mid-wall vs outer-wall window installation with continuous insulation
Context: we are building a 3-story addition in zone 6b/7, 6600-ft elevation, average winter temp 11F, 386 lb/sqf snow load.
We’re installing Loewen casement and awning windows (flange mount) in our new addition (windows arrive mid-January).
Doing things a little out of order, and debating whether to mount the windows mid-wall vs outer-wall. I know, we should have asked that question before the rough-ins and the sheathing (OSB, full shear), but here we are.
Questions are:
(1) mid-wall vs outer-wall window installation
(2) critique of the air/water barriers
(3) improvements in the window installation as shown
We used OSB for shear over 16″ OC 2×6 wall. Our original plan was to apply WRB, mount the window flanges on properly flashed openings, add the siding and call it a day. All with proper detailing, of course.
After the fact, we decided to add continuous insulation for an additional R6 and to get rid of thermal bridging. The OSB is done – no going back on that.
This isn’t a Passive House, so we are not squeezing every therm out of our build. Doing a mid-wall window is not so we can insulate the window frames. We just want good (good = high performance, high reliability) air/vapor/water barriers.
This is complicated a bit by wanting to match the addition to the existing building (reverse board-and-batten siding). The outermost layer is 1/2″ rough plywood + 1×8 cedar boards.
We know we are making compromises, but we’re going from R13 to R27 walls and from a R4 to R42 roof. We think this will be good enough without invoking the laws of diminishing returns.
We’re also not putting on a rain screen, hoping that the Comfortboard will be porous enough if we do a good job with our WRB and flashing.
ZIP is hard to cost justify with the OSB shear walls in place. Polyiso is cost effective. Maybe?
Rockwool seems to make sense – we’re surrounded by carpenter ants here in the WUI. Lots of damp stumps and some large logs – all defensible space compliant. But we’re surrounded by carpenter ants, legions of them.
I’m currently leaning toward the mid-wall window location, since it seems a little easier to flash in a reliable way. The rough openings are already done, and the sills are level (do not drain to the outside), so we need to use a backdam.
A picture is worth a thousand words. Attached is a PDF that has both the mid-wall and outer-wall installation design.
Any and all feedback appreciated. Learning as we go – I’ve already learned a ton from all the amazing folks here at GBA and we’ve made some great improvements to the project as a result.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Most builders I know prefer "outie" windows because their installation is more familiar and thus less expensive for the client. Mid-wall performs better and in my opinion is easier to integrate with control layers when you have exterior insulation. It does require exterior extension jambs and sill, but a benefit (in many cases) is the shadow lines the inset windows create, which give the facade much more life than a typical modern home that is very planar in appearance and resorts to things like changes in siding materials or adding fake shutters in an attempt to provide some sort of visual interest.
Yes, the builder does prefer the "outie" for exactly the reasons you point out.
But the "innie" is more consistent with the 1966 character of the existing home, for sure, which has "innie" windows everywhere else.
I'm really glad that you brought up the shadow lines. It's so easy to get caught up in cost and performance trade-offs, only to end up with a very thermally comfortable and functional, but slightly lifeless home.
Our architect - super brilliant and creative, we love him - designed hidden garage doors, which created this very clean front to the building. "Cool" in every sense of the word. But it was confusing, because there was a driveway to nowhere, and a wall under a gable with no windows or visible doors. I know much of this is cultural and what you are used to seeing, but the language of home (roof, windows, doors) hasn't fundamentally changed.
We ended up with a frame around our garage doors, and are happier for it. Maybe we lost an architecture reward for that change.
That said, we may just lash out once more against modern novelty on the windows (depending on cost, haha).
One thing I've learned in 20+ years of designing custom homes and renovations is that EVERYONE has emotional connections to certain things that mean "home" to them. Unfortunately it can be challenging to figure out what those are. For many of my clients, it's a somewhat traditional-looking home but not strictly traditional, but not super-modern either. It's often having some sort of wood-burning appliance indoors. It's often having the kitchen be a communal gathering space. Oddly to me, never having had a dedicated master/primary bathroom, I've had maybe three clients out of hundreds who didn't consider a dedicated master/primary bathroom to be an absolute necessity.
I got off track... there are certain things that say "home" to the owners or occupants, and having a blank wall with a driveway running into it would not fit that description for most people. Though it does sound clever.
zetetic,
Both your sections look good. I don’t see much to favour one over the other.
Thank you, Malcolm. Knowing that we're not making a mistake either way gives us some freedom of choice.
I'd match the existing house, regardless of extra complexity. But if you are relying on the Comfortboard for drainage, I think you should make some provision for water to exit from behind the siding at the top window buck.