GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

Floor above unconditioned space

Quinn Sievewright | Posted in GBA Pro Help on

Hello
Firstly thanks for the feedback on my previous roof assembly post. This question relates to the floor of the conditioned area (ceiling of a garage/carport). Thankfully no ducts, just wires and plumbing in a few locations.
By way of background, its climate 4C (near Vancouver, Canada) and a small building used as a holiday home, not fully occupied in Winter. The occupied conditioned space will sit above both an unconditioned garage and cantilevered over a carport.
Designers build up from conditioned side down:
– ¾ OSB T&G
– Poly sheet
– 9.5” I Joists on 16” centers
– R22 Roxul Comfortbatt in cavity (not clear how they keep the insulation ‘up’ against OSB with only about 6” spec’d)
– Air gap, ventilated above carport area
– Carport ‘ceiling’ will have T&G Pine, garage ceiling will be finished with ply or OSB
This build up raises some questions for me having read the Do I Need a Vapor Retarder and How to Insulate a Cold Floor :
1) Vapour barrier – House designer says they normally lay down a sheet of poly tight over the joists prior to laying the OSB subfloor. My reading here suggests this is not needed, OSB is sufficient. Assuming we can rely on OSB, I guess we should tape the seams and try to seal the locations under the joists?
If code forces our hand towards a ‘sheet’ barrier, would you suggest a more variable vapour barrier like Membrain ?
2) Air Barrier – Seems there is mixed info on OSB as an air barrier. I’m not targeting Passive standards by any means here, as such can I assume the OSB is sufficient to meet reasonable air tightness standards?
3) Related to the above, Martin suggests a thin layer of rigid foam directly under the subfloor, sealed with expanding foam. Is this required or a “nice to have”? Are the benefits the improved air barrier characteristics in addition to boosting the R Value?
4) Our exterior walls are planned to have R5 or R6 Roxul Comfortboard, would not insulating the underside of the joists (be it rigid foam like the article or Comfortboard) be a material loss in your views (purely budget restrictions).
5) The joists appear to only have about 6” in them based on the R22 Roxul spec and code is actually R28. I wonder if we are better off with R31 9.5” Fiberglass which is almost the same price as far as I can see.
6) If we fully fill the joists and follow Martin’s cross section, there appears to be no need for a ventilation gap above the carport soffit. However, if we can’t afford to fully fill, and therefore don’t insulate the underside (depending on your input as well), should we ventilate the remaining space?
Thanks very much.

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.

Replies

  1. GBA Editor
    Martin Holladay | | #1

    Quinn,
    Q. "House designer says they normally lay down a sheet of poly tight over the joists prior to laying the OSB subfloor."

    A. There is no need for polyethylene (which makes the installation of the subfloor awkward). OSB is already a perfectly adequate vapor retarder. OSB subflooring is installed with construction adhesive, so even if you want the OSB to serve as an air barrier, you really don't need to tape the seams.

    Q. "Can I assume the OSB is sufficient to meet reasonable air tightness standards?"

    A. You'll need two air barriers here: one above the mineral wool batts, and one below the mineral wool batts. In this location, the OSB (installed with construction adhesive) will be fine for the upper air barrier (especially if it is covered with flooring). For the lower air barrier, taped 5/8-inch drywall should be installed.

    Q. “Martin suggests a thin layer of rigid foam directly under the subfloor, sealed with expanding foam. Is this required or a 'nice to have'?”

    A. In some situations, it may be nice to have. But you may have misinterpreted one of the illustrations in my article, How to Insulate a Cold Floor. The illustration in that article (reproduced below) shows a duct with duct insulation near the subfloor -- not rigid foam near the subflor.

    Q. "Are the benefits the improved air barrier characteristics in addition to boosting the R-value?"

    A. Yes -- but mostly the improved R-value.

    Q. "Would not insulating the underside of the joists (be it rigid foam like the article or Comfortboard) be a material loss?"

    A. I'm not sure what you mean by a "material loss." The rigid foam improves the performance of the assembly. Especially if you will have plumbing in the floor assembly, the rigid foam makes sense. But if your budget is tight, your budget is tight. You have to determine which trade-offs to accept.

    Q. "The joists appear to only have about 6 inches of insulation in them based on the R-22 Roxul spec and code is actually R-28. I wonder if we are better off with R-31 9.5-inch fiberglass which is almost the same price as far as I can see."

    A. You are right to be concerned. Obviously, you don't want insulation that is less than code minimum. There isn't any good way to keep 6-inch-thick batts up against the subfloor in a 9.5-inch-deep cavity. It's very important to choose batts that are at least 9.5 inches thick.

    Q. "If we fully fill the joists and follow Martin’s cross section, there appears to be no need for a ventilation gap above the carport soffit."

    A. That's correct.

    Q. "If we can’t afford to fully fill, and therefore don’t insulate the underside (depending on your input as well), should we ventilate the remaining space?"

    A. No. You want to limit air movement through this floor assembly, especially if there is plumbing.

    -- Martin Holladay

    .

  2. Quinn Sievewright | | #2

    Hi Martin
    Thank you very much. I'm glad I've run this by you as we were clearly not going in the right direction.
    With respect to the lower air barrier, are there alternatives to taped dry wall? In the exterior carport area we'll finish with T&G pine and on the garage sealing hoping for ply or OSB. Perhaps Membrain above these elements (although I understand it isn't the most robust of air barriers)?
    Alternatively, if we used rigid foam under the joists, could we not just tape or can foam the seams?
    The image I was referring to is the second one in that article, attached below.
    Thanks again

  3. GBA Editor
    Martin Holladay | | #3

    Quinn,
    Most building codes require 5/8-inch drywall on garage ceilings for fire safety reasons. Check with your local code official -- I don't think that plywood or OSB will work. Taped drywall is important not only for fire safety reasons; taped drywall is also a durable air barrier.

    Q. "If we used rigid foam under the joists, could we not just tape or can foam the seams?"

    A. If you use rigid foam under the joists, then yes -- you should absolutely tape the seams (or seal the seams with canned spray foam). Conceivably, you could then install tongue-and-groove boards on the underside of the sealed rigid foam -- but only if your local code inspector approves of the plan.

    -- Martin Holladay

  4. Jon R | | #4

    I question how effective can spray foam sealed rigid foam will be after some shrinkage or movement.

  5. Expert Member
    Malcolm Taylor | | #5

    Quinn,
    I'm not sure it's even possible to lay a subfloor over poly. I've never seen or even heard of it being done in construction in BC. I don't want to disparage your designer, but based on some of their suggestions I'm not sure how much practical experience they have.

  6. Expert Member
    Dana Dorsett | | #6

    With TJI joists batts of any type and thickness are a pretty lousy choice, since the width of a TJI joist bay is about an inch wider than the milled lumber joists the batt was designed for. (Do they make batts specifically sized for TJIs?) The odds of ending up with voids and thermal bypasses are high.

    Blown cellulose (any density) at 9.5" would run about R33-R35, automatically fits to perfection and would be than a near-perfect batt job hand-sculpted for a near-perfect fit.

  7. Quinn Sievewright | | #7

    Thanks everyone for the responses.
    Malcolm - I see your point, to be far this was a discussion with the designer, however the crew chief seems to be very experienced and would not have necessarily done it this way. Thanks to your inputs at least now I can say I like it done a certain way from the start.
    Dana raises a good point as well. I haven't considered the size of the TJI flange versus the OSB web. We'll certainly ask the builder if blown might work here. However, it is in a fairly remote location so it may not be feasible.
    Thanks again.

  8. Quinn Sievewright | | #8

    Hello again

    Does anyone have any thoughts on putting adhesive in the groove prior to joining the boards? Is this necessary to maintain the air barrier? Or is adhesive along the joists sufficient?

    I found one thing by searching online...Weyerhauser states "a 1/8-inch glue bead applied at the tongue-and-groove joints can further improve floor performance."

    My builder will probably think I'm crazy to adhesive the T&G joint but I'm okay with that...

  9. GBA Editor
    Martin Holladay | | #9

    Quinn,
    Q. "Does anyone have any thoughts on putting adhesive in the groove prior to joining the boards?"

    A. Your reference to "boards" is confusing, but I'm guessing that you are talking about tongue-and-groove OSB subflooring.

    In my experience, the use of construction adhesive along the joists (in conjunction with the contribution of the finish flooring above the subfloor) is enough to create an air barrier. I've never used construction adhesive in the tongue-and-groove joint.

    That said, as long as you have a light touch with the caulk gun, and the temperature of the glue isn't too low (because cold adhesive gets very thick), I suppose you could try the Weyerhauser approach.

  10. Quinn Sievewright | | #10

    Hi Martin,
    Sorry yes the boards being the OSB subfloor. I should have perhaps mentioned the OSB will be our finished floor (sanded and stained) in this case. Temp will be warm at the time of install so perhaps we'll at least try if it doesn't end up being troublesome to do.
    Thanks again

  11. Expert Member
    Malcolm Taylor | | #11

    Quinn,
    I've glued the t&g edges on subfloors. How easy it is depends a lot on the manufacturer. Some have an almost semi-circular profile, some a much more narrow slot. Invariably, just as you do with the seams at joints, you will end up with adhesive on the surface of the OSB. Usually you smooth out the lumps with a piece of scrap lumber, or your boots, but you are going to need some strategy to get it off quickly and more cleanly if the subfloor is going to be your finished floor.

    i don't want to sound discouraging, and I understand where the impulse comes from, but I've never seen subfloor used as finished flooring work out well.

  12. Quinn Sievewright | | #12

    Hi Malcolm
    Interesting thanks for the info. We may try the first couple boards and see how it goes, as you say don't want PL everywhere!
    I understand your concern with the OSB being a finished floor. This is a holiday property and we are aiming for a fairly rustic and inexpensive finish, albeit with a well thought out building envelope that will last a long time. If the OSB doesn't turn out great, then we haven't lost much other than a bit of time sanding and oily it. We can always lay over the OSB in the future with traditional wood flooring is our thought.

  13. Expert Member
    Malcolm Taylor | | #13

    "We can always lay over the OSB in the future with traditional wood flooring is our thought."

    True enough Quinn. And hey - good luck with your build!

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |