Spray foam and ice and water shield
I’m currently in the midst of a fairly substantial interior remodel and part of the renovation is completely gutting and reinstalling a bathroom beneath an unvented dormer.
We found a roof leak near the center so I need to redo the shingles over this dormer (which is currently covered with asbestos tiles). I’m going to be doing the roof repair myself rather than using the contractor since they are only proposing to replace the entire roof. I’m trying to only do the problematic area with a plan of redoing the entire roof in the next couple of years.
The contractor is most likely going to install closed cell SPF in the ceiling joist cavities, although we haven’t seen the change order come through yet so I’m not sure what they are proposing. They finally finished demo and noticed that it wasn’t vented. They bid it as though it were a vented attic even though I have been telling the architect that it isn’t vented for 4 months now.
I’m only doing the center part that is circled in red. The roof area is so small and there are so many nearby valleys and transitions that I’m considering using ice and water shield for the entire thing (also it will save me from buying another underlayment). Does anyone foresee a problem with having closed cell on the underside and ice and water shield on the top side? Am I in double vapor barrier trouble here?
As far as the actual roofing, since it’s only temporary I plan on using three-tab shingles and doing my best to install them under the existing head flashing. Edit – I suppose I should mention that I’m in Climate Zone 5.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Flyingsock,
You are fine with the I&WS. There is no appreciable upward drying though the shingles anyway.
Here is what Martin wrote:
"I recently telephoned Joseph Lstiburek, a principal at Building Science Corporation, and asked his opinion on the matter. “We have double vapor barriers in commercial roofs all the time,” Lstiburek told me. “When people say, ‘The roof sheathing has to be able to dry in at least one direction,’ I say, ‘That’s ridiculous.’ It doesn’t have to be able to dry out. We have lots of historic experience with commercial roofs that can’t dry out in either direction. In commercial construction, we rarely have a situation where the sheathing can dry inward.”
Thank you. I appreciate the response. Joe Lstiburek has an article saying pretty much the same thing.
I have about 7 years now of an unvented roof over my "sunroom" here that has closed cell spray foam under the sheathing, and roll roofing on top (which is a bit like one massive shingle that is constructed by gluing shingle material to ice and water shield). The roof has a very low pitch, so regular overlapping shingles were not recommended.
I have had zero problems with the roof assembly. Just make sure you have a good spray foam contractor so that you get a good application without any voids.
Bill
Nice to hear.