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Old 09-03-2020, 09:09 AM   #1
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insulating knee walls on the interior?


I recently purchased a very old home (~110 years) in Minneapolis (climate zone 6). In all 4 corners of the 2nd level there's a 4' high knee wall (approx 6' wide) followed by a slanted ceiling that's approximately 6'x6'. I suppose you'd call it a "Cape Cod " style. Currently there's no insulation in the house except for a trace amount in the attic. [sigh] As you can imagine, the ice dams are intense!

I do plan to insulate the attic with cellulose this fall, but cellulose isn't a good option for these spaces between the slanted ceiling and the roof decking. There's is not much (if any) more than 3.5" (2x4 rafters) between the lathe&plaster and the roof decking. The highest point on these slanted ceilings is *basically* the edge of the attic that will be covered in cellulose. I will replace my roof soon, and I had been planning to have about 2" of closed-cell spray foam applied on the exterior side of the lath & plaster when the shingles & decking were removed. I'd apply more, but I also want these cavities to bring in fresh air. One concern that I have about the spray-foam option is that it reduces the air flow into the attic. The 3.5" between the 2x4 rafters in all 4 corners of the house is the *only* place where air currently enters the attic.

But recently I had another idea: adding interior rigid foam over the existing slanted lathe & plaster ceilings. To be clear, this would be the resulting construction: living space | drywall | horizontal strapping | 2" rigid foam insulation | lathe&plaster | rafters (providing attic ventilation) | roof decking | steel shingles

Are there moisture issues I'm under-thinking? It's not clear to me that the spray would actually achieve higher r-values than interior rigid foam when you factor in the space that would need to be left open for ventilation and the thermal bridging through the rafters. Thoughts very appreciated!
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Old 09-03-2020, 09:37 AM   #2
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Re: insulating knee walls on the interior?


Is the roof this shape.

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Old 09-03-2020, 09:43 AM   #3
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Re: insulating knee walls on the interior?


I've added a circle approximately in the place where the slanted ceilings are. I have one of these in each of the 4 corners, but I've only circled one of them.
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Last edited by brian85; 09-03-2020 at 09:46 AM. Reason: added circle
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Old 09-03-2020, 09:53 AM   #4
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Re: insulating knee walls on the interior?


Quote:
Originally Posted by brian85 View Post
I've added a circle approximately in the place where the slanted ceilings are. I have one of these in each of the 4 corners, but I've only circled one of them.
You likely don't have soffet vents, the areas from the valleys to the peaks is impossible to vent too.

You might be better to strap the roof for the new roof and provide the venting above the sheeting.
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Old 09-03-2020, 10:01 AM   #5
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Re: insulating knee walls on the interior?


https://www.buildingscience.com/docu...3-over-roofing
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Old 09-03-2020, 08:02 PM   #6
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Re: insulating knee walls on the interior?


Ice dams are usually near the eave where insulation is thin and house framing allows downstairs heat to melt the ice there.

1. you can add eave ventilation through the roof shingles (search for alternatives to soffit venting).
2. insulate the heck out of the eave area. You can add r50-60 fiberglass where it will fit then use closed cell spray foam to the eave area for as much insulation as you can get and close off air leaks. At least r19 for the knee walls.



Not sure how much living space you have up there. Rafter insulation is also important but gable fan venting could help. May have to use smaller fan. Dedicated gable fans could be over capacity there and may burn out too quick or such.
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Old 09-09-2020, 05:01 PM   #7
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Re: insulating knee walls on the interior?


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Originally Posted by Nealtw View Post
You might be better to strap the roof for the new roof and provide the venting above the sheeting.
Thanks Nealtw! Honestly I don't think I'm going this route. Though I have no doubt in its merits. Do you have any guidance on which approach is better between the 2 options I laid out (spray foam "outside" vs foam board "inside")?
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Old 09-09-2020, 05:19 PM   #8
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Re: insulating knee walls on the interior?


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Originally Posted by brian85 View Post
Thanks Nealtw! Honestly I don't think I'm going this route. Though I have no doubt in its merits. Do you have any guidance on which approach is better between the 2 options I laid out (spray foam "outside" vs foam board "inside")?
No I would strip the roof cut holes in the deck fill the inside with blown in add 2" of foam on top of the deck strap it and metal roof.



I don't think you would be happy with anything less.



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