GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Second floor of story and a half – insulation and smart vapour retarder

user-7444604 | Posted in General Questions on

These questions are regarding a one and a half story house, unvented, unconditioned attic.  Climate zone 7 Saskatchewan.
I’ve got some questions about how to put the second floor of the house back together.  (It was completely gutted.)
Some reassembly has already taken place.  I tried to follow as many of the air tight rules as possible ie) sealing the joist bays.
I’m seeking support and reassurance on the following:
1. The kneewall attic side has 3″ of EPS rigid foam with mineral wool insulation in the 2×4 stud bays of the kneewall.  Based on Martin’s article, I’m guessing it’s not right to put poly on the kneewall but would it be ok to put smart vapour on kneewall prior to drywall?
2. The sloped part of the roof (the 45s) are sprayed with closed cell spray foam.  The foam ends at the intersection of the 45s and collar ties (just below the collar ties).  This may be a flub.  I think the foam should have been applied so that it’s above the collar ties.  In any event, whether the foam stays as is or if I have the spray foam company return and apply more foam so that the foam goes above the collar ties towards the peak, do I butt the mineral wool right up to the underside of the roof sheathing?  I don’t if it’s ok to have the mineral wool batts touch the roof sheathing.
3.   Again, based on Martin’s article on 6 rules of poly, is it ok to install poly prior to drywalling the ceiling?


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    A smart vapor barrier on the interior side of your kneewalls is OK.

    Yes, tell the spray foam contractor to come back and extend the spray foam to the top of the ceiling joists. (Technically, they aren't really collar ties.)

    Once the spray foam has been extended, you can install mineral wool between the ceiling joists. Ideally, the spray foam extends a little above the top of the ceiling joists -- just as high as the mineral wool is thick.

    You can safely install interior polyethylene on your ceiling in your climate.

  2. Expert Member
    Akos | | #2


    If there are no vent baffles and the spray foamed 45 section, there is no way for interior moisture to get to the edge of the intersection there. Extending the insulation to the roof doesn't matter much.

    The important part is the roof section above the 1/2 story. This now becomes a mini attic and needs to be vented as such, easiest is probably a combination of gable and ridge vents.

    The area behind the knee wall section needs to be vented in a similar way as well as you will have air leaks from the main floor ceiling and 1/2 story floor.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |