Double stud wall Assembly – Moister issues?
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| Posted in Energy, Heating & Insulation on
Reading the FHBM artical ‘Designed for Success’ (link below) I noticed the 16″ double stud wall assembly had a layer of spray foam on the exterior sheeting (couple of inches) with the remaining wall filled with dense pack cellilos and closed with a 6mm poly vapor barrier . With the closed cell spray foam and the poly rap I am assuming moister in the wall is not a concern – why? Is this wall type safe in all climate zones including extream cold – we get 8000 hdd with many days sub -40. Why isn’t the due point a consern? Science based feedback would be helpful.
Thanks
Mark
https://www.finehomebuilding.com/pdf/021240050.pdf
Replies
Well, you can always plug estimates into this site: http://www.vesma.com/tutorial/uvalue01/uvalue01.htm and see if dewpoint ever drops below temperature. There are probably ways to be more precise, but they would use the same technology (and it's sound technology). (And it would be nice if there were the equivalent site that used US measurements.)
Valid question
Finaly took time to read that article and I think you have a valid question.
I don't have anything close to a valid answer but I will say that Passive House (Haus) design has only recently come here to North America. It's all the rage in the green building community right now, backed by 40 years of development in Europe.
Like the architect in that article saaid, it's stringent. It relies heavily on a very well coordinated series of air sealing methods, superinsulation, very high performance glazing. Screw up, or overlook any one small part of the perscribed process and you stand every chance of unforeseen consequences you don't want any part of.
I studied everything I could get my hands on about it a few years ago. I wanted to build one for my daughter. I finally had to cry uncle and revert back to what I understand and believe, can intuit from my life's work and what building science I can wrap my head around.
I do believe if you build a certified Passive Haus you'll have a state of the art energy saving building. For sure. But it's far more than a few steps, or techniques. It's almost a philosophy. You can't pick and choose a few details from it to integrate into a traditionally built home here in North America without running the risk of some unexpected consequences.
What seems clearly to be a vapor barrier on each side of the wall framing cavity is one such example.
Where is the poly?
I didn't see a reference to 6-mil poly in the text or in any of the captions, though I admit I could have missed it.
The only thing I see that looks like poly is in one of the drawings. But what looks like poly might actually be mesh fabric for blowing cells into the framing bays before the drywall is installed.
Jim
I sent a link to Andy, maybe he can find someone in those dark halls that might expand on this subject.
It's not my article, but I'll see if I can rattle someone's cage.
FWIW, I think PH is flawed because it's measured by method, not by results. That's not to say there aren't great lessons, such as the multiple blower door tests.
Mark,
Mongo's reply is correct. What you're mistaking for 6 mil. poly is actually a mesh netting that's applied to the studs as part of the dense-pack cellulose insulation process. The wall is in fact designed to dry to the inside.