Low slope roof, insulation in ceiling dilemma

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Old 03-22-20, 08:27 PM
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Low slope roof, insulation in ceiling dilemma

We have a 100 year old 3 floor house that we are renovating and want to put some insulation in the ceiling of the 3rd (top) floor. Above it is the space for the low slope roof where it gets cold in the winter.
These buildings weren't really designed to have any insulation at all. As such, they require some heat to melt all the snow on the roof in winter otherwise it becomes too heavy.
The experts advise putting some insulation at least due to the dew point in this space. So, they advise R8 or R10.

Now my dilemma is that I cannot easily find fiberglass in r8 locally. In addition, the joists are spaced irregularly, about 27" apart.
Is it possible to buy R20 and just split it in half to become R10?

Secondly, we were advised not to use cellulose on this ceiling since it would sag on the vapor barrier and touch the drywall but I'm wondering if cellulose wouldn't be better just due to the irregular joist spacing?
 
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Old 03-22-20, 08:34 PM
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I would buy 4x8 sheets of 1 1/2" rigid foam, cut them to size and fit them into place, use spray foam on the edges to air seal the pieces.
 
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Old 03-22-20, 11:01 PM
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Isn't it going to be very difficult to cut to an exact size? It's 1000sq ft to insulate, all with 27-29" joists.
how do I keep the stryofoam in place? Nails?
the joist bay is around 8" deep, do they go on the bottom of top of the bay?
 
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Old 03-23-20, 05:06 AM
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Cellulose will work, it fills voids and helps to air seal, that concern is not valid!
 
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Old 03-23-20, 05:11 AM
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Do you have plaster or drywall? When 5/8" drywall is used on the ceiling the weight of the cellulose isn't normally an issue. Never heard of it being an issue with plaster.
 
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Old 03-23-20, 06:07 AM
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It's 1000sq ft to insulate
you made it sound like you were just doing a small area where the space was tight.

yes, I agree that it you have 5/8 drywall cellulose should be fine. What's with the wide joist spacing? You will probanly need to add perpendicular 1x4 strapping to the ceiling so its 19.2 or 24 on center.
 
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Old 03-23-20, 07:37 AM
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Don't you have to address the snowload issue before thinking about insulation?
 
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Old 03-23-20, 08:28 AM
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Engineer says it can take R20 and snow load will be fine.
however, insulation specialist says r8 is better for old houses to ensure some snow melts.
The cellulose concern is not the weight but the fact that vapour barrier will touch the drywall, moisture issue.
If we put cellulose we can't fill fully since it will be more than r8 then.
it's an old house built in 1910, they are all spaced with joists like that. 3" thick joists.
can you only fill a cavity partially with cellulose instead of full?
​​​​​.

kind
 
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Old 03-23-20, 08:55 AM
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vapour barrier will touch the drywall,
I have no idea what this means, the insulation is not a vapor retarder (not barrier).

You paint is/can be your retarder!
 
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Old 03-23-20, 09:06 AM
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Insulation then vapor barrier ( plastic)then drywall
Cellulose will sag and make plastic touch the drywall + it becomes a condensation risk
 
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Old 03-23-20, 10:19 AM
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Your in Canada, different rules, no vapor barrier in ceiling in US!
 
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Old 03-25-20, 04:28 AM
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Ok aside from the vapor barrier question, should the insulation go at the bottom of the joist bay or the top? Imagine the cold air is just on the other side of the bay?
thermal bridge question I guess
 

Last edited by qwertyjjj; 03-25-20 at 05:44 AM.
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