# insulation & vapor barrier for joist cavities



## user_12345a (Nov 23, 2014)

i would skip the fiberglass and vapour barrier and put roxul board cut and placed in.

can put enough layers to get the r-value up high enough and foam in place.

vapour barriers suck in that area, impossible to do a good job.

you can also use foam faced with foil which u may be able to leave exposed.

with rigid u don't need vapour barrier.


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## jlhaslip (Dec 31, 2009)

user_12345a said:


> i would skip the fiberglass and vapour barrier and put roxul board cut and placed in.
> 
> can put enough layers to get the r-value up high enough and foam in place.
> 
> ...


Styrofoam / Foam to the outside would require a drywall layer on the inside.
Use closed cell foam and the plastic vapour barrier is not required, in fact it is wrong to use the plastic with closed cell because then you are trapping moisture between two sealed layers. Long story.


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## user_12345a (Nov 23, 2014)

That's all correct.

someone where mentioned that foil faced foam may not need drywall over it.

Roxul board isn't flammable so it doesn't need drywall over top of it. It also has good drying potential on both sides and doesn't allow air movement through it, so no condensation risk, no reduced r-value.

It's a truly great material - only issue is one layer won't get the r-value high enough.

Main point was, there are better options than batts and vapour barrier. the barrier if it's stapled always eventually fails and isn't air tight to begin with.


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