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What are the best fiberglass batts for 2×4 true dimensional 4 inch deep Old House Lumber? Double R-8, R-15, R-19 Unfaced?

Bryce Nesbitt | Posted in Energy Efficiency and Durability on

I’m referencing Owens-Corning Pub. No. 10017857-A “Building Insulation Compressed R-value Chart” and NAIMA “Estimated R-values for Compressed Fiber Glass Batt Insulation”

I have some small sections of true 4×6 lathe and plaster walls will be opened from the inside.  There’s no chance to adjust the dodgy tar paper or stucco outside.  Inside these stud walls are actually 4×6, rough cut old growth Douglass Fir studs from the 1930’s.  They’re actually 4″ deep.

What’s the best insulation approach, and are there published test results?

How about two layers of R-8 compressed to 4″?  Or a layer of something else vapor permeable, plus R-13 unfaced? Has anyone measured this?

I am aware that insulating old walls like this has risks ( “Just blowing insulation into the walls can create moisture problems that will rot the wood structure from the inside out.” ).

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  1. Charlie Sullivan | | #1

    As those charts you are looking at tell you, you at least want to stuff R19 in there and get R14 out of it, and you would do even a tad better to stuff R-20 or R-21 in. But not much more so it might not be worth it.

  2. Expert Member
    Akos | | #2

    Dodgy WRB and stucco is a recipe for trouble. I would treat the walls if it had no sheathing and insulate with an air gap on the exterior:

    For working with older walls with real 2" studs, I find the easiest is to get 24" wide batts, turn them sideways and cut them to fit the stud spacing. This is much easier and less waste than trying to trim an inch off the side. Mineral wool or HD fiberglass batts work the best for this.

  3. Bryce Nesbitt | | #3

    Note I've done fairly well with R-15 HD batts in a pinch in such walls. It seems to fluff up enough to fill. Good tip on the 24" wide batts sideways, I'd forgotten about that.

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