “Cold Roof” above the SIP panels and below the metal roofing? Is there a better way (than the SIP panel idea) to achieve a R50 insulation factor on top of the existing T&G boards? Please advise & regards. Don
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Replies
Donald,
Q. "I need to know the best way to seal the exterior (top) of the T&G boards, and then how to seal both the underside and top of the SIP panels."
A. You need an air barrier above your existing T&G boards. This could be peel-and-stick membrane like Ice & Water Shield, or a European membrane sold for this specific purpose. (For more information on European membranes, see 475 High Performance Building Supply.)
When it comes to sealing the seams between the SIPs, you should follow the recommendations of the SIP manufacturer.
Q. "I also need your suggestions on venting continuous soffit to ridge plenum. Perhaps a Cold Roof above the SIP panels and below the metal roofing?"
A. For information on this issue, see How to Install Rigid Foam On Top of Roof Sheathing.
Q. "Is there a better way (than the SIP panel idea) to achieve a R-50 insulation factor on top of the existing T&G boards?"
A. There are other ways, but only you can determine if the other ways are better. The other ways are to either use nailbase instead of SIPs, or to install a site-assembled sandwich of rigid foam and roof sheathing. Details are in the article I just linked to: How to Install Rigid Foam On Top of Roof Sheathing.
With a vented nailer deck you could hit R50 performance at lower cost using something other than foam blown cellulose or fiberglass), if you can tolerate adding 12-14" of thickness rather than 9-10" for a polyurethane SIP, at comparable thickness to an EPS core SIP.
12" TJIs or open web trusses 24" o.c. with 1.8lb fiberglass blown in netting, with a half-inch OSB air barrier under 2x furring + nailer deck may come in substantially lower cost than SIP solutions, depending on your labor costs. You'd have to bump that to 14" trusses to go with cellulose or 1lb fiberglass. (1lb fiberglass could be an issue in cooling dominated climates, due to the translucency of low density fiberglass to infra-red radiation.)
The structural requirements of the trusses are much much lower than a rafter truss, since it's only supporting the nailer deck, not the structural roof deck, so it can be a pretty wimpy truss, relatively speaking.) You would still want to put a low vapor permeance water proof membrane as the air barrier over the t & g decking.
At R50 polyurethane SIPs will likely never recoup the environmental hits of the polymer + blowing agents over it's lifecycle on reduced energy use. (It might if they use HFO1234_ _ blowing agents instead of the ubiquitous HFC245fa, commonly used, which has a global warming potential 1000x that of CO2.) EPS core SIPs or stacked EPS or polyiso might break even (due to less total polymer & much lower impact blowing agent) but it's still substantially less green than any fiber insulation solution.