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Community and Q&A

2nd floor rimjoists treated the same as basement/crawlspace?

sayn3ver | Posted in Green Building Techniques on

Zone 4.

I assume you want to treat the rim/band joist area of a light wood framed structure between the first and second floor the same as the basement/crawlspace articles covering the topic?

For traditional construction (ie no exterior insulation) and using an interior air barrier such as intello, do install and detail the barrier in each bay with batt insulation installed? 

Or do you simply cut and cobble rigid foam and caulk/spray foam in like a basement?

or can you use batt insulation in the cavity and just use a thinner piece of foam or drywall as an air barrier foamed or caulked in on the top facing the interior?

If you had zip sheathing taped/liquid flashed as an exterior air barrier, one still needs to be worried about moist indoor air reaching the sheathing in winter correct?

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  1. Expert Member


    See posts 3, 4, and 10 under this article:

    1. sayn3ver | | #2

      Thanks for the link.

      I am always left confused. In cathedral ceilings, martin argues that its impossible to air seal the interior ceiling and wall planes well enough to allow batt insulation in an unvented application against cold sheathing, that it would end in soggy roof sheathing in a few years. I am assuming the assumption is most roofing materials are vapor closed and installed flat on the roof sheathing?

      Yet, batt insulation against a cold framing member, behind probably OSB or Plywood sheathing that only has leaky non air tight drywall or subfloor protecting it from moist warm air, that probably has electrical or other unsealed or poorly sealed penetrations running from the first to second floor passing through the rim area.

      If I was not planning on finishing the first floor ceiling with drywall and left it either exposed with painted framing or some type of wood cladding or a drop ceiling or corrugated metal or whatever, the proper method would be similar to the foam basement rimboard treatment? or could I add small nailers to either side of the joists and run the drywall on my walls up into each bay and caulk it to the joists and subfloor?

  2. BrunoF | | #3

    Good question. I would think if you are using zip or taped sheathing that the 2nd floor rim joist area is no different then the rest of the wall cavity and wouldn’t need any extra attention. It is the top and bottom of the sheeting that would likely need extra attention to keep air out.

    Just my thoughts and I look forward to the replies as I am just now about to have my 2nd floor walls framed and am using taped OSB as my primary air barrier.

  3. Expert Member


    It's an interesting question - and one I hope others comment on. Here is my take:

    I think it was Joe Lstiburek who said something to the effect that "roofs are not just sloped walls". They experience very different conditions acting on them (the stack effect, impermeable exterior materials, night sky radiance, etc.) than walls, so I'm not sure it's useful to compare them. And to be fair to Martin, he has written that roof ventilation becomes increasingly less important as air-sealing improves.

    But what makes rim joists differ from other parts of walls framing? What's different about a rim joist than a piece of horizontal blocking in a stud bay? Really only the presence of an interior air-barrier and vapour-retarder, whose functions have been misunderstood a bit since the location of air-barriers started bouncing around in the past decade or so.

    From the perspective of air-leakage where the air-barrier is doesn't make any difference, but exterior ones don't stop moist interior air from coming in contact with the sheathing or rim joists unless there is also a secondary interior one - which is more effective if this material is also a vapour barrier or retarder.

    The problem with lower rim-joists is they lack both, and it's very hard to effectively install them. You could argue there is a big difference between these rim-joists where the framing runs parallel to them and those it is attached to, and that it's possible to treat the former much like second storey ones - and yes, if the second story ones are not effectively air-sealed by the ceiling and floor above, or some interior membrane, then they should probably be treated like the lower ones.

  4. Expert Member
    BILL WICHERS | | #5

    The "bottom" or "lower" rim joist, sitting on top of the foundation wall, is exposed to more moisture due to it's close proximity with the ground, and the fact that it's sitting right about the usually concrete foundation wall, which can wick moisture. This means the lower rim joist is at an increase risk for moisture problems compared to those higher up in the structure. As Malcolm mentioned, the lower rim joist is also usually exposed on the interior side, with no interior side air or vapor barrier to help protect it from interior side moisture drive.

    Higher rim joists, like those between the second and third floors, will have at least some level of air sealing from the ceiling and floor above and blow their location, so they won't be fully open on the interior in the way the lower rim joist is. They're also much higher above grade, which also helps to put them at reduced risk of moisture prooblems.

    It won't hurt to insulate the upper rim joist in a similar way to the lower rim joist, but the lower rim joist is the more critical location to get "right" to avoid the possibility of problems. The upper rim joist is more forgiving.


  5. sayn3ver | | #6

    Thanks for the replies. There is so many discussions about vapor diffusion, ports on unvented roof assemblies and moist interior air coming in contact with cold framing which got me thinking.

    I planned on at the minimum adding small nailers to each joist bay and installing a piece of fiberglass faced drywall/gyspum sheathing or using a vapor permeable air barrier and taping that so I can continue to use mineral wool batts in that cavity since it will already be on site.

    I can and would do caulked in rigid foam if that was required but would prefer not to.

    the ceiling below in this discussion will have numerous flush mounted lights in gypsum ceiling and open web trusses (with 2x8 solid end members) are being utilized for running electric and duct work so the drywall will not be air tight. however its not finalized and there is still a chance of leaving the ceiling exposed and painted.

    1. Expert Member
      MALCOLM TAYLOR | | #7


      What is still most common here are squares of poly bedded in acoustical sealant, and stapled in place. It's tedious, messy work and rarely is entirely air-tight.

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