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Best way to close gap between f/g insulation batts and flooring?

Benergy | Posted in Energy Efficiency and Durability on

My sister’s house has a 1996 addition done in 2×6 framing with 8″ I-joist for the flooring. Giving the builder the benefit of the doubt (cough), I suppose they didn’t know not to do this, but they installed the 6″ f/b insulation batts by simply nailing furring strips to the underside of the I-joists and laying the batts onto those strips within the respective joist gaps. That choice leaves ~2″ gap between the insulation and the underside of the subfloor, effectively eviscerating the insulation’s effectiveness.

I’m wondering what’s the best and/or easiest way to raise up the batts. I suppose I could staple a strap to the sides of the I-joists, about 2″ up from the bottom edge, and run that strap diagonally down each joist cavity—holding up the batts in the process. Or I could install those batt hanging wire pins to span the gap. I want to avoid removing the insulation as a step (it’s in good shape), unless it’s really the most effective way forward. I’m just not looking forward to a day spent suited up in the crawlspace.

Thoughts and comments welcome!

Ben in OR

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  1. Expert Member


    Apart from the amount of insulation in the floor I'm not sure there is much wrong with it. Joe Lstiburek advocated for a gap between the insulation and sub-floor to keep the floor closer to the temperature of the rest of the house. The batts are only ineffective if the perimeter is not insulated above the batts, and they are not protected by an air-barrier below.

  2. gusfhb | | #2

    In theory I am sure it is fine
    In practice, I am not so sure it is fine
    I had a house over an open crawl space with fiberglass over an inch of foam
    Routinely freezing
    Because fiberglass always has gaps
    If it is hard against the floor, the air has interference
    With a gap, it has access to the air in the space which is soon equal to the outside temperature
    I have lived it, don't do it
    Imagine this, 2 joist bays with a pair of 1 inch holes 'bored' through the batts at diagonal ends.
    In one the batts are tight against the floor
    In the other they are an inch or more away from the floor

    Which would you rather have under foot?
    An have you ever seen a batt installation with merely a pair of 1 inch holes?

  3. gusfhb | | #3

    And squirrels

  4. freyr_design | | #4

    Twine and staples like a web. Image search insulation twine

    1. Benergy | | #6

      Yes, this is looking like a candidate. Just not as easy as running the strapping along the exposed bottom edges of the I-beams, as would be typically done. I have to staple up 2" from the bottom edges, on the sidewalls of the I-beams (to close the gap), while holding the insulation batts out of the way as I do it. It will be a slow slog, whether I using strapping or wire pins across the width of each cavity. Ugh.

      1. Benergy | | #7

        Having to fix someone else's sloppy work makes me cross!

        1. freyr_design | | #9

          You could also just call an insulation contractor to come add r-13 batts to the bottom. I bet it would be pretty cheap

  5. Benergy | | #5

    OP here.
    I should say that all my BPI (& otherwise) training states that any gap between the pressure boundary and thermal envelope (i.e., between the wall/floor and its insulation) is a problem, and in energy-auditing such a space we must de-rate the value of the insulation for that component.

    That, plus the fact that the homeowners (sis & hubby) say this home addition floor is considerably colder in the winter than the rest of the house (built 1968) which also has 6" (R19) batt insulation under the floor, in its own crawlspace, that was installed correctly, in contact with the underside of the subfloor. In both spaces the f/g batt insulation is unfaced (facing belongs in contact with the pressure boundary, if it exists). I know that f/g batting commonly suffers from sloppy installation—there are deration formulas for auditors to account for that kind of thing.

    So that gap's gotta go. Just trying to figure the best/easiest way to snug it up.

    Thanks, all!

    1. Expert Member
      MALCOLM TAYLOR | | #8


      The effect of wind-washing and convective currents on permeable insulation is shown in tables 8 & 9 of this link.

      I think your efforts would be better spent on installing a good bottom side air-barrier - preferably foam - which would both increase the R-value and reduce thermal bridging.

      Here is what Martin says about these floors:
      "“The insulation that is doing most of the work in this assembly is the continuous layer of rigid foam under the joists. If you want a warmer floor, beef up the thickness of the rigid foam — and do an impeccable job of air sealing at the perimeter of the floor assembly (the rim joist area). Then it hardly matters how much fluffy stuff you put between the floor joists. You can put in a little (the Lstiburek approach) or a lot (my way).”

      1. freyr_design | | #10

        If you can afford it I would do this. If you value time it will probably be less than you’d think

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