Insulating a sloped wall & attic
We’re renovating a 2.5-story 1924 brick house in Brooklyn (right at the line of zones 4A and 5A). For historic and budgetary reasons, we’re not gutting the place. Instead, we’re trying to insulate/air-seal/tighten where we can maximize environmental impact and minimize sledgehammer impact. Something along the lines of the “Pretty Good [Retrofitted] House.”
Our first and major focus, then, is the top floor. It is mostly under the eaves, with knee-walls, sloped walls, a few dormers, and an attic above. Oh, yeah, and it has a leaky Spanish tile roof that we can’t replace right now. So we need a system that can be resilient to leaks itself and not rot our plank sheathing.
I’ve hammered out two plans (see attached image), and I’d really love to hear which seems wiser to you…and which is setting us up for mold/rot/condensation/apocalypse. See attached drawings to clarify these descriptions.
Both plans have Accuvent baffles to increase air circulation under the roof sheathing, directing any eventual leaking water out the eaves instead of into the cellulose, and providing circulating air to dry the roof planks and prevent rot. The downside of this is that because the water won’t be soaking cellulose/drywall, we won’t know about leaks as soon. But the pluses seem to have the upper hand.
Plan #1:
We baffle, dense-pack, and air-seal up the rafters (~9″ deep) all the way up to the ridge line. So after the sheathing, the next layer is baffles, then dense-pack cellulose, intello membrane, and then either air (in the attic) or drywall (in the 3rd floor).
Pros:
– Keeps the AC in conditioned space
Cons:
– More of the roof is obscured, so leaks are harder to find.
– The attic is conditioned space, so heat will be trapped there in the winter, instead of in the living space.
Plan #2:
We take down the plaster ceiling in the 3rd floor and bring the air barrier down to that level. We’ll do baffles and dense-pack in the knee-walls and sloped ceiling, and then a heck of a lot of loose cellulose in the flat joist bays of the attic.
Pros
– Less conditioned space!
– More of the roof sheathing is accessible for inspection & repair
– More air circulation to the covered sheathing, so it’ll dry faster.
Cons
– Cost more?
– Will we have to worry about condensation on either side?
– AC is in the unconditioned space–though we use AC rarely, so it’s not nearly as much of a concern as heat.
Another consideration is that given dormers & inaccessible walls, this won’t be a perfectly sealed top floor. Does that change the calculus?
Anything else that we seem to have overlooked?
[And, in case it’s relevant and/or you’re interested, we’re doing hot water radiator heat with a mod/con boiler, and the next lines of attack are insulating the basement & installing interior storm windows]
We’ve hit a bunch of dead-ends with our insulation plan: respected and credentialed contractors have given us plans that seemed solid, and in many cases, this forum has been the only thing between us and condensation/mold/death/destruction. Unfortunately, lurking as a reader has been more effective for shooting holes in plans that for coming up with one. So I’m here to ask for your input. So thanks in advance for any and all thoughts!
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Replies
Kimberly,
If you have AC ductwork in the attic, your insulation should follow the sloping roof plane, so that the attic is within the conditioned space.
I would suggest an improvement, however. You should scab on additional framing to deepen your rafter bays, so that you can insulate to a higher R-value than your planned R-33.
Thanks for the input--I was hoping you'd chime in. Very interesting to see that that jumps out as the most important factor here. Is that because it will improve the performance of the system while it's running, or because it will maintain the continuity of the insulation & air-sealing year-round?
Kimberly,
Ducts in an unconditioned attic are really, really, really bad. Even if you aren't blowing air through the ducts, they are leaking conditioned air.
All ducts need to be brought within your home's conditioned space.
Not an answer, but a request for clarification. I'm working on a similar project outside Philadelphia, with 3/8" plywood instead of the Intello running all the way to the top plate at base of rafters. I plan to add 2" of polyiso below the rafters, which are deep enough to allow for 8.5" of blown-in fiberglass insulation within the bays. My vent channels are 2" deep, made from 1" polyiso (should have used something with more permeance, but the deed is done). Continuous soffit and ridge vents.
Insulation in rafter bays on sloped ceiling will be dense-packed, with scrim attached across the top of slope where it meets the flat section to keep the dense-pack insulation from blowing out at the top. Will have 16" of loose-fill fiberglass above flat-section, with the 2"of polyiso under that as well. All of this is well-sealed at joints and edges, and unlike in Kimberly's case there are no ducts or other utilities above. One penetration in the plywood for the vent stack. Given that there are no ducts in the attic space, should this work okay?
William,
Q. "Given that there are no ducts in the attic space, should this work okay?"
A. Yes.
Martin, thanks for your input. I actually just found your recent post about creating a conditioned attic (perfect timing!). You mention that "Polystyrene “ProperVents” are too narrow, too shallow, and too flimsy." Accuvents are supposed to be stronger, and I believe, deeper. Would these be sufficient? Also, let me know if you want me to post this question over there.
Also, we have very reduced ventilation because we don't have a ridge vent. Are the baffles still adequate?
Kimberly,
I think that the AccuVents will be fine.
It's usually a fairly simple job to retrofit a ridge vent with most types of roofing.