[Greenbuilding] spray foam/fiberglas

Kat molasses at q.com
Tue May 10 19:29:29 PDT 2011

In general from what I understand on Building Science is that you should 
be fine using cellulose or fiberglass insulation if you have a thick 
enough layer of closed-cell foam next to the sheathing on the outside.  
The closed-cell, being vapor-impermeable, won't allow moisture to 
migrate through upwards, and if there's enough of it the inside of the 
foam will be warm enough so no condensation will occur on it.  Assembly 
dries to the interior.  Just *don't* make the mistake of putting a 
SECOND vapor barrier on the inside, as John seems to be suggesting.  You 
need that drying to the interior.  Check the code in your area to see 
how thick a layer they say is required in your area to keep that 
underside-of-foam-surface warm enough to prevent condensation.


> Seems the overriding concern with moisture in the roof assembly is 
> interior air leakage finding some path where it can condense. The 
> spray foam ‘should have’ air sealed but 2x6 framing is subject to 
> movement and foam can separate and shrink and crack. One option might 
> be another layer of spray foam to seal or if you don’t want to get a 
> contractor back for that - put any type of insulation and then an air 
> barrier.  A requirement for any ceiling here is a fire requirement 
> which could be 1.5” t&g or a layer of drywall before the wood t&g. The 
> drywall can be detailed as a decent air barrier with seams taped and 
> gaskets around pipes and electrical.
> If you are not planning on putting up gypsum or some other barrier 
> then I would carefully inspect and reseal the sprayed ceiling as 
> needed with canned spray foam and taper fill the perimeter edges and 
> caulk seal or foam gasket any wiring or pipe penetrations through the 
> foam and then put any insulation below.  As for the ‘any’ insulation I 
> would not use a batt insulation where the fibres are brittle and pose 
> a lung hazard (glass and rockwool) as they will migrate into the 
> living space thru the t&g. If you could find some wool or wool/poly 
> batts they would be great. Nice thing about the wood ceiling is it can 
> deal with small amounts of interior moisture and dry into the interior.
> John Salmen
> *From:* greenbuilding-bounces at lists.bioenergylists.org 
> [mailto:greenbuilding-bounces at lists.bioenergylists.org] *On Behalf Of 
> *tom at honeychrome.com
> *Sent:* May-10-11 6:19 PM
> *To:* greenbuilding at lists.bioenergylists.org
> *Subject:* Re: [Greenbuilding] spray foam/fiberglas
> Thanks Bob and Ron for the responses.  Yeah, when I had the foam 
> installed I should have told them to fill the cavities completely and 
> paid the price, but they were 'the experts' and told me it wasn't 
> worth it.  It didn't really make sense to me at the time, but I didn't 
> follow through.  I don't plan on selling this house, so resale and 
> short-term investment recovery isn't a priority.  I haven't installed 
> the ceiling yet, so I can still add insulation before I put the 
> paneling up.  The ceiling is t&g pine boards, so I could install most 
> of it up to the peak and blow in cellulose from the top, or put in 
> fiberglas bats, though the spaces are somewhat irregular.  Seems like 
> the cel. might be the better option.  But there is also the moisture 
> issue.  Even pretty well sealed, it is rather dry in the house in the 
> winter (wood-stove heated) and the roof isn't vented (which is why 
> blown-in closed cell was installed).  Will filling in the space left 
> with fiberglas or cellulose create a situation where moisture can collect?
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