Insulation in ceiling and low slope roof above

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Qwertyjjj

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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?

I could use stryofoam but then does it go at the top or bottom of joist bay? Bay is about 8" deep
 
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