Is skimping on window gap insulation going to bite me later?

Shoving fiberglass into those gaps isn't going to do much for you besides stop the biggest drafts. Fiberglass is essentially a filter, not an air barrier. Since you've alredy spent the money on closed-cell spray foam for the rest of the house, you're leaving a massive thermal bridge and air leak right at the windows. You’ll almost certainly feel the cold air pulling trough those gaps when the wind picks up this winter. The concern about removal is actualy a bit of a non-issue. If you use **low-expansion window and door foam** (usually the blue cans), it doesn't bond nearly as aggressively as the high-expansion stuff or the structural closed-cell foam used in your walls. When you finally go to swap those Andersons out, a simple handsaw or even a long serrated kitchen knife will zip right through it in seconds. It won't hold the window frame hostage or make the demo significantly harder. If you realy can't stand the idea of spray foam, at least use **backer rod**. It’s basically a foam noodle you jam into the gap to create a physical air seal, and then you can run a bead of sealant over it. But for a 1956 gut job, a dedicated foam gun with a pro-grade low-expansion canister is the way to go. It gives you way more control than the cheap plastic straws and makes a much cleaner bead.

Just make sure you don't overfill the cavity. You only need a bead about an inch deep in the center of the gap. That leaves enough room for the foam to expand without bowing the window jambs—wich is the real risk with using the wrong foam, not the removal process later. Leaving fiberglass in there is basically like wearing a heavy coat but leaving it unzipped.

I'm in a similar spot with an old fixer-upper and was actually looking into the backer rod option because I'm terrified of accidentally bowing my window frames. Does the backer rod really stop the drafts as well as the foam does? My gaps are super uneven—some spots are tiny and others are like an inch wide. Is there a specific size of backer rod that works best for those wierd varying gaps, or do you just double them up? I'm worried if I use the foam gun I'll still make a massive mess since I've never used one of those pro-style canisters before.

For those uneven gaps, you realy need a variety pack of backer rod sizes. I usally keep 3/8", 1/2", and 1" rolls in my kit. The rule of thumb is to use a rod that’s about 25% wider than the gap so it stays snug. If it’s loose, it’s not doing its job. Also, remember the rod is just the filler. You need to run a bead of acoustic sealant or a high-quality caulk over it to get a true air seal. Without the sealant, the backer rod is just a foam noodle sitting in a hole. It won't stop the micro-drafts that drive your heating bill up in January.

I honestly wouldn't risk the fiberglass. Since you're dealing with a 1956 house, those old rough openings can be moisture magnets. If that pink stuff gets damp from condensation—which it will, since it's not an air seal—it just holds that water against your framing. You coud end up with rot before you even have the budget for the new windows.

If you're worried about the mess, grab a cheap foam gun and some pro-grade window foam. It's way easier to control than the straw cans and literally peels right off the wood when you're ready to demo. It’s way better than finding moldy insulation later.