Wisdom of closed cell foam insulation in an unheated attic
I own a house in upstate NY due east of Albany. The plant hardiness zone is 5b.
The house is a mid-19th century 2-story federal style farm house, with a dug basement (dry with concrete floor), and an attic space that was finished by a prior owner into two bedrooms for some of the 8 kids in the family. (The house also has 1.5 story mid-19th century wing without a basement, and a mid-1980s addition consisting of two rooms off the back of the house and an attached 2 car garage, over which is a large bedroom and bath.)
My question relates to the attic in 2-story farmhouse. To save on energy consumption, I stopped heating the finished attic space and closed it off from the 2nd floor, but not tightly sealing it. The unheated old attic space was far from air tight, in fact had a lot of air leakage from both outside and inside the house. Not surprisingly, the sheetrock in this cold unheated space grew a nice mold crop from condensation from warmer, moist air rising from the second floor.
At this time, I am having all of the sheetrock removed, and the fiberglass insulation that was behind it removed. The exposed structural elements will be treated for mold.
When that is done, I have been contemplating bringing the attic back into the heated envelope of the house with spray foam insulation. My reading is leading me to the conclusion that my best bet is closed cell spray foam. The aim will be to eliminate the migration of cold air from outside into the space.
This is being done in conjunction with a larger project of tightening up the house and installing a geothermal heating system to be the primary heat source for the house (but keeping my current oil burner in reserve status). The geothermal will be primarily water-to-water for heating, with a water-to-air auxiliary unit in the (to be foamed) attic, which will also provide cooling for part of the house in the summer.
Getting to the point(s):
1. Is the overall attic plan generally sound?
2. Is closed cell the way to go in the unvented, within the envelope attic-space?
3. Is there any need for ventilation between the underside of the roof sheathing and the spray foam insulation? Would it be best practice to have ventilation between the sheathing and the foam? If so, what are the concerns? FYI, the roof sheathing to be foamed consists of the original 150+ year old planks, on top of which was laid pressed chip board (in sure that’s not the right name) panels when the house was re-roofed in the mid-1980s. The current roof was installed in 2008, so it’s 12 years old.
4. Am I correct that with a properly air tight (vis-a-vis the outside), insulated within-the-envelope 3rd floor space, the only heating required is that which migrates up from the second floor, which is closed off, but not sealed off, from the second floor?
5. Am I also correct that the third floor, if properly sealed-from-outside air intrusion, should not have humidity/condensation issues?
6. A leaky roof insulated with closed cell spray insulation is reported by some to be a concern. Is this a real risk? What can be done to mitigate the risk?
7. A roofer has mentioned the risk of spray foam insulation lessening the useful life of roof shingles because they can’t radiate summer heat inward. Is that a real risk? I’ve read that was a concern that has been overblown.
Thanks. And sorry for the verbosity.
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