We recently took down our wood paneling and are tempted to replace the current R-7 insulation while we have the walls open.

Currently hanging is fiberglass R-7 paper backed insulation that looks to be reading as 1.5in thickness.

Our home was built in 1965 and the walls are a true 1.5inx3.5in likewise cavity depth is 3.5in.

We are still thinking we should upgrade to at least R13 as R15 has been hard to find locally.

Does this sound out of line?


2 Answers 2


You'll find that the name for 2 by 4 lumber is an exaggeration of the actual size which is 1.5" x 3.5" as you get it from lumber store.

So the batting designed to go into a wall stick framed with 2 by 4 will fit perfectly fine in your wall assuming they used the same spacing.

  • Thank you. We have variable spacing because of our large windows sbut understand we'll need to cut it down
    – Tiara
    Commented Mar 20 at 19:28
  • Every home has "variable spacing". That's just as normal and common as 3-1/2" 2x4s, and you'll cut your insulation accordingly. You want R-13 or R-15 batts, but the latter tend to be much more expensive.
    – isherwood
    Commented Mar 20 at 19:53
  • 1
    A very lazy search at my local home derpo has unfaced r-15 at $0.94/sf and r-13 at $0.85/sf. So not nothing, but not much imho. (This might be a supply/demand quirk, as most of my municipalities are demanding r-15.) Commented Mar 20 at 23:48
  • R15 has been a bit tricky to to find in rolls but we have considered batts
    – Tiara
    Commented Mar 21 at 2:46

2x4 is a nominal size. It's the size of the rough sawn lumber at the mill before it gets kiln dried and then "surfaced" down to the actual dimensions, 1-1/2"x3-1/2".

The marketing material for Owens Corning fiberglass batts names the 3.5" dimension. I assume that any other brand does the same. The maximum marketed R-value that you'll find for fiberglass batts in (nominal) 2x4 wall cavities is R-15.

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