Insulating a Roof over a Vented, Unconditioned Attic
I am having my low pitched roof changed from tar and gravel to asphalt shingles. When the contractor removed the existing tar we discovered that, (surprise!) the roof decking used (plywood) when the roof was put on was far too thin and is now sagging on most of the roof and rotting in others. So, we have to replace the plywood at least OR, our contractor recommends, fix just the rotting spots, put polyiso rigid boards right on top of the current decking, and then another layer of plywood over that and then shingles. I am very interested in addressing some of the significant heat loss/cooling issues of our home (it has no insulation in the walls, NONE!) but am wondering if it will actually do anything to solve the issue given how our home is built: we have an attic with blown in insulation on the floor, I do not know the R-value. There are vents at the soffits but no vents on the roof (our contractor wishes to add a ridge vent). The attic is completely sealed from the rest of the house (unconditioned) with drywall. So, with this set up, given the soffit vents and unconditioned, vented attic will insulating the roof with rigid ployiso boards do anything to assist us with keeping the house cooler in summer and warmer in winter or should we just replace the plywood, put on the shingles and be done with it?
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