TPO Membrane over Flat SIPS Roof
This is a question related to the thread on “How to Make a SIP Roof Better”. I am building a SIPS house with a nearly flat roof structure- a butterfly roof at 1.25″ per foot and a companion roof at 0.5″ per foot. The house is in Fredericksburg, VA. Both roofs will have 8″ EPS (Neopor) panels. Directly on top of these panels a TPO membrane will be installed.
I am concerned about potential water vapor exfiltration from the interior moving to the top OSB skin causing it to eventually rot. I dont believe this will be a problem over most of the panel- just at the seams between panels and at roof penetrations (two 4″ vents). I would appreciate any ideas for mitigating the potential failure modes presented by this design. Do I need to ventilate this roof- build a lattice atop the SIPS panels? What I dont want is a bunch of stuff sticking up from this roof. It needs to be near invisible (I may be able to move the vents to a side wall). Thanks.
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I've said it before and I'll say it again: you don't have to worry about diffusion, only air movement. (Building scientist Bill Rose recently said about the same thing in a comment to my blog on cathedral ceilings.)
Don't rely on spray foam alone to seal the seams of the SIPs. Instead, use a belt-and-suspenders approach by sealing the seams with both spray foam and SIP tape.
A nearly flat SIP roof would not be my choice of roofs, if the top of the SIPS get wet and deteriorate you will have one of the biggest headaches of your life. A nearly flat roof of any kind would not be my choice but if there are no other options I would use 60 mil EDPM.
Thanks for your advice.
Put a double lock mechanically seam metal roof over the top. Best of both worlds in that it is a 50 year application and rated for near flat roof applications (only when mechanical double locked). We do the for people on flat roofs all the time.
As Martin said, make the deck air tight and that will eliminate 99% of the moisture.
If you are still concerned, go with an overdeck vented structure with a vapor permeable top (i.e. Tyvek) to the structure deck and a vapor impermeable wrap (i.e. Titanium) to the vented deck.
If any moisture ever comes up through the penetration, it will dry to outside and the overdeck venting will improve summer performance as well with convection behind the panels.
Plywood and 2x4s are pretty cheap so it is not a huge deal breaker to do it.