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Questions on sealing rim joists (and a few other items)

anontemp123 | Posted in General Questions on

I have fiberglass insulation acting as a seal between my foundation and sill plate. Can air still get through there?

The foundation on the front of the house is below grade, and the rim joists are well above the foundation. Is sealing here as important as the if the rim joists and sill plate are directly on the foundation?

There are no rim joists on the sides of the basements. Sometimes the sill plates are obstructed. These also have fiberglass between the sill plate and foundation. Are these as important to seal as sill plates with rim joists? If so, I can probably foam seal about 50% of these sill plates. Would that make a difference?

Any special treatment for the chimney area?

How can I fill very large gaps? Is foam sufficient or should I use mortar?

The crawlspace is below grade and surrounded by conditioned space on all sides except the exterior wall. I’d appreciate any comments on if sealing the sill plates is important here.

Rear basement
Rear basement
Front basement
Front basement
Front basement
Front basement
Front basement
Front basement
Right basement
Right basement
Right basement
Left basement
Left basement
Sliding door framing

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  1. anontemp123 | | #1

    (Apologies for the typos and confusing parts. Seems I can't edit my question after posting.)

  2. LucyF | | #2

    Can you tell us a little bit more about the house (age of the house, location) and what stage in the renovation(??) you're in?

    Fiberglass does not help with air sealing. It does not stop air movement. In order for it to function as insulation it is important to prevent air from flowing through it.

    In general, you have do the air sealing before the fiberglass or any insulation other than foam is installed. I guess another exception is if it is covered with drywall and you are going to do the Airtight Drywall Approach, but I don't think that applies to the rim joist. It is a notoriously leaky place in a home. It's got to be sealed with foam or the cut and cobble method or something more than fiberglass.

  3. user-4524083 | | #3

    Dear "forum user" - Too many questions. Fiberglass is air permeable and a lousy sealer. One remedy, a lot of work, would be to trim off the excess on the inside of the sills,where you could,and then caulk or foam the wood/cement interface. Foam can fill fairly large gaps, certainly up to an inch. Wood sills should not be below grade, and I couldn't tell from the pictures or your words what was meant by "The foundation below grade,and the rim joists are well above the foundation." Fiberglass is a good insulator when it is fluffed up to fill the whole cavity without voids and there is something in place, preferably on both sides of it that was pretty air tight but vapor permeable, like wood or drywall. The fiberglass batts in the cavities of the rim joists, between the floor joists, is a common practice and an almost complete waste of time. Fluffing those batts up to completely fill the space, if possible ,and then cutting a piece of wood,drywall or EPS or polyiso foam a little smaller than the space, and caulking or foaming that in place,would work. If the fiberglass batts don't fill the space pretty completely, take them out and just do the foam as mentioned above, maybe more layers of it , depending on your climate.( Where are you?) Good luck - you have a big job ahead of you but mostly time, and not too expensive.

  4. anontemp123 | | #4

    Sorry for all the questions. I thought about making multiple posts and that's what I should've done.

    The house is in Maryland and was built in the mid-1980s. We bought it on foreclosure. We stripped half of the house down. All the flooring is gone, and most of the basement drywall is off. We've replaced all the windows. We've repaired the roof. We've solved all our water issues. (Water was coming into the house before whenever there was heavy rain.) All the HVAC ducts are exposed right now. Tomorrow a company is coming to seal all the HVAC ducts and registers with mastic, foam, caulk, etc. Please let me know if there are other specifics that might help.

    If the fiberglass is compressed (like in the second picture) between the sill plate and the foundation, it still won't create a seal?

    On that front side of the house, the foundation is not built as high as the other three sides. Where the foundation comes up to is still below grade, then there's a sill on top of that, then the studs and window, and finally then the rim joists. (It's possible I'm using the term "sill" incorrectly.) I tried to show this in the pictures. In fact, before we replaced the gutters, every time it rained, the area below that window would be wet. There was even termite damage on the studs and the sill, and we had most of that wood replaced. Anyway, on this front side, there is OSB (or maybe homasote) and brick on the other side of the rim joists. SInce the rim joists are not resting on the foundation, I wasn't sure if I should be as dedicated to sealing those.

    Many energy efficiency companies here don't use a board to seal the rim joists like in all the YouTube videos. Rather, they will either (1) foam around the perimeter of the rim joist and then put the batts back in place, or, (2) as a more expensive option, use 2-part foam and go at it. I'm guessing cutting the foam boards and foaming those in place is too labor intensive. Is Option 1 an almost complete waste of time, too?

  5. GBA Editor
    Martin Holladay | | #5

    Dear Forum User,
    You should learn to use the search box on GBA. You also might want to read some relevant articles listed on this page: How To Do Everything.

    There is lots of information on GBA. The more you read, the fewer questions you will need to post.

    Here are some links to relevant articles to get you started:

    Insulating rim joists

    Air-Sealing a Basement

    Building an Unvented Crawl Space

    How to Insulate a Basement Wall

    Installing Fiberglass Right

    Q. "If the fiberglass is compressed (like in the second picture) between the sill plate and the foundation, it still won't create a seal?"

    A. No. To create an air seal at this location, you need high-quality caulk, a high-quality European tape like Siga Wugluv, or at least one inch of closed-cell spray polyurethane foam.

  6. anontemp123 | | #6

    Martin Holladay, thanks for your reply. I have searched everywhere, read material for more than week and spent quite some time learning to uploading pictures and learning code to make this post. I asked questions here on items I was confused about or didn't find answers to online.

  7. charlie_sullivan | | #7

    There are enough potential air leaks that you may want to get a blower door test to help figure out which are really the biggest leaks, and then you'll also have an expert who can answer your questions while looking at everything in person. An alternative might be if you have a monster 1000 CFM range hood, you can run that on full and feel for where the air is leaking in the most.

    Having wood below grade is a bad idea. Possible solutions include:
    1. Re-grading outside if you can do that and still have the grade slope away from the house.
    2. Replacing the below-grade stud wall section with either CMU (concrete blocks) or poured concrete.
    3. Less of a sure-fire solution, but one that might work, temporarily excavate to put EPS foam boards on the exterior of the wood, between it and the dirt, with a robust waterproof layer between the EPS and the wood.

    Fiberglass works fine as an insulator but does essentially nothing to stop airflow, even when it's compressed.

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