I’ve definitely run into this with my 1970s split-level. The first time I tried to install an aluminum window, I was convinced the opening was close enough to square, but once the frame went in... nope. It wanted to twist just a hair, and when I tried to force it flush with shims, the sliding sash started sticking. I ended up pulling it back out and re-shimming less aggressively, but then there was a gap on one side that needed extra trim.
I’ve noticed aluminum doesn’t really “forgive” like wood does—if you try to muscle it into place, it’ll flex or bow and then you’re stuck with a weird bind or gaps that never quite seal right. Is there a trick to getting the frame plumb without over-shimming? I’ve wondered if leaving a little play and letting the trim do more of the work is better long-term, but then you risk drafts. Anyone have luck with those expanding foam tapes instead of caulk? I’m always worried about overdoing it and making things worse down the line.
I get where you’re coming from, but honestly, I think people overthink the shimming with aluminum. The trick isn’t to let the trim hide the problem—it’s to get the rough opening as close to square as possible before you even start. If you fudge it with shims or foam, you’re just masking a bigger issue, and that’s where you get drafts or sticky sashes down the road. I’ve tried those expanding foam tapes and, yeah, they can fill gaps, but they don’t really solve the core problem if your frame’s torqued. In my experience, it’s worth taking the time to fix the opening itself—even if it means reframing a bit—rather than relying on shims or extra trim. Aluminum just doesn’t have the give for shortcuts. Wood’s way more forgiving, but if you’re set on aluminum, precision is everything.