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Community and Q&A

Squishing batt insulation

cwoodford | Posted in General Questions on

I am renovating a house more than 100 years old. The exterior walls are not insulated but I do have full access to the cavity.

I was going to insulate with regular batt insulation but the stud cavity is actually 4″, (not 3-1/2″ like it would be for a newer home with modern lumber).

The batt insulation at any local store I have checked out only really sells exterior wall insulation in either 3-1/2″ or 5 1/2″ thicknesses.

My question is this: Is it better to install the 3-1/2″ insulation knowing that the wall won’t be completely filled with the insulation, or is it better to stuff the 5-1/2″ insulation into the cavity knowing the insulation may be squished and lose some of its R-value?


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  1. Expert Member
    Dana Dorsett | | #1

    It's absolutely better to make it a compression fit. An R19 batt is really an R13 batt stretched out to ~6", and only performance at R18 in a 5.5" deep cavity. Squishing it to 4.0" it would run ~R14-R15.

    Better yet, an R21 batt compressed to 4" would deliver ~R16-R17 due to the higher density. See:

    Do the arithmetic: Compressing it to a higher density lowers the total R but gives it a much higher R/inch.

    But depth is not the only problem- it has to go full-width as well without buckling or forming voids.

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