Old home, half-story insulation
I’m in Maine. I’ve got an old 1.5 story farm house, built in the 1860s. We have done a lot of work to the downstairs, but this is our first time working on any of the rooms upstairs.
The room I’m working on is part of a newer (but but still quite old) addition.
Here’s a photo of what we have after removing floor/wall/ceiling covering
Those boards are holding up blown cellulose insulation. When I get home tonight, I’ll find out how deep it is, but I’m guessing it’s not enough. I will be adding more blown insulation in the attic, but I’m wondering what I can do for those diagonal walls (the roof). The room gets warm in the summer, and it’s cool in the winter, but not drastically so.
This is a small part of a very large home renovation project and there are some pretty tight budget constraints, so while I’d really like to pull down those boards and have a contractor spray closed cell foam, I’m not sure it’s in the budget.
I was thinking about adding Roxul Comfortboard panels on top of those boards to act as a thermal break, and then attach drywall on top.
All suggestions and advice is welcome!
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Are the rafter bays vented?
I might also mention that we're redoing the roof as part of this work. Stripping off the old shingles and putting on new.
It's very hard to get to in the attic above this addition, so I'm not sure if they're vented. I guess I'll have to find out, but if I were to guess, I'd say that the rafters are completely packed with cellulose.
The rafter bays are full of cellulose, except where it may have settled. With no interior side air-barrier or vapor retarder moisture readily moves into & out of both the cellulose and the roof deck on the exterior side. Hopefully this hasn't compromised the roof deck in a serious way, but you'll know when you go to re-roof.
It's far better to put the thermal break on the exterior side of the roof deck, not on the interior, since that keeps the roof deck warmer (= drier). If you want to put a thermal break on the interior side you'll first have to install an air barrier over those planks to limit convective moisture transfer to the roof deck, preferably a smart vapor retarder and definitely NOT polyethylene sheeting.
Most of Maine is in US climate zone 6, but parts are in zone 7, and it matters. In zone 6 it only takes ~50% of the total-R on the exterior to be moisture-safe at the structural roof deck, but in zone 7 you'd need 60%. So if those rafters are 6" deep cellulose you have about R22 in the rafters, and would need at least R22 on the exterior, possibly R33 if you're in zone 7.
If you are NOT going to install exterior insulation, strip it down to the planks and install a permeable underlayment such as Solitex Mento as a WRB & air barrier over the original roof deck, then install 2x4 furring and a new nailer deck with #30 felt & shingles or whatever so that the structural roof deck is air-tight, but can dry toward the exterior. Then install a smart vapor retarder on the interior side planking prior to the Comfortboard. If the Comfortboard is thick enough to be more than about 1/3 of the total R use Intello Plus, not MemBrain, which may become too vapor open at that colder layer due the higher RH. Alternatively, you could put MemBrain on the attic space side of the Comfortboard where it's warmer, and protect it with a layer of gypsum board.