Roof foam
I am having a clients cape style home re-roofed. The asphalt shingles are not very old but they were installed on top of wood shingles. We are going to strip the roof down to the purlins that the wood shingles are attached to. The crawlspace has been encapsulated but foundation walls are not insulated. The upper level has the typical knee walls but only has one access into the space beyond. This space is detailed in the typical incorrect way that causes so many issues with these Cape Cod style homes. It is not feasible to remove the interior wall and ceiling coverings but since we are doing the roof I am thinking about exterior rigid foam. I think that I could manage about 2″ of foam without causing major issues with the dormer window. I am guessing that we will find some cavity insulation between the rafters but not much(2×6). I know that 2″ will not give us enough insulation value but it will be much better than what is there. My plan would be to install the foam on top of the purlins and then plywood over that. We would, of course, have to create a seal from the outside wall plane up to the rigid foam either from inside or by removing enough purlins in this location to do it. It is my understanding that in my climate zone(coastal Maryland) 2″ of foam will be enough to control the point of condensation but how concerned should I be about actual r-value here? The main thing that I am trying to take advantage of is the roof being re-done.
Mike Labesky
The Cypress Group Limited
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Replies
Mike, why foam before plywood? That sounds reversed of what is typical, and seems like the foam could be crushed against the purlins.
Mike could you put up just enough of the rigid foam along the soffit ends to create a backer and spray foam the areas that meet the knee walls? This would be a good way to air seal and insulate those areas, that is if you can actually get good access. I live in Minnesota and we have many 1 1/2 story homes with similar difficult roof/dormer/wall transitions.
Are your clients on board? It's not easy to go from what they have now to something with decent performance without some commitment.
Doing this without going down to the rafters seems like it will never play out well.
I, too, had asphalt shingles over wood shingles, over purlins/sleepers, over rafters. Even with my relatively simple gable roof, I knew if I didn't go down to the rafters, resheath, and work from there, I would never be happy with the air sealing and water management details.
It wasn't cheap, but I'm glad I did it.
I have never seen the need to remove the purlins in this case. They are 1x3 with about an inch gap between them. I don't think I am going to be able to get my roofer to do the rigid foam anyway so I am going to explore a different option. I think that I am going to have to work with spray foam from the eaves to the top of the knee walls and am going to make sure that the sloped ceiling is completely air-tight on the room side. This will not yield the r-value but should stop condensation issues. Air-sealing is going to be the critical component for the sloped ceiling to keep interior air out of this closed cavity in the winter.
Mike, I misunderstood your use of the term purlins.
If you are redoing the roof, but not tearing into the interior, exterior rigid foam as you suggest seems to make perfect sense. Just tape it for air control. And perhaps find some creative way to bring that air barrier down to a component of the wall for continuity.
I don't personally understand why messing with spray foam is regarded as less hassle than throwing up some foam on top of a roof already undergoing a reroof, but I'm neither a roofer nor spray foam contractor so... I do think people might default to spraying often because it sounds easy to just blast polymers onto old crap.