Because of the four inches of exterior foam board and full one-inch-thick strapping to hold it on and provide a nailer for the siding, we needed to create extension jambs for the windows and doors. As with most tasks, there are a number of ways in which that task could have been handled, and the solution is partially dependent upon the location of the window within the exterior wall.
In our case, the architect decided that the windows should be installed within the 2×6 framing (i.e. flush with the Zip sheathing), rather than even with the exterior plane of the wall/siding structure. The architect did this to achieve better thermal performance, and also to provide a more appealing exterior by providing depth and shadow lines.
Given that position, we had to come up with a method for extending the window (and door) jambs five inches to place the exterior jamb surface to a place where it could be properly trimmed out; trimming the jambs out that far, would put them in the same plane as the surface of the 1×4 strapping.
Rather than building plywood extension jambs that extend into the rough opening and attach to the wide surface of the 2x6s that form the opening, we decided to use Azek, and build the boxes so they would attach to the outside of the building structure, with screws driven through the sheathing and into the 2x6s that form the rough window opening.
The jamb extension boxes are glued and tacked together. We cut a 3/8″ wide by 7/16″ deep rabbet into the inside rear edge of the sides and top of the boxes.
This allows the Intus windows (which don’t use a flange) to be installed 1/2″ proud of the wall sheathing and fit into the extension jamb and over the sill. We also put a 5 degree pitch on the box sill, and made the sill a half inch wider than the sides and top to allow water to drain over the trim sill.
Attachment flanges (1 x 1 3/4″ Azek strips) are tacked and glued to the sides of the boxes to allow the boxes to be screwed through the sheathing and into the studs. On the smaller windows, we only put these flanges on the sides of the boxes (as shown above). On the larger windows, we also put them on the top and bottom for added support.
Here are a couple of photos of the boxes installed (no nailers or trim yet):
Once the box is attached to the house and the 4″ of foam board is applied to the exterior of the walls, we glued and pocket screwed 1×8 Azek “nailers” to the sides and top of the boxes to serve as an attachment point for both the trim hoop and the siding. A 1 x 3 “nailer” is attached to the bottom.
Here’s what the nailers look like when attached:
And here’s what they look like on the house:
Once the nailers are attached, we’re ready to attach the trim. The trim, which will be in the “Arts and Crafts” style, will look like this:
The sides are 5/4 x 4 1/2. The top is 6/4 x 5 1/2, and the sill is 1 1/2 x 2 1/2, with a 15 degree pitch. The inside dimension of the sill is 1/8″ wider than the sill on the box. When installed, the trim sill is pushed up against the bottom of the sill on the box. The trim hoop is sized to result in a 3/8″ reveal all the way around.
All in all, it’s an extremely sturdy structure, and they’re very quick to build (much quicker than this explanation conveys). However, care must be taken to ensure that they’re installed square (as we built them, there is about a quarter inch of play each way that can result in an out-if-square installation if you don’t pay attention). Also, the Azek is flexible enough that variations in the sheathing (e.g. where the wall bows slightly) can result in a bowed front edge on the box. If the bow is in the sides or the top, the 1 x 8 nailers will take it out when they’re attached. But if the bow occurs in the sill, the 1 x 3 nailer may not provide enough rigidity to remove it. The easier solution might be to shim between the attachment flange and the sheathing if necessary.