Combining Foamboard and Roxul- Air and Moisture Issues?

Howdy Ya'll- My first post on the internets' best forum...

First- The Situation:
I just purchased an old house (originally coal miners' quarters owned by "The Company" built around 1900). I've taken most of the house down to studs. It was framed with true 2x4's and 1x12's running vertically from foundation to roof along the exterior. Currently (traveling from interior to exterior) there is 1x12, tarpaper, 3-tab roof shingles used as siding then vinyl-siding.

I plan to add 2x2 furring strips (making a 5 1/2" wall cavity) to thicken the exterior walls so I can increase the insulation.

second, The Plan:
My idea was to put 2" of foam board in the wall cavity and fill around the edges with 'Great Stuff'. Then add 3 1/2" Roxul (mineral wool batts) to fill the cavity and bring me up to r-25 in the walls.

Finally The Questions:
Should I add a vapor barrier (tyvek?), an air barrier (plastic?) or nothing along the interior before sheetrock?

Should I install the insulation the other way around(mineral wool towards the exterior and foam board along the interior)?

Man oh man i want it to be perfect. Oh, and I'm in West Virginia at over 3000 feet.

Thanks a bunch Ya'll!

I would do 6" batts and plastic [6 mill poly] on the inside next to drywall. As I understand it Tyvec is a rain gaurd not a vapour bearer and should be outside where you have tar paper.

Welcome to the forum!

With the asphalt roofing shingles (vapor barrier) on the exterior wall, don't add another vapor barrier (plastic) on the inside, you will have problems.

You could add the foam board inside to the studs but--- it will keep your cavity colder (wetter) because you will stop the heat from the room warming it. I'd rather cut-n-fit the foam on the inside with Roxul in the rest of the cavity, tight to the drywall face to limit convective loops there. Add some foam (fanfold, sill sealer, 1/2" foam board) to the studs for a thermal break from the outside temperatures (wood is R-1.25 per inch) using proper layers to end up with a good fit, no air gaps. http://www.buildingscience.com/documents/information-sheets/installation-of-cavity-insulation/

Limit the air moving from the room to the wall because of pressure differences by air sealing the drywall well. http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/ You lose 100 times more from a small leak than from diffusion. Caulk the sheathing to studs/plates for air sealing once it's exposed. Seal the wiring/plumbing holes in the plates and exterior light boxes. http://www.wag-aic.org/1999/WAG_99_baker.pdf

If in Zone 4, you don't want/need a v.b. at all. http://www.buildingscience.com/docu...heet-310-vapor-control-layer-recommendations/

Gary

Thanks guys- I appreciate it.

Great thought on the thermal break Gary.

p.s. buildingscience.com is amazing!