Wet polyiso and plywood on roof
pbout
| Posted in Green Building Techniques on
My roof build up has plywood, 3” of polyiso, and a second plywood deck. We didn’t get the roof on yet, and even though we tried to rig up some protection, we had rain and then a few leaks. Next week it should be clear, and we will finish the install.
Should I worry about the wet roof? I imagine the plywood will dry up and down (vapor permeable underlay going on), but what about the polyiso?
Clinate zone 3C, pretty mild.
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Replies
How wet is the plywood? Just a little bit? It’ll dry, but you really want it to dry before you seal up the assembly so that you don’t trap the moisture.
For the polyiso, I usually stick my finger in the end of one of the sheets. If it’s just a little damp at the very edge, it’s not a big deal. If it seems wet as far in as your finger goes you have a problem. Basically a small amount of dampness will dry, but if the polyiso is saturated you probably need to replace it. One rain storm shouldn’t normally be enough to cause major problems unless the polyiso was immersed in water (as in the water pooled up along the edge of the polyiso and stayed that way for a while).
Bill
Foil faced polyiso would be very unlikely to have taken on enough water to matter in a single rain event. Some paper-faced polyiso might take on more, but most paper faced polyiso is asphalted and moisture resistant (it's used in roofs, after all). The paper might feel slightly wet, but the polyiso takes on almost nothing.
Thanks Dana, Bill.
The polyiso is foil faced. I don't have access to an edge to check how wet the polyiso is. There is no indication that there was immersion, but hard to know for sure if there was something that I can't see. The only indicators are leaks through the bottom deck, but even those are infrequent, as the bottom deck had all seams taped.
I will get the roof assembly up next week, when it is supposed to be dry. I won't insulate below. Because my underlay material is vapor permeable, I think the top plywood will still be able to dry up towards the roof. The bottom plywood can dry down; I won't insulate for another 4-6 weeks.
I'm not some expert on polyiso, but I put 3" of it from a school roof on the walls of my house. They were exposed to the rain to some degree for months. I have a huge stack of scraps sitting outside in the elements, for probably close to 2 years now, and it still seems fine to me.
I used some of it for one final portion of the siding last fall and it was dry as a bone.
Nice, thanks Nathan