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Community and Q&A

Insulating and Air Sealing Rim Joist / No Sill Plate

advice_yesplease | Posted in Energy Efficiency and Durability on

Hello there

I have a (fairly) newly built house (build around 2000) and am looking to do some efficiency improvements starting with the basement – had a few questions to make sure I don’t introduce any problems. We are in the Atlantic Canada (Zone 6)

For some reason the house doesn’t appear to have a sill plate/gasket installed – the house uses OSB I-joists that are resting directly on the foundation. See:

I can’t see any kind of capillary break there at all unless it’s really tiny

The house does not appear to have symptoms of sagging joists around perimeter, uneven floor etc., and foundation is 2-3 feet above the soil-level so I can only presume that moisture wicking into the floor joists/rim joists has not been a major issue – I spot-checked several places where accessible and did not observe any softness in the I-joist contact points.

In the spots where the rim joist makes contact I don’t observe any gaps whatsoever, however part of the basement is finished so I can’t do a thorough check.

So, my questions are:
1) Is it worth it to air-seal (spray foam) the edges of the rim joist given that there are no obvious gaps in the areas that I can see/access?
2) If I do this air sealing, will it significantly inhibit any of the natural drying that the house has enjoyed in its 20 year existence and introduce any problems?
3) When I go one step further and insulate the basement walls, thereby making foundation wall colder overall (presumably at the top even if it’s exposed) will it create condensation issues up there and expose the joists to moisture?

Note that in the area between floor joists and rim-joists, there are fibreglass batts shoved in there at the moment but no air sealing.

I will also note that the OSB sheathing around the house seems to overlap and is flush with the foundation walls in most of the areas I checked. Whether this is good or bad, I guess that it also provides a bit more air-tightness

Any insight or alternative recommendations? I could perhaps seal off the joist bays entirely to prevent condensation but my worry would be that any moisture making its way to the joists due to capillary action would have trouble dissipating (even accounting for using something slightly vapour permeable and I suppose accounting for the fact that it is vapour permeable to the exterior there would be a lot less airflow)

Thanks for any help!

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  1. GBA Editor
    Kiley Jacques | | #1

    In my opinion, your moisture concerns are valid and I look forward to seeing how experts respond. In terms of air leakage, have you had an energy audit/blower door test done? Have you used an thermal camera to get a reading on the leakage at the rim joist? I ask because information yielded from these two diagnostic efforts will help you weigh your options.

  2. advice_yesplease | | #2

    Thanks so much for your response!

    Yes I did have an energy audit/blower door test a few weeks back but did not get much insight from the auditor at the time other than him saying the house was not super-tightly sealed - did not mention the sill directly just said we have what would be equivalent to approx 16x16 inch opening in the house based on the test results. Not really certain whether that's good or bad without knowing the ACH value. I am still awaiting a report with actual test results, hopefully that comes through!

    I've since found some obvious culprits (open fresh air intake for oil boiler 5" dia, cold storage with two 5" dia open vents, and 5" dryer vent from former location that was open), and there were also 4 attic hatches (a bunch leading to 1.5 storey section) that were unsealed that I've now sealed up - so I'm guessing that must have affected the results during testing and maybe obscured any impact of the sill.

    I do have a thermal camera and will take a look at this from the inside - that's a good idea. I looked at the outside before and the infrared coming off the foundation itself tends to overwhelm the sensor (that glowing foundation was kind of my basis for wanting to insulate the remainder of the exposed foundation!)

  3. charlie_sullivan | | #3

    I think you should expect that there's significant air leakage even if you don't see a gap. I think you could seal it with caulk, and deal with insulation there separately. If you use fluffy insulation, having some solid material over it to block air flow into it is important--that's what's missing now as I understand it.

    Is it possible to consider mineral wool board insulation on the outside of the foundation wall, at least down a few feet? That could help keep the concrete warmer in winter and aid drying to the outside.

  4. Expert Member
    BILL WICHERS | | #4

    For 1, I pretty much guarantee you there are gaps, even if you can't see them. Hold your hand up in there on a windy day and you'll probably feel the air leaking through those gaps. In my own home, almost every single joist bay felt drafty as I was going down sealing the rim joist. Canned foam isn't going to work well for small cracks, you'll want to use caulk for that. I like polyurethane sealants. Even better is to use canned foam to "foam in" blocks of rigid foam cut to fit along the rim joist. I used 2" EPS in my own home, and I used EPS because it is the most vapor permeable of the common rigid foam types, so it allows for a little bit of drying for the rim joist.

    For 2, yes, air sealing does limit drying somewhat. I don't know if I'd say "significantly", but that is a complicated thing to try to answer. Less air movement does mean less drying, but it also potentially means less wetting in the first place. I would try using EPS and canned foam here so that you get the air sealing, but you also preserve some drying ability at the same time.

    For 3, I don't think it would make a difference for condensation risk, but insulating/sealing the basement walls will increase the likelihood of moisture wicking higher up the wall and potentially resulting in a wetter rim joist. You can limit this somewhat by not completely sealing the entire wall and leaving a small "drying gap" just below the rim joist. It's not much, but it's a bit of insurance. The tradeoff is a cold spot where you can get condensation. There is no free lunch, unfortunately. The best option is to add a capillary break between the top of the masonry wall and the framing. If you can do that, and make sure all your exterior drainage is good, then you're in good shape and can safely seal up the entire wall.


  5. advice_yesplease | | #5

    Cheers all for this advice - I think I will have to wait for a windy day to see if I can pick any air movement up in the joist bays. Funnily enough the RIM joist itself does not seem super cold to the touch despite it being about 2degC outside this morning - further adding to my confusion!

    I uploaded a clearer photo showing both joists, and also the sheathing overhang with the foundation:
    Note that there is just under 7 inches from the inside edge of the concrete to the RIM joist, suggesting to me that the remainder (of the 8 inch foundation)

    Not that it would be a good idea but I was vaguely thinking that maybe I can't see any gaps because there may be some kind of lip in the foundation that the RIM joist OSB overlaps with, like (b) in this figure with the difference being that my joists are not embedded in concrete
    Was this type of construction done anywhere?

    I will consider these other improvements you mention (caulk, capillary break, exterior insulation)

    I guess that it seems to make sense to insulate the joist bays with something vapour permeable in this case (EPS foam board with spray foam). I need to think of a way to treat the already-finished portion of the basement separately as there's no way I'm getting in there to seal/insulate - maybe I should put in some sealed blocking to that area and insulate that part from the outside

  6. advice_yesplease | | #6

    Alright I now feel somewhat foolish - I finally stuck my head in the joist by to get a closer look and cleaned off some dirt:

    There actually is wood that the rim joist is resting on, that is level with a lip in the concrete - it was just so dirty that I couldn't distinguish it from concrete lip in my photo..
    I had not seen any foundations that have this lip in searching around so it never occurred to me. The photo also reveals a gap between sill plate and rim joist in the new area I inspected which suggests it would benefit from some sealing.

    This still doesn't explain why the joists are resting on both the wood sill plate and concrete lip without any capillary break but I feel slightly better about the situation knowing there is less contact between joists and direct to foundation involved. I would have to pull some siding off to see if there is a gasket under the still plate but since the drawings spec a PT sill plate with gasket I am hoping that's the case.

    Anyone ever seen such a system before? Does this change my approach to insulating?
    It seems like it would have been extra trouble to add that lip in there when building the foundation - I don't know why they would do it. I am also somewhat concerned by the fact that there's nothing in place to keep the joists from twisting so wondering if I should try and remediate before trying to insulate.

    Appreciate your help and patience!

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