Attic insulation in 100-year-old house
I have a 100 year old house in Northern Iowa. We have a walk-up full height attic with cellulose blown in the floor of the attic, about 7 inches. We have an AC unit in the attic that cools the upstairs. Our heat doesn’t come from this attic unit. Our roof is wood shingles over spaced sheeting, in good condition, replaced about 15 years ago. I would like to place rigid foam on the floor of the attic and cover with plywood so that we can still use the attic for storage. This would also help me seal all of the penetrations of the ceiling. I am planning to build a box of foam around my air ducts to include them in the insulated space. I would like to leave the roof deck as it is to preserve the life of the shingles by allowing them to breathe. Does this seem like a reasonable plan? I’m planning on 4-6 inches of foam covered with plywood. I’m estimating the current blown in cellulose at R-21. Thanks for your thoughts and experiences. I’m in zone 6 according to the insulation values on the map from the dept of energy.
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