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Rim joist is hidden by floor joist blocking

doubleare | Posted in General Questions on

Hi, I am about to take on a basement spray foam insulation project in a house in PA built in 1934.  What I recently found out is that when I look up above my foundation/sill plate in between my floor joists what I thought was my rim joist is actually just blocking between the floor joists.  There was an old dry vent in place that I took out in preparation and that is when I realized the rim joist is another 6″ or so back behind this blocking.  I am thinking if I just have the blocking sprayed it won’t be as good as if the actual rim joist was sprayed.  Spent a lot of time googling this but found nothing so my first question is there a term for this type of basement/floor framing?  Second, am I overthinking it and should I just have the blocking sprayed?  Lastly, wondering if I can cut a hole in each bay so that rim joist can be sprayed?  Not sure how big of a hole the insulation guy would need to get the gun in there to spray.  Thanks in advance for any replies.

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  1. BrianPontolilo | | #1

    I'm sure that there are some GBA members who have faced a similar dilemma; there are many situations where rims joists can be tough to access in older houses. So, I bet you'll get some helpful responses.

    It seems to me that if you could drill holes to give the spray foam contractor access to the rim joist, that would be ideal. Not only because that is where you want the thermal barrier, but also for the air sealing that it will provide. This is a notoriously leaky area. You may also be able to do this yourself with a two-part spray foam kit.

  2. Expert Member
    BILL WICHERS | | #2

    You could cut maybe a 2” hole in the middle of each piece of blocking with a hole saw. With a small gun, the insulator could probably get the entire cavity filled through that hole. The foam will expand a bit and plug the holes for you.

    You might want to swing a wire around inside each cavity before spraying foam. Thick cobwebs might keep the spray foam from applying evenly.


  3. raptorman | | #3

    What did you end up doing here? I have the same situation with blocking between each joist. I see the previous owner removed a few for furnace venting, but I don't want to do that all the way around.

  4. doubleare | | #4

    Project got delayed so I haven’t done anything yet. However, even if I were ready for the insulation now, I don’t know what I’d do. I plan on talking to the insulation contractor about it when the time comes later this summer.

  5. rhl_ | | #5

    I have the same issue. Hole drilling is the preferred method.

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