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Community and Q&A

Insulating rim joists in condtioned crawlspace

Dentside | Posted in General Questions on

Hello, All…

Zone 5.  Southern Interior region of BC, Canada.  Hot summers, cold winters, little to no humidity any season.

I understand the concern regarding the difference in outside temps vs. inside temps in the winter and the potential for condensation on the inside of rim joists.  I’ve seen and read a lot of information regarding techniques for insulating rim joists, particularly the use of foam board and spray foam; however, I’m a little confused regarding my particular application.

My home is essentially built on a large deck.  A dozen buried concrete piers (three rows of four) are supporting three beams and the house sits on top of the beams.  The floor joists and beams overhang the piers on all four sides, such that no part of the “foundation” is exposed to the elements.

I’ve enclosed the space below the house by building an insulated skirt wall between the bottom of the floor joists and a short, concrete sill (to keep rodents out and to have something to build the skirt wall upon).  The resulting crawlspace will be encapsulated (poly on the ground, no exterior vents) and conditioned (open to the main house and a small exhaust vent in the crawlspace).  The crawlspace is at grade level…not below or above grade.

Given the above, it strikes me that if the house and crawlspace are both conditioned, and the walls – above and below the rim joists – are insulated in a similar manner (Roxul batts with barrier on the warm side), then the rim joists do not require any “special” treatment.  Of course, they would be insulated and barrier-ed, but the use of foam board or spray foam would be unnecessary. 

Am I missing something here?

Cheers,

Roy

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Roy,

    What you are suggesting - batts and poly - is how rim-joists are typically insulated by builders in BC. Once the batts are installed, the poly is bedded in acoustical sealant and stapled in place. The weakness is air-sealing. If you look at most of the joist-bays on an average house you'll find gaps between the poly and sealant, because it is very difficult to do well.

    That's why foam is recommended. If you can seal the bays diligently your approach will work fine.

  2. Dentside | | #2

    Malcolm,

    Much appreciated. Is it the conditioned air leaking out that's the issue...or is it exterior air leaking in? Or both? And is the concern that the leaked air would cause condensation on the interior of the rim/band joist?

    Roy

  3. Expert Member
    Zephyr7 | | #3

    Leaks can go both ways. Which way the air actually moves depends on the relative pressure between inside the house and outside. If the pressure inside is lower, the air will leak inwards. If the house is higher pressure relative to outside, the air will leak outwards. I’d guess that the majority of the leaking for a typically rim joist is going to be outside air coming into crawlspace due to stack effect in the house.

    Don’t worry so much about which direction the air will move, just try to minimize how much air can move at all. Air sealing works in both directions after all :-)

    Bill

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