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Discounting the R-value of fiberglass batts

Curtis Dean | Posted in Energy Efficiency and Durability on

I do energy assessments for our utility customers (Zone 6A). I have a question about a customer looking at an insulation retrofit. Her home was built in 1979 with 2×4 exterior walls. Using a probe, I was able to determine that the exterior walls have fiberglass batts. Thermal imaging did not reveal any major gaps or voids.

I know a perfect installation should achieve an R value of around 11-12 (assuming a 3.5 inch thick batt), but we all know that it is highly unlikely to be perfect. Can anyone give me a good number to use to discount for imperfect insulation? R10? R7?

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  1. GBA Editor
    Martin Holladay | | #1

    Two researchers at the Oak Ridge National Laboratory, Jeffrey Christian and Jan Kosny, have measured the whole-wall R-value of a 2x4 wall insulated with fiberglass batts; it's R-9.7.

    For more information, see "Calculating Whole Wall R-Values on the Net".

  2. Curtis Dean | | #2

    Thanks for the info, Martin. As always, very helpful.

    This customer is planning on having RetroFoam injected into the exterior walls to increase R value. Being a utility, I can't recommend particular products, but I do know that the customer is going to spend $3,000 to go from an R9.7 to maybe R15 and that's not very cost effective.

    Can dense pack cellulose be put into walls where batts already exist? My guess is no, but you guys know more than I do!

  3. GBA Editor
    Martin Holladay | | #3

    RetroFoam has an R-value of R-4.1 per inch, so the center of an insulated 2x4 bay will have an R-value of R-14.3. However, the whole-wall R-value of the RetroFoam-insulated wall will be less, because of thermal bridging through the studs -- perhaps R-11 or R-12, but I'm just guessing.

  4. Keith Gustafson | | #4

    >>>>but I do know that the customer is going to spend $3,000 to go from an R9.7 to maybe R15 and that's not very cost effective<<<<

    I don't know about the money, but do the math. Going from R10 to R15 is more cost effective than going from R15 to R22. IOW the lower the existing R value, the more important it is to increase it, even by a little.

  5. Robert Hronek | | #5

    Whether the wall is dense packed or retrofoamed the wall will become much tighter and will add to its efficiency. When we talk R value we seem to loose focus of other factors affecting performance.

    I would say that the effective R value of a wall is going to be highly affected by several things. When using batt insulation, the hardest insulation to install properly, you have to look at the quality of installation. If it is the so called econo batt that doesn't fill the cavity its R value will be very low. I have heard that 40% of stud cavities are not the full 16" width which complicates the issue of proper install. Then you have to factor in how air tight the cavity is. Other factors are things like soffits that don't include an air barrier over the insulation. Or porches that don't have an exterior air barrier above the ceiling of the porch.

    I think it would be safe to say that unless the home was built very recently that the walls perform much lower than the calculated R values of its components.

    A wall that is dense packed or foamed is going to perform much closer to the calculated R value of its components.

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