Getting precise measurements for weird-shaped windows (arches, triangles, etc.)

I get what you mean about foam board being easy, but I actually had a different experience with corrugated plastic—mine warped a bit in the summer heat, especially on south-facing windows. Maybe it was just the cheap stuff I grabbed? I’ve gone back to doubled-up cardboard for quick jobs, even though it’s not as fancy. Sometimes the low-tech route just feels more forgiving when you mess up a cut.

Yeah, I’ve noticed the same with corrugated plastic—mine bowed out after just one summer on a west-facing window. Think it was the combo of cheap material and direct sun, honestly. Cardboard’s not exactly glamorous but it’s forgiving when you’re learning, and it’s easy to redo if you mess up a cut.

For weird-shaped windows, here’s what worked for me: I taped a big sheet of kraft paper over the window and traced the outline with a pencil. Then I cut out that shape and used it as a template for the cardboard. It’s not super precise, but it gets close enough that you don’t have big gaps. Sometimes I’ll trim a little extra once it’s in place.

One thing I’m still figuring out is how to get a really clean edge around arches—my cuts always look kind of jagged. Maybe there’s a trick with a sharper blade or something? Anyway, low-tech has its perks... especially when you’re just trying to block out the sun without spending much.

I get the appeal of cardboard for testing, but honestly, I’ve had better luck using foam board for weird shapes. It’s sturdier, cuts cleaner, and doesn’t warp as fast. Have you tried scoring your arches with a utility knife and then snapping off the excess? That usually keeps the edges smoother for me. The paper template idea works, but sometimes I just measure a bunch of points and connect the dots—takes longer, but feels more precise in the end.

I get why foam board’s popular, but have you ever tried using thin plywood for templates? It’s a bit more work to cut, but it holds its shape way better than cardboard or foam, especially if you need to check the fit multiple times. I find the “connect-the-dots” method a little tedious, honestly—maybe I’m just impatient, but tracing with a flexible ruler saves me some headaches. Anyone else think foam board can crumble at the edges if you’re not careful?

I get what you’re saying about foam board flaking apart, especially if you’re handling it a bunch. It’s cheap and easy to cut, but I’ve had the edges get all mashed up or start crumbling after a few test fits. That said, I’m not totally sold on thin plywood either. Last time I tried using 1/8" ply for a window arch, it warped just from sitting in my garage for a couple days—humidity here is no joke. Plus, cutting tight curves with a jigsaw isn’t always as clean as I’d like. You end up sanding forever.

Cardboard is flimsy, sure, but for weird shapes where you need to tweak the fit over and over, it’s hard to beat how quickly you can adjust it. I usually double up cereal boxes or use something like a pizza box so it’s a bit sturdier. Once I’m happy with the shape, then I’ll transfer that to whatever material I’m actually using—plywood, MDF, whatever.

The connect-the-dots method drives me nuts too, but sometimes those flexible rulers just don’t want to hold the curve right and you end up with a wavy line anyway. I’ve had luck with that contour gauge tool (the one with all the sliding pins) for copying weird profiles—just press it against the trim or wall and trace that onto your template. Not perfect for big windows but handy for tricky corners.

Foam board has its place if you’re careful and don’t mind replacing your template once or twice. But yeah, nothing’s perfect—seems like every material has its own little headache attached.

You nailed it—every material has its own way of making you want to pull your hair out. I’ve done the foam board shuffle more times than I care to admit. It’s like, you get the template just right, and then by the third test fit, it’s shedding little white crumbs everywhere and the corners look like a mouse chewed on them. But hey, it’s cheap, and sometimes that’s the deciding factor when you’re just trying to get something roughed in.

I hear you on the plywood too. I live in a spot where the humidity makes everything curl up like a potato chip if you leave it in the garage for more than a weekend. Tried to use 1/8" ply for a curved window sash once, and by the time I got around to installing it, it looked like I’d tried to make a boomerang instead. And don’t even get me started on sanding... I swear, I’ve spent more time sanding than actually building anything some weeks.

Cardboard is underrated, honestly. I’ve used everything from cereal boxes to Amazon packaging. My wife thinks I’m running a recycling center out of the basement. But you’re right, for dialing in a weird curve or angle, nothing beats being able to just snip off a sliver or tape on another chunk until it fits. Then you can go to town on the “real” material with a little more confidence.

Those contour gauges are neat for tight spots, but I always end up with pins getting stuck or the thing not quite matching up on bigger profiles. Still, it’s better than eyeballing it and hoping for the best. Sometimes I’ll just tape a bunch of pencils together and try to trace the curve that way—looks ridiculous but it gets the job done.

Honestly, half the battle is just accepting that there’s no perfect way to do it. You find what works for your sanity and your budget, and call it a win when you don’t have to redo the whole thing. If only windows would come in normal shapes... but where’s the fun in that?

- That “mouse chewed on them” look with foam board is way too familiar.
- Plywood warping is wild—humidity here turns my scraps into modern art.
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Cardboard is underrated, honestly.

That contour gauge thing—yeah, I’ve had it jam up too, especially on thicker trim. Sometimes I just trace the outline with stiff paper or even cereal box cardboard, then transfer that to my insulation board. Not perfect, but it saves a lot of cleanup. Don’t give up—these odd windows are tricky for everyone.

Sometimes I just trace the outline with stiff paper or even cereal box cardboard, then transfer that to my insulation board. Not perfect, but it saves a lot of cleanup.

Yeah, I’ve done the cardboard trick too—honestly, sometimes it ends up working better than those fancy gauges, especially when you’re dealing with old, uneven trim. One thing I’ve found helpful is using painter’s tape to hold the template in place while you trace, so it doesn’t shift around and mess up the outline. It’s not foolproof, but it definitely cuts down on the frustration. Those weird angles and arches never seem to match up with anything off the shelf... I guess that’s part of the fun (or headache) of older houses.

Funny how the simplest methods end up being the most reliable sometimes. I’ve tried those contour gauges too, and honestly, they just don’t play nice with wavy old trim or those weird little bumps you get in plaster. Cardboard templates have saved me a ton of headaches, especially when I was fitting a new window insert into an arch that was anything but symmetrical.

Painter’s tape is a solid move—I’ve used masking tape in a pinch, but it can peel up fibers if you’re not careful. One thing I started doing is tracing the outline, then flipping the template over to double-check for symmetry before cutting anything expensive. Not foolproof, but it’s caught a few mistakes.

I’ve also heard of folks using kraft paper or even freezer paper since it’s a bit more flexible than cardboard and can hug the curves better. Never tried it myself, but might be worth a shot next time I’m wrestling with one of these “unique” windows...