Rough opening size ?

[rustytoolss]

Got a question about rough cut opening size for windows etc, when using canvas on foam. If your window needs a rough opening of 24"x24". Did you allow more room for the canvas ? If so how much extra ?... Or did you go for a real tight fit 24x24 ?

[KCStudly]

RO generally allows the proper amount of extra clearance for an easy fit. I would not plan on leaving extra. For just one layer of canvas (each side) I would not bother leaving any extra as there should be plenty of tolerance for the window to still fit and I would rather miss a little on the "less clearance side" (not snug) than on the loose/gaps showing side.

Best to have the window in hand first to make sure you have accounted for the correct radius, if any, in the corners. Double check your clamp/trim flange against the window frame, too. IIRC, some people have been caught out by using the trim ring as a template and then found the window doesn't fit.

[ghcoe]

I used the inside trim ring as a template for my window cutouts, after verifying that they were dimensionally the same as the window frame. The cutouts were less than perfect with the saw blade deflection. Sanding helped but, still had a real tight fit in some areas and nearly too loose fit in other locations. Luckily the outside flange and inside ring managed to cover the imperfections. My canvas wrapped into the window locations. Since the fit was a bit tight the foam did give some to allow the install. Saw blade deflection is a pain to deal with on the 2" foam. I have not tried one yet, but I wonder if a electric carving knife might work better for foam cutouts. Good luck.

[rustytoolss]

I tried a electric carving knife (used). Did not work to well. As I stripped the gears in it. The 2" foam was more that the knife could handle. But to be fair, the blades were dull. I should have sharpened them before pushing the knife to the point of destroying it.

[ghcoe]

Ok, so the next step maybe, in my thinking, is to use a rough cut saw blade. Something where the blades splay out from the shaft (if that makes sense). In thick foam it seems the fine and medium cut blades can not clean the pathway out enough and begin to walk excessively on the far side cut. A rough cut blade with a heavy shaft I think will work OK in clearing out the pathway and keep walking down to a minimum. The problem is that they may not make jig saw blades that are long enough to go through 2" foam. I do think that sawzall blades can be converted to be used in a jig saw. This is just a few ideas I have running though my mind, but have not tested yet.

[KCStudly]

Windows aren't that big. With a little practice and a straight edge you can make a pretty decent plumb cut with a thin straight edged steak knife. I find a low angle drawing stroke yields a better result than an up and down sawing motion.

[ghcoe]

I was just at the store and saw that there are jig saw blades in the 3-4 inch range. I guess that I was thinking of when I cutting two 2" pieces of foam. Also there are jig saws that reciprocate and it is suppose to make it easier to cut through softer material.

[GPW]

Drywall saw works well for cutting window holes in 2” ...

[QueticoBill]

Why wouldn't you over cut and use spray foam?

[KCStudly]

Spray foam fills gaps and is good for rough build up, but is not as easy to carve or sand neatly. Also, I find that only the outside crust has a fine bubble structure, while the inside usually has large void bubbles, at least in the relatively humid climate that I use it in. I'm told if you knock it down first and stir it after dispensing, then apply, that it doesn't foam up as much, but in my testing it didn't make enough of a difference to bother with the mess.

Frankly, I'm in the camp that would build a wooden frame and glue that in to a larger RO; that way the clamp (and/or screws) has something solid to compress against.

[GPW]

It’s pretty easy to “leave the line” when cutting foam , and equally easy to take a sanding block or rasp and true it up to an almost exact fit ( just like wood eh ? ) ... We glued our windows in with that 3M marine sealer , canvas outside skin was already on , so when we mounted the window and inner frame , it did compress the surrounding outer surface slightly , but not actually much as we thought ... been 2+ years now outside ... still no leaks , no problems .

Wood frames are nice and give you More Strength ... I’ve been thinkingv of an outer wooden frame hybrid framing , a thinner wood ( considering the new PT clear and dry we recently got ... NICE!!! ) ... that would be inset into the surrounding foam ... ? A wooden window “liner” so to speak ...

That’s the trouble with these darn Foamies , too many ways to do the same thing , so we have to choose which one works for us ...

[lthomas987]

I traced my window and then made my cuts with a jig saw. Because my windows were used I didn't have inner trim rings. So on the inside I inlet for a 3/4" x 1-1/2" frame that I made to fit around the window's inner rubber flange using a hot wire cutter made from a 100W soldering gun.

I test fit everything several times before canvas, and after canvas, and then after primer and paint things were too tight and I ended up having to beat the heck out of my aluminum frames with a mallet to get everything in, despite everything fitting pretty much great before that.

So while everything was pretty snug to begin with I took a few passes with a surform plane and opened it up a bit. I am pretty happy with how it all went together in the end but getting the final window install in place was a total pain that could have been solved by havivng an extra 1/16" inch.

In the end I'd say, cut a bit small, then test fit a few times with a layer of your canvas on all 4 edges. Can even be just scraps but especially near corners where you might get overlapping layers. It's really easy to plane/sand off a bit of foam, when you think you're right lay it all up again with a couple extra sheets of paper in there. I also had door problems where the door worked fine until I painted. So possibly I just had all my clearances too tight.