I need to use 1/2" plywood or OSB on both vertical sides of my retrofitted window frame. I was wondering if there is any difference in OSB (cheaper) vs plywood (more pricey) for this application considering that if somehow moisture got in (which I'm making sure it doesn't but who knows) and the material gets soaked and swells (like I think OSB is more likely to in moist conditions), it may squeeze the window on the sides too tight, causing it to break.

  • OSB should always been the last option when given an option. In this case, though, I'd suggest just using shims...either ply or composite, and fill the rest of the gap with foam insulation.
    – DA01
    Nov 10 '15 at 22:06
  • Too big gap for shims
    – amphibient
    Nov 10 '15 at 22:06
  • I meant use 1/2" plywood shims.
    – DA01
    Nov 10 '15 at 22:10
  • also, re: moisture, be sure to wrap the entire frame with stick-and-peel membrane before putting the window in.
    – DA01
    Nov 10 '15 at 22:11

You are correct about the swelling. If it is just padding and will not get wet, it will do just fine.

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