Does it make sense to insulate a roof when walls have no insulation?
We need to replace the cathedral roof on a 1930s addition to our home that we use as a family room. The room has exterior walls on three sides. The wood panel walls have no insulation and there is less than 2″ between the exterior and interior. We have been told that there is no possibility of insulating these walls without replacing the siding, which we are not planning to do. The windows are original and single pane. We are not planning on replacing the windows – which take up about 70% of the east facing wall – but plan to purchase mini-blinds that add insulation. Below the floor, there is spray foam insulation that looks pretty deep and thorough. The ceiling in the family room is 22 feet tall at its greatest height.
I include all this detail about what we are not replacing and what we have to see if it makes sense to add insulation to our roof when the rest of the room is probably very leaky and the walls have no insulation. Like most folks, we have a limited budget, but want to do what we can to improve the energy efficiency of our house.
I have been told that our current roof is simply plywood applied directly on top of the ceiling and then the asphalt shingles are on top of the plywood. So there is not an easy way to add insulation. To replace the roof on the whole house it would be about $25,000. To add insulation to the roof above the family room and a flat section above our bedroom it costs an additional $25,000. The insulation the contractor proposed is 2.5″ of Polyisocyanuarte rigid foam with an R value of 14.4. We had a proposal for 3″ of closed cell spray foam, but it was even more expensive. We live in Seattle, so it very rarely gets below freezing. Currently, we keep the family room at 58 degrees in the winter (use blankets to keep warm) and roast in the summer.
My questions are (1) will adding the 2.5″ rigid foam roof insulation reduce energy usage by much when the walls are not insulated and will it keep it cooler in the summer so it is more comfortable to use. (2) will it make much of a difference if we added more rigid foam, e.g. if I asked the roofer to add two layers of 2.5″ rigid foam insulation?
Replacing our roof is our chance to improve the comfort energy efficiency But I don’t want to double the cost of a new roof, if it will not have much of an impact. We have other green projects, like those insulating mini-blinds, that we could spend our money on if the roof does not make sense.
Thanks for any advice!
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