Closed-cell spray foam against OSB sheathing
Hi everyone,
I’m looking into spray foaming my basement with Closed Cell (2lb) foam.
My concern is the effects that the spray foam would have on the breath ability of the OSB Sheathing on the areas of the pony walls as they could no longer breathe in.. currently outside I have Tyvek WRB and vinyl siding. The wall in question faces south-west so there would be almost full day sun exposure.
Would there be any concerns with ventilation/breathing of the OSB under this wall system?
There is another wall that is not finished yet on exterior (just Tyvek’d) that will be stucco’d… would the stucco require an air/ventilation gap behind it if I was to apply closed Cell foam to the interior of OSB?
Climate Zone 7
Thanks
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Replies
Rick,
Vinyl siding is a well-ventilated type of cladding. Plastic housewrap is vapor-permeable. So your OSB will easily dry to the exterior.
That said, it doesn't make much sense to install closed-cell spray foam between studs. This article explains why: Installing Closed-Cell Spray Foam Between Studs is a Waste.
-- Martin Holladay
Well I used closed cell spray foam in my bedroom walls up against my zip sheathing. It wasn't as easy to trim as open cell but I did it. Since I did it myself using a tiger foam kit the extra cost was partially made up and I feel the bit if extra work was worth it
A vented air gap with the stucco is highly recommended, independently of whether you blow the budget on closed cell foam (for no good reason) or not.
Rick,
Dana is right. The air gap is essential, as explained in this article: To Install Stucco Right, Include an Air Gap.
-- Martin Holladay
thanks Dana and Martin,
the reason I was looking into closed Cell foam for the basement was because the foundation walls step from 9ft down to 5ft concrete.. the back wall is full 9ft and it steps down about half way through the side wall. I wasn't to keen on batts against the concrete so was looking to spray that area..then figured I would take it down the whole wall, which got me to the pony wall area where I wasn't too sure if I should be spraying.
Based on the article and Danas comment, it isn't really necessary to foam the above grade pony walll.. would it be advisable to foam the high foundation wall area and below grade only?
Currently if I leave as is, it is just poly and batts as insulation and vb against the concrete and the ponywalls
Thanks
Rick,
Below-grade basement walls (usually made of concrete or CMUs) are insulated on the interior with either closed-cell spray foam or with rigid foam. So it's OK to use closed-cell spray foam for that purpose (although it isn't the most environmentally friendly option). For more information, see How to Insulate a Basement Wall.
The framed walls above your basement walls can be insulated like any above-grade wall. There are lots of options here. At GBA, we often recommend the installation of continuous rigid foam or mineral wool on the exterior side of your wall sheathing (to reduce thermal bridging through the studs), but in your case it may be too late for that option.
A wide variety of insulation materials will work between your studs -- but without a layer of continuous exterior insulation, you'll still have thermal bridging.
-- Martin Holladay
thanks Martin.
What would you suggest for concrete foundation walls exposed above grade?
I have about a 30-40foot run of concrete wall that is 2-3ft above grade before stepping down to 6inches above grade.