First of all, great site. I wish I would have found this sooner.
I’m doing a home addition in region 4 (Northern Virginia). The house is about 70 years old. I’m pretty clear on insulating most of the house. We’re using the Zip r-5 product on the exterior, sprayed cellulose inside the walls, and blown cellulose in the attic. Also, I plan to do the rim joists with open cell spray foam.
There are three tricky areas I can’t figure out.
1. The basement walls – The basement already has been framed with 2x4s. There’s an inch air gap with the concrete. In hindsight, I would have liked to do rigid foam, but I think that ship has sailed. Am I stuck with closed cell spray foam as the only preferred option. Is open cell not recommended for basement walls?
2. While most of the house has an attic, one bathroom is just drywall directly under the rafters (sloped 3:12). Can I still use the blown cellulose there? Or is there a mold/rot concern with moisture from the bathroom and the difficultly of ensuring an air gap between the roof and insulation? The roof is just standard plywood, paper, and shingles if that matters.
3. One small room is being built on the slab of an existing concrete porch. To match the floor level in the rest of the house, the contractor built the porch floor up with 2x6s on end. These are supported with periodic flat 2x6s on the concrete floor. I can easily put several inches of rigid foam here between the 2x6s. Can I, or is there a risk of rotting the 2x6s that will be covered with the foam? The porch is 4 feet above grade. It’s bricked all the way around so I don’t know what’s under the slab, dirt or air.
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