Vapour retarders are required by Code (and p.2 here) in your climate zone above open cell foam. That's why many builders will use closed cell instead.

However, the unusually thick application would very likely meet the classification of a vapour retarder. To put yourself at ease, simply check with the building authority that issued the permit.

I'm no cold climate expert, but was sure that cold
climates use closed cell, not open cell.

the reasoning behind foam insulating attics is
to make an air tight, unvented, semi-conditioned
attic space.
no baffles, no soffit, rigde, passive or gable vents.
air tight.
the foam seal between the roof & the attic floor
has to be continuous & air tight.
depth of the foam should also be continuous.

we do this here in the south, open cell unvented
attic, because in 90% of homes, the ductwork is
in the attic. putting insulation on the roof line creates
an insulated space around the ducts, allowing them
to perform better. it also solves the issues of condensation
of high efficiency hvac systems here.

nationwide, the unvented attic solves the issues of
thermal bypasses. (openings between attic space &
living spaces.) trades people make lots of holes in
ceilings & cover the openings with covers, trim kits,
supply grills etc.

however the foam sealed addition attic would have to
be separated from the rest of the house, unless
you are foaming the whole roof.

I'd check with other spray foam insulators about
type of foam, and get a few more bids while
you are at it.

do you have ductwork in the attic?
adding hvac to addition? know that
foam insulated attics allow for smaller
more efficient hvac systems.
have system sized (manual J) for
addition.

best of luck.

Especially the confirmation that in IECC Building Climate zones 5 and higher Code requires a Class II vapour retarder. (Chicago is in Climate Zone 5A.)

worthy, I think the insulator is suggesting that the thickness of the insulation itself constitutes a class II vapor retarder and so I don't need to add an additional barrier.

It seems to be the norm in MA to use open cell foam in unvented attics without an additional vapor barrier. I sure hope it works, that's how our attic is done. None of the companies that bid our project specified an additional vapor barrier, nor did our energy consultant or architect.

is 10" the average install depth for these climates?
it isn't something you have to argue with foam companies
about to meet code R-value requirements?


you learn something new everyday!
I truly thought northern (heating) climates
all used closed cell.

thanks guys!

Still warm, dry and happy here in MA, but it hasn't really been all that long.

Thanks Rwiegand. We are getting open form insulation installed today and I live in Chicago area as well. We are not required to have vapor barrier either and I just had the same concerns. We were told that there is paint with a vapor barrier that we can use for priming the drywall. We are having ProTech out of Skokie doing the work.

ask the question at greenbuildingadvisor.com and see what they say.