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What is a good R-factor estimate for a wall assembly with old UFFI foam from the 1970s?

TimL_in_ND | Posted in General Questions on

I’m doing some modeling to guide a DER of a 1900’s house. Bore-and-fill urea/formaldehyde insulation was added in the 70’s. It’s shrunk a bit and is quite fragile as expected. I’m looking for a realistic R value to use for this assembly. The wall is 2×4, plaster and lathe on the inside, 2 layers of siding on the outside. My guess is that the old foam has an R of 12 total (3″ at R4 per inch). Any inputs on what R to use for this assembly?



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  1. LukeSkaff | | #1

    It has an R-value of 4.6 per inch in good condition the problem is many old installs will be very cracked as the wall has expanded, contracted, and shifted over time which lowers the R value.

  2. GBA Editor
    Martin Holladay | | #2

    Why do you need to estimate the wall assembly's R-value? If the main purpose of the estimate is to get a handle on your future energy costs, I would go for a low estimate. As Luke said, the old insulation is likely to be cracked and deteriorated. There is thermal bridging through the studs. Maybe you can call the 2x4 assembly R-10. Just a wild-ass guess, but it's better to low-ball it than overestimate its performance.

    If this is a deep energy retrofit, plan to add significant R-value to this crummy old wall, and don't assume that the old UFFI is doing much of anything to help you.

  3. TimL_in_ND | | #3

    Thanks Martin, Luke.
    That was my gut feel as well.

    I've been using BEopt as a tool in hopes it can guide selection of the 'sweet-spot' for exterior insulation of the wall assembly; I needed the existing wall R-factor to input to BEopt.

    Trying to decide between 2 layers of foam board and doing spray foam. Any opinion on how thick to go for each of these options? I'll post this question in another thread to avoid having a confusing subject line for this answer.


  4. STEPHEN SHEEHY | | #4

    UFFI insulation is a big problem for many people trying to sell a house. In many states, you are required to disclose it. The feds banned its use in homes in the 80's. If you have the walls open anyway, I'd remove it completely, even though the formaldehyde has almost certainly dissipated by now.

  5. Expert Member
    Michael Maines | | #5

    I've worked on a few renovations that had UFFI. In every case, there were a lot of gaps where the foam met the framing. To be on the safe side, I would estimate at least a 50% reduction in theoretical R-value, or about R-8 (hr•ft²F/Btu) in your case.

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