So I plan to install my windows as INNIE's, utilizing the tyvek against the sheathing with flexible flashing at the bottom and then straight flashing for sides and head then installing the exterior rigid foam.  I have seen in several videos that guys are using metal drip headers installed under the tyvek as well as metal window sills.  Since I am only installing 1.5 inches of foam is there a reason to use the metal flashing to creat a window box like that?  I want to make sure i'm not creating problems by running the water under my siding or creating a probelm with the drainiage plane.  Please show me how it should be done if I am missing something.

Innie details start on page 14.

Most can do just fine with flexible flashing, properly lapped. Be sure to give it a bit of slope under the window to direct water toward the exterior.

Using a crinkle-type housewrap between the foam & sheathing is recommended, which offers both a path for bulk water to drain via gravity and a better capillary break between layers. Don't sweat the miniscule amount of thermal by pass convection of that micro-cavity would allow.

Thanks Dana. I was thinking about using the horseshoe shaped shims I have seen a guy using that posts a lot on the greenbuilding advisor website. That will give some pitch and also sets the window up a little higher in the box. I was considering using the commercial tyvek because of the crinkle face it has for the same reason you mentioned. I was going to use the flex
Flashing that tyvek makes for the sill, then straight flashing for the sides and the head.
So does the siding j channel finish proud enough to the window that there is no need for an extended widow box or framing of any kind with the 1.5 inches of foam???

I am seeing some options for the flashing detail online and on greenbuilding advisor. Looks like some guys are installing the windows after the picture frame the window box with 1x material. In my case since I am using 1.5 inch foam I would need to use a 2x ripped down to achieve the same deepened window box. I am concerned that all they are doing is creating a thermal bridge by doing that. Essentially they have extended the stud all the way outside the envelope and stopped the foam. Is that going to be an issue and make my windows cold? I am considering doing 3/8 plywood inside the window box but that seems like a lot of flashing material to ensure that plywood is protected. I am beginning to wonder if the 2x material around the window box would be the best way? Any experience with this?

Yes, use a heavier plywood box; I'd suggest 5/8" or 3/4" Advantech, nailed to the framing, taped at the seams. Fasten the windows with "masonry clips" which you can order with the windows, through the box into the framing. (You can make your own, too if you forget). There is no good reason to use 2xs so bring the foam to the window box and tape the plywood>foam joint with Siga. Be sure to extend the box out to the face of the strapping. Minimizes the thermal bridge.

So I am really undecided on the mounting of my windows. Seems that I could just as easily mount the windows on top of the 1.5" rigid foam because that has been acceptable for years. Trouble is, if I want my water barrier to be under the foam, what do I use to flash the window and what protects the foam from moisture other than the 3/4 of an inch I am creating for the air movement??? I am using White windows and white siding so I really don't see a reason to install lineals because they won't accent anything.

the foam doesn't have to be "protected from moisture", so that isn't an issue. polyiso cannot be saturated, so it isn't used in the ground, but on a vertical surface it'll be fine. The rainscreen should the placement for your water barrier. you can make the water barrier under the foam if you want, but it makes flashing that much more complicated, and this is all complicated enough as it is.

Where/why/how would water (not moisture, but water) get behind the foam? If it is under an overhang, properly flashed, the joints taped, possibly covered by a fabric, there should be no way for water to get behind the foam, any more than it would get behind the sheathing. If there is a path for water, fix it now.

I agree with Bob. Why wait for the moisture to penetrate the foam? Stop it at the back side of your air gap and tie it to all flashings.

Ok I see, so what I'm hearing also is that it would probably be better and less complicated to flash the window if it were mounted outside the foam as well. Since I am only using 1.5 inch foam I could easily mount the window on top of the foam. I do however have some pretty large windows, one of which is almost 12 ft across. Would it be held just as well mounted through the foam as it would be under it on the sheathing?

Hi, I know this post is four years old, but I was wondering if you would care to share how you ended up installing your "INNIE" windows? Looks like I am in just the same spot as you were: non-flanged, uPVC windows, some 12ft wide, sitting in a plywood box with house wrap and 1 1/2 inches rigid foam continuous insulation ontop. Did you pull the rigid foam all the way to the inside, and mount the window over it? How did you flash the foam? Did you pull the WRB over it or under it? And, my most pressing question, did you build trim around the window box outside, to protect the rigid foam? Or did you find another solution for that? Thank you!

MikeE: This has been covered fairly thoroughly in a few places over on GBA:

https://www.greenbuildingadvisor.com/article/innie-windows-or-outie-windows

https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2018/08/08054045/Innie%20window%20detail%201.jpg