I’m renovating a house built in 1950 that has a finished attic with very little insulation. I’ve been trying to figure out the best compromise for insulating my attic that is going to be both cost effective and energy efficient.

I’ve gotten estimates for installing closed cell spray insulation (very expensive) and applying a couple layers of polyiso above my roof sheeting (expensive and I can’t find an experienced person in my area who has adequate experience doing it).

What I’m considering doing is insulating from the inside with 1.5″ of space for ventilation, 1″ of polyiso, 6.5″ of fiberglass or cellulose, then 3″ of polyiso covering the bottom of rafters. I would use the first layer of 1″ polyiso to make the space for the ventilation gap and seal all of the edges with canned spray foam. I’m leaning towards using fiberglass because it’s cheaper and easier to install, and the polyiso + foam should create a tight air barrier (negating the advantage of tightly packed cellulose). I can buy recycled polyiso in my region for a deep discount, which makes this very cost effective compared to spray foam. It has paper on two sides (rather than foil). Should I worry about using two layers and creating a moisture “sandwich”? Would it be better to use something more permeable like EPS for the first layer?

This method would give me an R-value of about 45 (a little short of code, which is 49 for my area).

I’ve attached a diagram to explain my plan. I’d appreciate any insights of comments.

– N