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Community and Q&A

How to insulate/seal in tight spaces

[email protected] | Posted in Energy Efficiency and Durability on

Hi, I’ve got about 14″ where the ground floor was bumped out 2.5″ over the walkout basement. The joists run perpendicular, so there are about a dozen little “bays” roughly 30″x14″ where the outside ir circulates and makes it feel like you’re stepping on a slab ice in the kitchen above. I’ve caulked outside, but bridging fitted at the openings to the basement, along with various wiring runs and vents make it virtually impossibly to get a caulk gun or expanding tape into all the corners inside the bays. I was going to use expanding foam, but when I found out it’s VOCs, I didn’t like the idea of having those gases circulating through the room, which is the laundry room, and into the rest of the house. Has anyone found a sealant to use in tight, inaccessible places, or do you just stuff it with formaldehyde-free insulation (my current solution, using recycled denim)?

Appreciate your thoughts!


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  1. GBA Editor
    Martin Holladay | | #1

    This is a classic example of a situation where closed-cell spray foam is the best solution. I would suggest that you buy a two-component spray foam kit at your local lumberyard.

    If you really want to avoid spray foam, you have a difficult task ahead. You can use rectangles of foil-faced polyiso (rigid foam) with all edges sealed with high-quality European tape, but that will be fussy work, for sure.

    For more information, see How to Insulate a Cold Floor.

  2. [email protected] | | #2

    Thanks, Martin. If I do go this route, is there a difference in the effects on air quality among brands? And are the off-gassing effects done after 48 hours, or is there minimum off-gassing for life?


  3. GBA Editor
    Martin Holladay | | #3

    Lingering odors are rare with two-component spray kits, but possible. If you consider yourself chemically sensitive, only you can make this decision.

  4. user-945061 | | #4

    I think Martin's recommendations are appropriate for your situation. However, for really low access joists and cantilevers, we'll often dense-pack cellulose instead of spray foam. We dense-pack through sealed 1" polyiso. Spray foam is limited to areas that you can hit with the gun, and consequently it can be pretty easy to get voids in coverage. Dense-packing fills inaccessible cavities a bit better.

  5. [email protected] | | #5

    Thanks to both of you. I was wondering about cellulose, too. This is exactly the info I need to decide.

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