Devil’s Triangle update
After reading the great information on this site, I went to go tackle the attic in my inefficient old house (40s, brick, slate roof, plaster and lath in zone 5A) where icicles are causing repeated gutter damage.
The actual situation is a bit more complicated than expected. Relevant info:
– This is a 2.5 story, so there are kneewalls on either side.
– One triangle is unconditioned, with 4″ of blackened fiberglass underneath the attic floor boards between the joists (joist space measures 7.5″, joists and rafters are both 16″ o.c.)
– One triangle is conditioned: there is fiberglass between the rafters, behind stapled up cardboard. This side has plumbing and ducts going to the top floor rooms.
– The angled roof over the room: rafters were stuffed with 6″ of fiberglass, which is now black and falling out in clumps. (Rafter space measures 5.5″)
There are gable vents, but I can’t see where they open out into the attic. At the end of the kneewall space there is a wall of black substance which might be fiberglass over a gable vent. I’m not sure.
From everything I’ve read, the major problem here is air sealing (hence black fiberglass) and inappropriate insulation putting the dewpoint in a bad spot. Some of the roof sheathing has dark black stains on it, presumably from where there was once fiberglass that adsorbed water and held it against the sheathing.
So here are the questions that would be wonderful if anyone could help answer:
1) What’s best to do about the angled roof over the living space? If I fill that space with insulation, it doesn’t look like there will be any ventilation to the roof in that area. Could I use spray foam to fill that and do a partial ‘hot roof?’ With 5.5″ that could be about R30 maybe. Not perfect but better than what’s there.
2) On the unconditioned side, would sealing with great stuff, then EPS boards, then regular batt insulation be an OK method?
3) On the conditioned side, is it better to attempt something on the roof to keep the whole space conditioned? Another option I considered was to make an additional box/wall around the ducting to bring it into conditioned space and then back-converting the remaining area to be unconditioned and working out the gable vent situation.
Thank you very much! The concepts took a while to get down. The practical steps are even harder.
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First of all, can you tell us your name?
Q. "What's best to do about the angled roof over the living space? If I fill that space with insulation, it doesn't look like there will be any ventilation to the roof in that area."
A. This is an insulated cathedral ceiling. To read about all of the options for insulating this type of sloped roof assembly, see this article: How to Build an Insulated Cathedral Ceiling.
Note that a good solution may require you to demolish the plaster or drywall ceiling, and lower the ceiling height. If you go with a vented assembly, you need to install ventilation baffles from your soffits to the ridge (or to the attic under your ridge vent). For more information on this aspect of the work, see Site-Built Ventilation Baffles for Roofs.
Q. "Could I use spray foam to fill that and do a partial 'hot roof?' With 5.5" that could be about R-30 maybe. Not perfect but better than what's there."
A. Yes, you could do that. Remember, however, that you don't want to install spray foam on the underside of slate roofing, especially if the slate is installed over skip sheathing. (Spray foam glues everything together and makes a mess. It also prevents the slate from drying on the underside.) So you'll probably need to make this a vented assembly rather than an unvented assembly.
Q. "On the unconditioned side, would sealing with Great Stuff, then EPS boards, then regular batt insulation be an OK method?"
A. Are you talking about insulating a kneewall (a vertical wall)? If so, that approach would probably work. The trickiest part of insulating a kneewall is installing blocking between the joists under the kneewall bottom plate, and installing blocking between the rafters above the kneewall top plate. For more information, see this article: “Two Ways to Insulate Attic Kneewalls.”
Q. "On the conditioned side, is it better to attempt something on the roof to keep the whole space conditioned?"
A. In almost all cases, the best solution to your dilemma is to install rigid foam above the roof sheathing, followed by OSB or plywood sheathing and new roofing. For more information on this approach, see these two articles:
How to Install Rigid Foam On Top of Roof Sheathing
Insulating a Cape Cod House
However, I would be reluctant to advise that approach for a house with a slate roof. A slate roof is rare, beautiful, and long-lasting. I would think long and hard before removing any roofing slate.
Hi Martin, and thank you for your reply. My name is Bryan Benson.
Thank you for the links to the articles. They are very helpful, though I'm still a bit unsure about translating into practice. For example. I didn't know that slate was unique in that it should always be vented and wouldn't work in an unvented assembly. That's very good to know! The roof is totally out of scope here. I'm not doing anything to it unless I find that all the sheathing on the conditioned side is rotted out when I pull down the fiberglass that's there.
I've attached some pictures just so the current situation is clear. Edit: Please note they are all turned 90 degrees for some reason. I'm sure this is something you've seen a thousand times before but there it is anyways. Side 1 is the 'unconditioned attic' side with fiberglass on the floor and kneewalls and side 2 is the 'conditioned attic' side with fiberglass in the rafter bays. The "unconditioned" side, by the way, is not cold at present. I'd say 50-60 degrees and it is in the 20s now outside.
To answer your answers:
A) "Note that a good solution may require you to demolish the plaster or drywall ceiling, and lower the ceiling height. If you go with a vented assembly, you need to install ventilation baffles from your soffits to the ridge (or to the attic under your ridge vent). For more information on this aspect of the work, see Site-Built Ventilation Baffles for Roofs."
I think you're right that the only way to address the angled part of the ceiling is to move the ceiling down. The ceiling doesn't go all the way to the point, but rather forms an inverted 'U' shape with about a two foot span where the rafters directly contact the gypsum board. The big question is whether that two foot span with direct thermal bridging and reduced insulation space is worth moving the ceiling for. I may be able to get access to the top of the U from the outside since that's where the vents are.
B) "Are you talking about insulating a kneewall (a vertical wall)? If so, that approach would probably work."
I'm talking about the knee walls and between the joists of the floor of the unconditioned side (side 1) using a 'cut and cobble' approach. The local big box sells EPS sheets (Insulfoam) that come pre cut and fit in the bay between the joists, reducing the labor intensiveness. I would do two layers of that then lay over that with batts to fill the joist space and then rolls crossways once I'm over the top of the joists. I am leery about long-term air/moisture from below sealing issues with this method though. Would it make sense to lay down some barrier (housewrap) first? That would be difficult but doable.
C) What about the idea of building a box around the pipes on the conditioned side, then converting the conditioned side to an unconditioned attic space, then leaving the whole roof vented?
Thank you again very much! This is just a bit over my head and your experience is very valuable.