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Community and Q&A

Insulation options for exposed floor joists

rsoup | Posted in Energy Efficiency and Durability on

I’m working on the plans for my bedroom addition. I have everything nailed down(thanks to the people on this site) except for my floor details. It’s a 14×16 bedroom perpendicular off the back of the house. It going to be built on block piers about 8’ off the ground. Issue is I’m going to have to stick with 9.5” joists. My plan was I-joists but open to options. I’m in climate zone 4(central md 21797). What are my insulation options? I prefer not to use spray foam but I think it’s my best choice. Can I put plastic on top off the joists sealed with caulk before subfloor goes on to prevent off gassing of the foam? What’s my best choice for under the joists? The finished ceiling will be t&g cedar. Thanks

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that discusses your options: How to Insulate a Cold Floor.

  2. rsoup | | #2

    Ok so I will do 1.5” XPs in between joists under subfloor, then 8” of dense pack cellulose. Underneath should I use polyiso or xps? Can I then nail t&g cedar boards directly to the foam(nailed to joists obviously) or do I need any additional barriers or support for cellulose. Thanks

  3. GBA Editor
    Martin Holladay | | #3

    Q. "I will do 1.5 inch of XPS in between joists under subfloor, then 8 inches of dense-packed cellulose."

    A. Green builders try to avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential. Either EPS or polyiso is a better choice.

    I would skip the rigid foam against the underside of the subfloor if I were you, especially if you plan to install a continuous layer of rigid foam under the joists. If you want to address thermal bridging through the joists, the continuous layer of rigid foam makes sense. Narrow rectangles of rigid foam (installed cut-and-cobble style) between the joists -- not so much.

    Q. "Underneath, should I use polyiso or XPS?"

    A. From an environmental perspective, polyiso is far preferable.

    Q. "Can I then nail tongue-and-groove cedar boards directly to the foam (nailed to joists obviously) or do I need any additional barriers or support for cellulose?"

    A. You certainly need an air barrier, so if you go that route, you have to be very sure that the rigid foam is perfectly air-sealed. Foil-faced polyiso with foil tape at the seams is one option. You may find it easier to install the tongue-and-groove boards if you install 1x4 strapping, 16 inches on center, between the polyiso and the boards.

    Lastly, make sure that your cellulose installer agrees with your plan, so that the dense packing can be planned.

  4. Expert Member
    Dana Dorsett | | #4

    Continuous rigid foam UNDER the joists is superior to installing it between joists under the subfloor. A 3/4" plywood or OSB subfloor is already a Class-II vapor retarder sufficient for managing interior moisture drives. High R/inch foam applied between framing is a waste of foam, since the R1.2/inch wood would be conducting almost as as much heat as the R6/inch foam.

    I assume by "ceiling" you are referring to the exposed cladding of the under side of the floor, exposed to the outdoor air?

    You can't nail the cladding to the foam since it has really lousy nail retention, but you can nail THROUGH the foam into something structural, which can work for foam up to 1" thick. Beyond that the nail lengths would become awkward nailing up through t&g cedar + foam. With the 1x4 furring option through-screwing the furring to the joists with pancake head timber screws 24" o.c. should be enough to support the weight of the cedar without sagging too much. If flatness is important you can tighten that to 16" o.c. With only have 3/4" of wood depth to nail into you'll be better off using ring shank nails for long term retention (not that t&g cedar weighs a whole lot.)

    IRC 2015 code min R for floor insulation over crawlspaces and pier foundations in your climate zone is R19 between joists. Dense packing 9.5" of cellulose would give you R35, which at ~2x code is arguably already R-overkill even before adding a layer of continuous foam. Installing half-inch CDX or OSB directly to the bottom side of the joists would be easy to air seal, stands up to the pressure of dense-packing, offers superior drying capacity toward the interior, and is an easy to hit continuous nailing surface for any cladding. If the subfloor & band joists are well-caulked to for air tightness you might even consider less than dense packed fiberglass in there. (Dense-packed fiberglass starts at 1.8lbs, and 2.2lbs density is pretty common.) At just 1.4lbs density L77 or Optima gives you R36-39 in a 2x10 cavity (more than 2x code), and won't sag or settle. It's not as air-retardent as cellulose, but it's not bad.

    In your climate with cellulose you're probably looking at 2.8-3lbs minimum density to prevent settling in floor assembly over a pier foundation, 3.2lbs tops. It would take a bit longer to install than 1.4lbs fiberglass, not that it matters much. With just 225 square feet of floor area it's as much about setup and break down time and making the trip to the job site as it is about blowing time.

    With lower density non-dense-packed fiberglass you don't need the robustness against flex of CDX or OSB on the under side, but it would still need an air barrier. That COULD be an inch of polyiso (long-nailing the cladding top the joists) if you wanted to, but thermally breaking a 2x10 joist isn't nearly as critical at your +13F 99% outside design temp as it is in locations where the design temps are in negative double-digits.

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