I need help.

I'm installing a bunch of Andersen windows on an extension. I want to use Owens Corning foamboard over the sheathing. The plans call for R-7, I'll probably go for 3/4" maybe up to 2". I've been trying to figure out the best way to install windows with the foamboard. I'de like to laydown the board, then Tyek and install windws over that and nail through the boad into the sheathng with long nails. Can it be done this way? If so, does it affect the installation procedure at all?

Or, do I have to shim out the sheathing same depth as foamboard for a nailing surface. If so, any special instructions on doing this?

Any help would be appreciated. Can't find any information on this topic. Even tried calling Andersen, they said they don't have an answer.

Thanks.

 

Anderson doesn't have an answer??? Interesting. Maybe you just need to keep calling until you find someone who can give you the answer. Surely they have guidelines... or maybe they don't?

All I can say is that installing windows on top of foam is asking for trouble. Is the foam going to be 100% watertight? Meaning, no water from any seam anywhere will get behind the foam? When you install windows on top of foam, with no building paper to flash the window to, you're just asking for a leak, in my opinion. But some builders do it all the time, so who am I to argue? In my professional opinion, building paper should be installed over foam to ensure the sheathing (your foam- and thus the top of your windows...) is not exposed to water. I don't think you'll find that in any code book, and I don't think you'll find too many contractors doing it that way, but it makes sense to me.

At any rate, back to your question. The other problem with installing windows onto the foam is that you may run into problems installing your windows as you try to keep them exact with your wall thickness. If the foam compresses on one side, but you're hitting cap nails on the other side, then youre going to have trouble when you go to put on your extension jambs. 4" on one corner, 4 1/8" on the other, 4 1/4" on the other, 3 7/8" on the next.... that sort of thing. Of course that can happen on any wall, but since foam isn't solid, it's a little more likely to occur. Framing the edges of each rough opening with a 2x4 would be a good idea, since it would serve 3 purposes. It will make your wall thickness solid and hopefully even all the way around. It also gives you something soild to nail (and caulk) the windows to- otherwise the window is basically HANGING on 2" of roofing nail. (how secure is that???) And then later, if you need to put on j-channel or window trim, you'll have something solid to nail the trim to. If you planned on caulking the windows to the foam, you would either have to use latex caulk or PL300, neither of which are probably approved for the elasticity and adherance required for vinyl nailing fins... not sure about that. Don't forget that most oil based caulk will EAT foamboard. Caulking the windows to the 2x4 on the other hand, does not present a problem.

Either way you do it, I'd highly recommend that you use peel and stick membranes to seal the nailing fin of the window to the foam- added insurance against leaks. Adding the 2x4 around each opening is what I'd do. Then again, i'd also paper it. I'm going to get a reputation for being fanatical before long. Or is it too late?

 

Thanks XSleeper. That was exactly the type of advice I was looking for, and it makes a lot of sense.

I assume that if I Tyvek over the foam and 2x4's that would meet your suggestion for building paper?

By the way, surprisingly, Andersen didn't seem to have any answer. I called both their Andersen Direct number and the number for builders/design professional. I'm wondering if they're staying silent on the foamboard issue on purpose. If they have guidance and something goes wrong (heaven forbid moisture problems), maybe that would make them partially liable(?)

Thanks again.

 

Yes. Before you install the windows into the rough opening, you'll want to cut a head flap in the tyvek, temporarily tape it up out of the way w/ a small piece of tape (so that you can run your bead of caulk along the sides and top) then install the window plumb, level and square, apply peel and stick to the sides and top and then fold that flap down on top, cut it to the correct length and skip tape it with contractor's tape.

I think you've got Anderson pegged. But its silly they don't have an approved installation practice. I suppose they figure that if they leak, it's the contractor's problem, or whoever put them in.

If you are really worried about moisture behind the foam board, as an option, it wouldn't hurt to install lathe on the sheathing, on top of your studs, to provide a small airspace / drainage plane. A continuous strip of aluminum screening along the bottom would help keep the bugs out. Before you side, you'll probably want to cover the bottom edges of the foam with trim coil.

 

Thanks again for some much needed advice.