Sloped ceilings from inside: Removing drywall
We just uncovered the final instance of corner-cutting by a local/franchised insulation contractor. (T Carlson, this is the same project you and I discussed, but the situation is messier than previously thought). Rather than insulate the entire sloped ceiling between the kneewall and the top attic, this group only insulated the top portion and left the lower 3 feet of sloped ceiling entirely uninsulated (and hidden). This unscrupulous crew made a conscious decision thinking we would never notice.
For solutions to insulate the sloped ceiling, we’ve explored working from the kneewall area up, which is impossible, and also from the top attic down, which is nearly impossible. (They foamed in 4″ of polyiso in the top of the sloped ceiling bays, and there is 18″ of cellulose in the top attic.)
This leaves removing drywall and working from the inside.
The rafter bays are 2×8″. At the bottom of sloped ceiling is foam board with a 1″ air channel, and at the top of the sloped ceiling is 4″ of polyiso (2×2″ boards), foamed into place and with uneven edges. Even if we had top access, removing the foamed in polyiso would be a project. So, whatever we do will need to be made continuous with what is currently in place at the top and bottom of the slopes.
Assuming that we open roughly 2-3′ of sloped ceiling along a 40′ run of closets and bedroom, what is our best approach to get this right once and for all? We are in CZ4.
1. Make baffles from rigid foam, air seal the edges, maintain the 1″ channel, and fill the bays with batts?
2. Make baffles from rigid foam or 1/4″ plywood, maintain the 1″ channel, and spray closed cell foam (either flash and batt or 4″ of foam)?
3. Some other approach?
If we use rigid foam, what kind?
Of course we could also dense packed the sloped ceilings, but 1) this incurs moisture risk, 2) wastes the 1″ air channel we worked so hard to maintain when we moved the thermal boundary in the kneewall, below, 3) isn’t that great R value, 4) will still involve drywall repair and painting to clean.
Thank you.
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Replies
Emerson,
Assuming you open up the ceilings from the interior, the second option is probably better than the first, simply because you are more likely to end up with a decent job of air sealing. When it comes to performance, air sealing is the key.
Emerson,
Home Depot seems to be discontinuing Roxul mineral wool in favor of Owens Corning Thermafiber. The Roxul is currently heavily discounted. If going with batts, I suggest you snap some up via online order. Others can suggest baffle material, but I think Menard's has cheap unfaced thin EPS that might work well with the rock wool.
Regards,
Joe
Martin, Thanks. Woukd you use rigid foam or plywood for baffle? Which will last longer? Emerson
Joe, Thanks. Checking it out.
Emerson,
Q. "Would you use rigid foam or plywood for baffle? Which will last longer?"
A. Either product will work fine, as long as you choose rigid foam that is stiff enough to resist the pressure of your chosen insulation. Airtightness matters, so you want to use a product that is stiff enough that it won't crack.
Assuming care during installation, either product will last indefinitely.
Martin
Thank you
In CZ4, is there any reason not to use foil faced polyiso as the baffle? It seems the jury is out whether Eps or polyiso performs better in cold weather, but perhaps polyiso would outperform in our CZ?
Emerson,
Q. "Is there any reason not to use foil-faced polyiso as the baffle?"
A. Some builders prefer unfaced EPS because it is somewhat vapor-permeable. (Foil-faced polyiso is a vapor barrier.) I would certainly argue that R-value per inch doesn't matter very much when choosing a ventilation baffle.
For a complete discussion of whether the vapor permeance of a ventilation baffle matters, see "Site-Built Ventilation Baffles for Roofs." Spoiler alert: I don't think it matters.