I know there was a discussion about tapering the inside of window openings to reduce the tunnel effect. The design offered was a sub-buck in a much larger opening. What about something more direct? Here's a drawing of an idea I had. I would only do the sides, with a flat top and bottom. The problems I see would be that the angled buck would have to be fairly precise, and would be tricky to secure. I'd fasten the angled piece and the straight piece together, and then insert it into the opening in the foam. The inner wall of the ICF would be trimmed with a hand saw, using the angled buck as a guide. Temporary cross braces would be cut to match the angle. Screws on the inside of the angled buck would anchor it to the concrete. The window opening is sized for a typical 6-9/16" window return, but the 4-9/16" could be done with different dimensions.

Do you think this could work? See any problems holding it in place during the pour? The method with the larger opening and the sub-buck looks poorly insulated to me, and uses a lot more lumber.

The problems I see would be that the angled buck would have to be fairly precise, and would be tricky to secure

That does look time-consuming to construct and to my eye it makes the windows look even more like they are in a blockhouse or bunker,

How would you attach the inside trim? All I see is drywall and foam.

I think the idea is good if you plan on doing a drywall return. Regarding the comment on using a lot of lumber and poor insulating value of other designs, realize that you are placing a window in that opening, which will be far worse than the buck, regardless of size, in terms of insulation value. The amount of lumber you will use in your window bucks will seem like a drop in the bucket by the time you build your house. Hell, I think I used over twenty 2x4s to build a simple stinkin' closet.

I asked my contractor about fastening trim, not only by the windows, but anywhere where this isn't a web (it's gonna happen). He said something about glue, but I wouldn't trust glue to hold wood trim on drywall. If it warps, it's just going to peel the paper off of the drywall. I could embed some PT lumber or plywood inside the ICF pre-pour. My CMU house in FL has that problem- very hard to mount anything around the windows. Alternately, I could use Tap-Cons and counterbore and plug the holes in the trim.

It's funny; I was watching an old British show last night, and one of the buildings had tapered returns just like we are discussing. They were wood raised panels, and looked very nice. You could tell that the walls were very thick; probably stone. Even if I paint everything, I will still probably do the raised panels. It gave a very traditional look to an unusual window design.

I'm not planning to do all of the windows this way; just the ones across the back of the house facing the view. t's actually only two large windows. Maybe I'll do the two basement windows first as a practice run, and then make a decision on how well we like it. The good news is that its easily reversible if we don't like it, especially compared to wanting to make the angled returns after the pour!

Great, drawing should work well. If you intend to have a lot of these openings, consider using IngegraSpec . They make tapered bucks and need no additional bracing most of the time.