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Question about cut and cobble rim joist insulation

tim08108 | Posted in Energy Efficiency and Durability on

I’m planning on insulating around the rim joist of my 1901 home using the cut and cobble method with rigid foam insulation sealed shut with spray foam. I know that normally you fit the pieces right up against the sheathing and between the joists. However, mine are a bit different than all of the diagrams and photos I’ve seen. My studs seem to go all the way down through the sub-floor to the rim joist. So between most (but not all) of my floor joist, there’s one stud, either against the floor joist or in some cases right in the middle of the two.

My question is, should I still cut the foam panels for the whole width between the joists? In most cases, this would leave an air cavity between the panel and the sheathing the depth of the studs. This would be easier to install, but would the air cavity lead to moisture or other issues? Or, should I cut the foam to fit around the studs so it goes flush with the sheathing? In this case, I worry about the studs acting as a thermal bridge and diminishing the effectiveness of the insulation job.

In case my description wasn’t clear, I’m posting some photos of what I’m talking about.

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  1. tim08108 | | #1

    Forgot to post the photo. In this one, the stud is in the middle, but in a lot of them, it's flush with one of the joists.

  2. Expert Member
    Dana Dorsett | | #2

    You don't have a rim joist- you have balloon framing with both notched floor joists and studs resting on it. It looks like you have ship-lap plank sheathing installed at a 45 degree angles to counter racking forces on the wall too.

    I expect that if you stuck your hand in there to feel around you would find that the sub-floor planking doesn't block air movement into the wall cavities above (?). It may have a 2x4 fire-stop blocking access to the wall cavities, or not. You may or may not have batts or other insulation in the wall cavities, but can probably figure that out with a bit of probing. DO figure that out, and report back- it may make a difference on how to proceed.

    What's on the exterior side of the ship-lap plank sheathing?

  3. tim08108 | | #3

    The wall cavities above have some kind of spray-in insulation from the previous owners, except for one small section where there are fiberglass batts. On the other side of the ship-lap is foam insulation (maybe about an inch thick or so) and vinyl siding.

  4. Expert Member
    Dana Dorsett | | #4

    What type and how thick is the foam on the exterior side?

    And where are you located (which climate zone)?

  5. tim08108 | | #5

    It's 1/2 inch thermax sheathing outside under the vinyl siding. And I live in South Jersey.

  6. GBA Editor
    Martin Holladay | | #6

    I think that the best solution in this case is to use spray foam. If I were you, I would buy a two-component spray foam kit. Here is a link to an article that describes using a two-component spray foam kit to insulate the rim joist area: Basement Insulation — Part 2.

  7. tim08108 | | #7

    Thanks for the feedback everyone. I got a quote for spray foam insulation, but had been hoping to save a few bucks and do it myself. (And I know you can get the 2-part spray foam kits, but that seems like a messy and unfun job, and also slightly more expensive than cut-and-cobble). So is cut and cobble a bad idea for balloon-framed houses in particular?

  8. GBA Editor
    Martin Holladay | | #8

    Q. "So is cut and cobble a bad idea for balloon-framed houses in particular?"

    A. Yes, because you have to address air leakage upward, through the stud bays, as well as ordinary air leakage through the exposed wall sheathing and the sill plate.

    In the case of areas like the one in your photo, you would need two horizontal rectangles of foam (to address the stud bays) and two vertical pieces of foam (to address the wall sheathing). That's four rectangles of rigid foam, each one of which needs to be air-sealed at the perimeter. It looks like access to do this work might be very awkward.

  9. tim08108 | | #9

    Thanks for the clarification. Looks like it's the ol' tyvek suit for me after all....

  10. Expert Member
    Dana Dorsett | | #10

    The foil-faced Thermax is R3, and probably performs at R2.5-ish during the coldest weeks of winter. That's (just barely) enough exterior R for dew point control on a 2x4 wall in a zone 4 climate, but it's marginal.

    With the Thermax on the exterior it must dry toward the interior, which means you don't want more than an inch or two of closed cell foam on the interior or it becomes a potential moisture trap at the sheathing. In fact, the foundation sill might be better off limiting it to 1" if it's resting on a poured concrete foundation with no sill gasket or other capillary break. You can fill the gaps to fatten up the R-value with cut'n'cobbled rock wool batt.

    Before spraying the foam, installing cut'n'cobbled rigid foam air-dams at the sub-floor level to block convection up the balloon framing bays is a good idea. If you then did a flash-inch over all of it, air-dam, sheathing, foundation sill and finished it out with cut up high density batt it would be fine. You'd have something between R8-R10 between the fiber insulation and the great outdoors, which is enough dew point control for R15-R20 of fiber even without interior side air barriers (though interior air barriers are still a good idea, even if it's just cut-up Tyvec or some other high permeance material.

  11. user-4243359 | | #11

    Balloon frame basics:
    As others have observed you have a balloon frame house. The studs and stud cavities are continuous from the foundation to the attic, which I presume is two stories. The wall cavities inadvertently act as chimneys, drawing air from your basement and out your attic. Because the interior walls of most old houses aren't well sealed, air is also drawn into those "chimneys" from interior living spaces as well as the basement. The net result is an extremely leaky home, particularly when the temperature is low and the stack effect greatest. You really need to ensure that the wall cavities are sealed off at the bottom (basement) and top (attic). Some balloon frames have a top plate, some do not. Since yours doesn't have a sole or bottom plate, it is likely doesn't have a top plate either. After insulating the ring and sealing off the bottom of the wall cavities with spray foam, as suggested, you should move your efforts to the attic and repeat the sealing effort there.
    If all of these assumptions are true and you are successful in reducing the draw through the wall cavities you will have a much, much more comfortable home. However, you need to be vigilant of the changes that will have occurred in your basement. Closing off the wall cavities will in effect be turning off the ventilation system for your basement which may result in dampness, mold and radon problems that will need to be rectified.
    You mentioned that there was some type of "spray-in" insulation in some of the wall cavities. I presume that this means foam. While you have access to this material, it will be worthwhile to asses what it is. Given the vintage of your house, it is possible that it is UFFI (Urea Formaldehyde Foam Insulation). This form of foam was used for a short time in the early 1970's before being banned for the health risks associated with off-gassing formaldehyde. It can be easily identified by handling a sample. If it crumbles and turns into a dusty powder, its probably UFFI. The good news is that the off-gassing is long gone. The bad news is that the product will have shrunk considerably and created significant voids thereby negating its benefits as an insulation. The upshot is that if you have UFFI in your wall cavities, effectively they are un-insulated. Not good but good to know.
    Along those same lines, if the 1/2" foam board insulation under the vinyl siding is installed over the original clapboard siding, as was commonly done to create a smooth surface, this insulation too will provide little of its potential R factor to your wall system. Again, not good but good to know.
    Best of luck. Improving these old balloon frame structures can be a challenge but the savings and increased comfort will be dramatic.

  12. tim08108 | | #12

    Thanks for the info, Edward. I'm guessing the spray-in foam is more recent than that based on what I know about the previous owners, but I'll see if I can sample it at all. And the original clapboards were removed, when the vinyl siding was put on, so the thermax panels should be flush.

    Our attic is finished (though not connected to our heating/cooling system), so we can't do a lot of work up there without ripping out the walls,. But one of the kneewalls had a small door installed for storage presumable, so I can crawl in there and see if I can see what the situation is up there.

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