Does that mean when someone installs vinyl siding w/ the fanfold underneath that they should not have an interior vapor barrier? Since the fanfold is the same material as insulating foam sheathing, no?
No, 1/2" is fine; but 1-1/2", you've got a vapor retarder.
Permeability decreases as thickness increases: 1-1/2" blocks moisture transport 3 times more than 1/2".
A vapor barrier is required on the inside of the insulation, you can not do it without it.
That depends on his local climate, and codes.
It's a code requirement here (6mil), and Alberta is extremely dry ambient.
You mean,
because Alberta is extremely dry ambient. Vapor pressure is related to the difference in humidity between inside and outside. So for the same indoor humidity, a drier outside means there's more vapor pressure.
i'd listen to the guy from Alberta. His climate is closest to yours.
Not sure I agree. Alberta's drier, and colder, than Wisconsin; I think they're on different sides of the line for plastic. But I'd still check local codes.
Anyways, the point is that you can't do
both, 1-1/2" of foam on the outside, and plastic vb on the inside, in
any climate, and not cause problems.
Nate -
Another thing to look out for - you'll pick it up in those links - is that if you want foam on the outside, you can only put so much insulation inside the wall. The inside of the foam has to be kept warm enough to not get condensation. If you over-insulate inside, you move the dewpoint* further in, and the framing rots.
*"first condensing surface", if you want to get technical.
Is this new construction, or retrofit? There are other ways to get high-R-value walls:
- spray foam
- dense-pack cellullose
-Thicker walls, either by using bigger studs, or (if you want to avoid thermal bridging) strapping cold spots
- Insulated Structural Panel construction
- rigid foam on the
inside, would work pretty well in your climate; use the foil-faced, and it's a vapor barrier on the correct side...
...Lstiburek's climate-zone-specific builder's guides, from the buildingscience.com site, are the bible for this sort of thing, and well worth the price. Written for contactors, rather than academics: clear explations, detailed recommended specifications and drawings... makes it all very easy to understand how everything plays out in your specific climate.
Last note: bear in mind that nominal R-value is just one piece of the equation. The average house loses a lot more heat through drafts, than through not having enough insulation. I remember Calgary as pretty freakin windy in the winter...