Exterior continuous insulation and mold
Hey Folks,
We are going to build a passive house-lite in Marine 4c. Wall assembly is gypsum, 2×6 advanced framing with 24” oc filled with blown-in fiberglass, taped and sealed Zip panels, 11/2” Type I EPS, then a rainscreen and Hardie.
We just met with a Huber rep who said some Seattle insurers are no longer insuring exterior rigid foam insulation because too many houses are eventually developing mold IN the rigid foam. Apparently the low permeability of rigid foam CI is holding enough moisture for it to eventually mold. They discover this when homes have needed repairs which have revealed the rigid foam, upon which they make the unpleasant discovery of moldy foam.
To be fair, the Huber rep would prefer we use Zip-R instead of just Zip sheathing, but we trust their intentions were good and they wanted us to be aware. More importantly, is this something we should be concerned with? They also mentioned something about the compression psi of the foam being important—said a minimum of 20psi is desired (the Type I EPS is ~12).
Should we be worried about the dew point and moisture accumulation in the foam itself? And/or the psi of the EPS when used in this manner?
Sterling
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Replies
Sound like hearsay from a biased source.
Ask for documentation but my guess is the walls in question were not code compliant in more than one way.
Walta
Sterling,
Like Walta I'd like to hear this from more than one source who doesn't sell a competing product before becoming too concerned.
I'm not dismissing the possibility it is occurring in Seattle, but the PNW is a very different climate that Maine, with very different moisture challenges.
That said, I've renovated foam assemblies in my similar Vancouver Island conditions - everything from buried EPS, EPS foam in hot tub lids, on exterior walls and roofs, and the interior of basement walls - and while I have on occasion found sheets which had become saturated and heavy from the water they took on, I've never seen anything but incidental mold on them.
I find this very unlikely. I've seen mold ON rigid foam before, but NEVER INSIDE the foam. Mold needs moisture AND FOOD to grow, and there isn't any food for it inside the foam. I think it's far more likely the moldy walls weren't built correctly, possibly with too little rigid foam, or a double vapor barrier, or other way to trap moisture, and then the batt insulation in the walls got moldy. That I've seen many times.
The easy way to find out if this is a real thing is to call your insurance agent and ask. If they say it's not a problem to insure a structure insulated the way you plan, then you known you're good to go.
Bill
Thanks, everyone. Much appreciated! Confirms what I was thinking.