Ontario Canada requires R29 on exterior walls of new construction. The local contractors are achieving this is adding 1 1/2" XPS to exterior of wall sheeting (OSB). However...the local building inspectors also REQUIRE 6 mil poly on the interior. Q- is this a problem? I seem to recall any amount of XPX on the exterior serves as a vapour barrier.... we are creating a wall with no ability to dry. Am I wrong... Is there a solution.. thanks. Would EPS work? I don't think the math works out for EPS .. R22 in the wall (Roxul) + 1 1/2 EPS (at R4.5 per inch) 4.5 plus 2.25 = R28.75

Moderator...please move this if I've posted in the wrong forum...perhaps Building and Construction?

My plan preparer is chomping at the bit for an answer

I prefer the XPS (I already have it)..but the concern is screwing up the wall and creating a mold problem

What you need is a vapor retarder/ membrane. Will the inspector consider this?

The bone heads here want 6 mil poly

I found a sympathetic ear..it looks like membrain will, be OK... will that be sufficient to prevent moisture issues in a 2x6 wood framed wall in zone 7

Attached is technical information about the vapor permanence ratings for both products. Hope this helps. https://www.foam-control.com/?pdf=Foam-Control-Insulation-Update-Molded-Polystyrene-Compared-to-XPS.pdf&id=4035 Kevin Casey

Thanks for the information..