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Highest R-value for under floor in attic area above ceilings

us-scott | Posted in Green Products and Materials on

Is there a product that could be poured into holes drilled in floor with high R factor? I have looked at spray in foam, but it has to be installed in perfect temperature and is very expensive.

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  1. GBA Editor
    Martin Holladay | | #1

    You have a choice. You can choose the product with the highest R-value per inch -- which in this case means closed-cell spray polyurethane foam -- or you can choose a product that is not "very expensive." It's your choice.

    A less expensive product like dense-packed cellulose is easier to install through drilled holes in a closed cavity than spray foam.

    Closed-cell spray foam will give you about R-6.5 per inch, while cellulose will give you about R-3.5 or R-3.7 per inch.

  2. Expert Member
    Dana Dorsett | | #2

    Mind you, in VERY cold climates (say, US climate zone 7+, or even the cold-edge of zone 6) filling the joist bays with insulation leads to striping of extra-cold floor along the joist edges. If you filled the joist bays by only half the coolth gets spread more evenly since the air adjacent to the joists above the insulation are pre-heated via the sides, by nearly room temp air above the insulation. If you blow cellulose in a 1.5lbs density rather than dense packing the striping will be noticeable the first few years, but by the time it has settled an inch it will be much more subdued.

    Or, if you're in a warmer climate zone the striping would only be significant when the temperatures below the floor was 25F or cooler.

    From an energy use point of view there isn't much point to going with a high R/inch insulation, since the floor's average performance is undercut severely by the very thermal bridging that creates the cold-edge striping. If it's possible to install rigid foam on the underside, where it can thermally break the joists, the average performance goes up (and the cool-striping goes down.)

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