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Insulation options for Climate Zone 5

PeteChuk | Posted in Energy Efficiency and Durability on

I’m looking for input on an insulation question with a new house build. Im a contractor in Upstate NY / Zone 5 in the Hudson Valley. 3000 SQ ft single level modern home. We are maybe 75% done framing it now.

For context, the plan shows closed cell foam 6.5″ / R-49 directly applied to the underside of the roof sheathing (5/8 Zip). Low pitch roofs with rubber roofing.

The walls are 2×6 16″ oc with 1/2″ Zip, 3″ of closed cell foam in cavity, and 2″ of Rigid Foam directly applied to the outside of the 1/2″ Zip…. then 3/4″ furring strips, and vertical metal cladding.

The floors are the main question– maybe a third of the house (one room 28′ x 28′ and a 16′ x 16′ room both approx.) sits over concrete footers and 8″ x 8″ steel columns rather than a conventional foundation. The floor deck, which consists of flush (4) 9.5″ LVL beams (around the perimeter, and one down the middle of the floor joist span on the bigger room) and 12″ floor trusses, sits roughly 4′ above grade.

The plans call for 5″ of closed cell foam to be applied in the floor joist cavities.
I am attempting to advocate instead for using Fiberglass or Rock Wool in the cavities , and then use 2″ + 7/16″ Zip R sheathing, seams taped.. underneath both rooms’ floor systems. I feel like it will result in a better overall floor.

The main concern is airtightness, overall R Value, and something that works for our area. What do you guys think?
Dropping the beams below the joists would have changed the look for the worse and was never really discussed, before anyone asks…

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Replies

  1. Expert Member
    Michael Maines | | #1

    What do you mean by "floor trusses?" If they are wood open-web joists, batt insulation won't work well due to air leaks around the truss webs. Fibrous blown insulation would work well if one side of each joists were netted to contain the insulation. If you mean I-joists, batts would be fine, just get ones that are full width, not the typical ones cut down 1.5" to allow for framing.

    In either case, with 12" joists, filling them with fibrous insulation would result in very little thermal bridging and roughly R-40 insulation, so there is no need to add a rigid foam layer (in the form of Zip-R)--the ROI would be miniscule. Closed cell foam alone would give you roughly R-30.

    What kind of rigid foam are you planning for the exterior of your walls? Assuming XPS, which ages to about R-4.3/in, your walls will perform at roughly R-25.5. That's quite good for your zone. They would perform just as well if you were to install 5.5" fiberglass batts in the bays. Better, actually, because there would be no danger of trapping moisture at the sheathing, and the ROI would almost certainly be better. I'd use dense-packed cellulose for various reasons but that costs more than fiberglass batts, and with exterior insulation, fiberglass batts would work well.

    Your roof won't really be R-49 for more than a short time after the foam leaves the spray gun; it MAY be R7.54/in initially--that's a stretch, because testing is done under highly controlled conditions--but will degrade over time as the gaseous blowing agent is displaced by air, eventually leaving you with about R-5.6/in, or R-36.4. Compared to R-30 to R-40 in your floor and R-25+ walls, having only R-36 in your roof is not proportional. But if the house is airtight, with above-code levels of insulation elsewhere, skimping on roof insulation is probably fine. You could add some fiberglass batts to increase the R-value there.

    1. PeteChuk | | #2

      Thank you very much. Extremely helpful. I had a similar feeling about the wall assembly, that it doesn’t make sense to use both closed cell and 2” xps. The floor trusses are the “K” truss style ie. A 2x4 on the flat for your nailing surface

  2. PeteChuk | | #3

    Do you have any ideas for the walls? The homeowner is really interested in the spray foam. I’ll grant that it will help with some rigidity. But if we had to do the 3” of spf, should we tape the seams of the 2” xps? No tape? The moisture is my main concern . Thanks

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