I totally get the appeal of leaving a bit of extra space and just dealing with shims and foam. My place is a 1920s bungalow, and the walls are basically a patchwork at this point—nothing is plumb or level anymore. I tried to go for a snug fit on one window because I read it helps with drafts, but all I got was a ton of stress and a frame that still isn’t totally square. Ended up with gaps anyway, plus a bunch of time spent fussing with the opening.
But here’s the thing: I’ve always wondered if those bigger gaps, even when filled with foam, make a big difference in energy efficiency compared to a precision fit. Like, does the foam actually insulate well enough, or are we just trading one problem for another? I know caulk and foam are rated pretty high for sealing, but sometimes I worry about long-term settling or shrinkage.
I guess in old houses you just have to balance sanity with efficiency. For me, as long as it’s sealed up tight and not leaking air, I’ll take an easier install over fighting ancient framing any day. Still makes me wonder if there’s a “best practice” for these situations, though...