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Batt insulation

user-1129453 | Posted in Energy Efficiency and Durability on

A while back I read an article that was talking about how batt insulation is not an air barrier. Air can just flow right through it lowering the R-vaule to almost nothing. A fact that I was already aware of. I got to thinking about underneath my own house, a vented crawlspace, I have insulation in between my joists that is exposed to air, without an air barrier on the underside of it. Is the R-19 batt insulation I have under my house preforming at a much lower value because the air can get through it? Please forgive my ignorance. This is so common Im sure there is a reasonable explanation but it got me thinking. Thanks in advance!

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  1. Expert Member
    Dana Dorsett | | #1

    In a warm-side up configuration R19s do OK, since the denser cooler air on the cold side stays there, and doesn't create a convection force within the batt. In an air conditioned house where the floor is cooler than the air in the crawlspace it will convect a little, but it's rarely more than 30F warmer (the ASTM C518 tested delta-T) in the crawlspace than it is inside the house. With an air-tight air-barrier below the batts, and with air-sealed band joists to limit wind-washing infiltration, it'll pretty much be performing at R19, if it's at it's full manufactured loft which is ~6", not 5.5". At 5.5" compression (as in a 2x6 wall/rafter/joist) it's performance would be about R18 at a delta-T of 30F.

    Without the air barrier it is susceptible to wind-washing losses from below in a vented crawlspace, but in calm air it'll be pretty close to R19 performance during the winter, a bit less in the air-conditioned hottest part of the summer.

  2. jklingel | | #2

    And, without rigid foam on the bottom of the floor joists, is it not more likely to get condensation on the bottom edges of the joists, leading to mold/rot? Depending on how cold he is? ???

  3. user-1129453 | | #3

    Thanks for your answers. I live in North Carolina and this is all that I have ever seen. Vented crawlspaces with exposed insulation. Im not sure if its humidity concerns or cost issues but no contractors that I have seen have done anything to stop the wind washing on crawlspace insulation. I have done a lot of research on sealed and conditioned crawlspaces. I think after hearing this, it is an obvious solution. I just wonder why contractors haven't corrected this problem.

  4. GBA Editor
    Martin Holladay | | #4

    There are lots of reasons why exposed fiberglass batts make a poor choice to insulate a crawl space ceiling. Fiberglass batts are hard to secure in place and often fall down. Batts can get moldy in damp conditions. And, as you point out, any time that fiberglass is installed without an air barrier on both sides, it doesn't perform as well as other insulation products (like rigid foam).

    For more information on crawl space insulation, see Building an Unvented Crawl Space

    For more information on insulating floors, see: How to Insulate a Cold Floor

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