Cathedral ceiling efficiency retrofit
I live in NH (zone 5). I am trying to make some efficiency improvements to a roughly 14ft by 20ft room with a cathedral ceiling that is 14ft at the apex. The room connects the main house and the garage. There is one skylight in the ceiling and two light fixtures. There are no canned lights. The room is on its own heating zone (oil). The room has never been very warm, and like most 1986 built houses, it was not built with an eye towards efficiency. The walls are 2×6 and are insulated with standard fiberglass batts. The 2×10 ceiling is also insulated with fiberglass batts. At most, the ceiling is R25. I recently made the energy efficiency situation worse by adding soffit vents and a ridge vent to this section of the house. For one, the batt insulation is not properly air blocked at the soffit. I can feel the ceiling and wall tops being quite cold around the air intake vent areas, but it seemed unwise from a rot potential perspective to keep a ‘hot roof’ with fiberglass batts. Indeed, when the ridge was cut, the plywood showed some minor signs of moisture trouble. I was thinking that it would help to put 3 inches of polyiso below the current Sheetrock ceiling and then drywall over it. I was also thinking about getting rid of the venting altogether and dense packing the ceiling with cellulose, possibly in combination with a few inches of polyiso under the current Sheetrock. How much polyiso would it take to ensure that the old (now colder) Sheetrock ceiling wouldn’t become a condensing surface in either of my scenarios? Re-roofing to go at the problem from the outside seems out of the question as the roof is fairly new. What about my walls? Is dense packing them prudent? I am thinking this could help with the convective losses that the soffit vents may have introduced. Any suggestions as to a reasonably affordable and prudent course of action given the sad state of affairs? Thanks for your thoughts.
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Replies
Michael,
It's safe to guess that the R-value of your existing ceiling is close to zero. That's an exaggeration, but not much of one.
If you want to really fix this problem, it's going to take some money.
The best way to fix the problem is to install a thick layer of rigid foam above the roof sheathing, followed by a second layer of sheathing and new roofing. But you don't want to do that.
The second best way to fix the problem is to demolish your drywall ceiling and install a layer of closed-cell spray foam against the underside of your roof sheathing, and to then fill the rafter bays with dense-packed cellulose, mineral wool, or fiberglass batts.
The worst way (OK, maybe the third best way rather than the worst way) to try to fix your ceiling is to install a layer of rigid foam on the interior side of your existing drywall. The reason this isn't a good approach is that it's hard to install a thick enough layer of rigid foam in this location to get the R-value you need.
Before going with an unvented approach, you first have to verify that there is no interior side vapor barrier such as polysheeting or foil facers on the batts (kraft is OK.) As long as there is no vapor barrier it's safe to close up teh venting and dense pack the ceiling blowing cellulose in from holes drilled on exterior side, then put the polyiso on the exterior under new roofing.
With 2 x10 rafters with dense-pack over compressed fiberglass the cavity fill would then hit ~R35. In climatezone 5 as long as the exterior-R is at least 40% of the total you don't need anything more vapor retardent than standard latex paint on the interior to protect the roof from interior moisture drives. That means you would need at least R23 on the exterior, which would take 4" of polyiso, not 3", but the mid-winter performance of the outer inch or two is pretty low with polyiso- you'd actually be better off making that last inch EPS, which will outperform polyiso when the average temp through that last inch is below roughly 40F. If you want some margin, go with 3" polyiso (R18 nominal) + 2" EPS (R8 nominal).
To get around the high expense of that much foam, there are several vendors in the region selling reclaimed roofing foam and factory-seconds foam at a deep discount. You'd still have to spring for a half-inch OSB/plywood nailer deck and the timber screws to secure the nailer deck to the rafters, 24" o.c. penetrating the rafters by 1.5" min, but unlike gutting it from the interior you would
A: not have to live in a construction zone while the work is going on
and
B: actually meet/beat code minimum (R49, under IRC 2012)
Using reclaimed foam even with the new roofing and nailer deck it will be cheaper and higher performance than a gut-job with a 40%/60% ratio of closed cell foam/fiber in the 9.5" deep rafter bays. At R58+ it means you'd have nearly zero chance of ice dam formation on that roof, since you'd have R23+ of thermal break over the rafters, which would otherwise be only ~ R11 stripes of thermal bridge, stripes that would be obvious when viewed from the exterior only a few days after a snow storm as the leaking heat melted the snow from below on those paths.
If you take a gut-the-ceiling approach it would take about 2.5" of closed cell polyurethane (R15) to the tune of $2.50-$2.80 per square foot (for just the foam) to reliably hit the prescriptive 40/60 ratio for foam/fiber, with blown cellulose or fiberglass for the remaining 7", and you'd end up with a center-cavity R below code (in the low 40s), and you'd still have the thermal striping of the rafters to drive ice dam formation.