I've always been lead to believe that you never mix two types of insulation. Condensation will form between the two resulting in less R value than you would have with using just one.

No problem using two different types of insulation. It is very common to use spray foam as a flash coat to seal up crawlspaces and attics. Then a lot of folks use blown in cellulose on top of the foam in attics. In crawl spaces many people add foam board over the flash coat of spray foam. No problems doing either of those things

It used to be common(i think it still is) for builders to used rigid foam sheathing on the exterior everywhere except on the corners where osb is used for bracing. The walls are insulated with kraft faced fiberglass between the interior finished wall and the foamboard. I have never heard of anyone having a problem doing it that way,. And yes, the vapor barrier is always on the heated side of the walls.

I am not sure if I missed it but what is your wall depth 2x4 or 2x6? If you truly wanted warm walls 6 is the way to go. If they are 2x4 your only going to get so much out Of them. Do what you can and put your efforts into the overhead insulation and your base perimeter insulation. You will lose most out your lid. Get that really well insulated and just as importantly really well vented so moisture is not a problem up there. Your base perimeter can let a ton of cold in if not addressed.

Osky

We can no longer use 2x4 exterior walls here.

You can if you can reach R21.

Looking at the above posts... If you have 2x6 walls with a house wrap on the outside and 6" insulation Kraft face on the inside I'd leave it at that. Make sure you have enough room around your window and door jambs for insulation, do it right, and use good doors and glass. If the interior is a wood tounge and groove type product I'd put it right over the craft face. Done.
I would not put poly any where near the place if I didn't have to.

Osky

I think this is the route I'm going to go with, Osky. They are 2x6 walls and electric is already in. I still may put osb or at the very least strapping because I'm going vertical with the tongue and groove. Attic is well insulated so heat loss out of the ceiling should be minimal.

Thanks everyone for your responses, looks like I will avoid placing two vapor barriers over each other. If it means I have to bring in a little extra firewood for winter then so be it. I would rather have a less insulated, dry house than a super insulated house with moisture issues.

Make the house really tight and then put a HRV in. I built my house and used icfs from the basement to the rafters. If you have the air exchanger it should take care of the moisture inside.

Funny how different places are different. Some areas require a poly vapor barrier and others prohibit it.

Personally, in a cold climate, you couldn't give me a house without a poly VB !

We're it mine, using vertical tongue and groove, the interior osb is crappy backing for nailing. I would just put horizontal blocking in. 2 rows min, maybe 3. 2 x 4s set wide face to the inside flush to the inner studs. Toe nail them in. End nail and stagger if easier.


Osky

OSB is not a good backer for T&G if you are using finish nails in the tongue. OSB just doesn't grip nails very well.

Adding 2x4 nailers on flat between the studs will compress your insulation and diminish your R value.......something you are trying to protect. With batted insulation, you're going to have drafty walls if you simply install T&G without some sort of solid backing.
Skip the insulation board, skip the OSB, and just add 1/2" drywall......taped and mudded with one coat to seal things up.
I would save yourself a lot of work and run those finish boards horizontal. If not, fir ontop the drywall and run it vertical.

Hopefully you electrical boxes aren't installed and rough wire yet, or else you'll be moving everything.

I think engeneers have WAY too much time on their hands

I think 6" inulsation will do just fine despit a couple of 1 1/2 inch compressions where the nailing blocks are. The wood that replaces that 1 1/2 inch of insulation has good r valu itself.

Osky

. I enjoy working with a good engineer ! You can keep all the architects though. They think just because they can draw something two-dimensional that it can be built with wood in three dimensions.

I received a set of signed, stamped prints from an architect for a new museum for the University of Alaska. He had the floor below grade but wouldn't believe me until he sent out a surveyor !

A lot of engineers have actually worked in the trades. Architects not so much.

Lot of truth right here boys.

Don't get me started about architects.

I can't tell you how many times I've had to pore over prints trying to find simple addition/subtraction errors when total inside dimensions don't equal outside dimensions. You would think basic math skills would be prerequisite to being an architect.

One of my current jobs; converting a back porch into a master bath/walk-in closet/dressing room, the existing footing does not meet current code (not deep enough). In my opinion it is deep enough to have no movement. My architect agrees. He was shocked when the code enforcement officer wouldn't let him sign off on it instead requiring a report from an engineer. It was a real blow to his ego! LOL

Electrical boxes are installed but every one of them in the house is an adjustable box so wall thickness can be up to around 2 1/2" should I choose

Would plywood be a better backing for T&G than osb?