This is the story of how we ended up with our window box detail.
We decided early on to use advanced framing techniques and work on a 24″ module. In early drafts of the design I used window widths that fit between studs. The advantage to this approach is that you use less wood, smaller headers and fewer jack studs. Less wood means a bit less money spent on framing materials and fewer opportunities for thermal bridging and cold spots around openings in the envelope.
Sounds great in theory, but then I discovered how difficult it is to fit windows into a 24″ module in a small house. Aesthetically we wanted the windows to line up with doorways and the spaces inside and frame the view, and also make use of passive solar. Eventually I gave up on fitting narrow windows into the structural grid.
The tradeoff was that almost every window interrupted the path of the load, so the headers above the windows had to be beefed up and more jack studs were required. Adding more structure around the window opening means more bridging from the outside to the inside of the window box, short-circuiting the insulating value of the double-wall construction.
We also decided to use a rainscreen detail on the outside of the wall (see Final Energy Analysis and Recommendations), which pushes the 3 1/4″ thick window frames out a bit, leaving a deeper window box inside. This exposes 4 1/4″ of exterior structure (2×6′s, 1 king stud and 2 jack studs) around the window frame. Doing the math, 4.25″ x 1.25 R/inch = 5.3 R, which is not a lot compared to the 40 R of the double stud wall.
Even though there are 12 window openings in the 2 bearing walls (north and south facing) it is only a small amount of bridging. Doing the math again, we have roughly 863 sf of interior walled surface area on the north and south walls, and 29.5 sf of bridging area next to the windows. That is roughly 3.5% of the overall interior wall surface.
Our contractor, Warren, came up with several good ideas to minimize the cold spots created by the bridging. First, move the header up to the top plate to add some space and insulation between the header and the top of the window frame. Second, add a layer of 1″ rigid insulation around the inside of the window box, to which the drywall will be attached. This layer of insulation is small (R5) but it doubles the R-value at these locations and breaks the bridge between the outer structural wall and the interior window box.
Earlier this week Warren added the bevel framing and insulation for one window. It goes together quite well, but in hindsight I would have just insulated the problem areas, the 2 sides of the window next to the studs, rather than the entire window box.
This weekend we are working on the remaining windows. Luckily the sun is out this weekend which makes working in the house fairly easy even when the temps outside are still below freezing. The passive solar is already working!