Insulating window framing (Passive House style) worth it?
Building an addition in CZ 6b/7, elevation 6600′, 386 lb/sqf ground snow load. Average winter temp is 11F (4 months). 11F is not that cold, but the temps regularly drop below 32F throughout the year, including shoulder season, which puts us in CZ 6b/7 (8600 HDD (65F) in warm years, 9200 HDD (65F) in cold years). So not extreme cold, just lots of regular cold.
We are installing Loewen double-pane casements/awnings/direct sets with i89 (S4) and e180 (S2) coatings.
We are trying to figure out if it makes sense to buck out the windows so that they are flush with the exterior (current thinking), or take a more passive house approach and (a) install the windows mid-wall for better performance and (b) then further insulate the frames.
Our concern is that the energy ROI on the mid-wall install will likely be measured in decades, seeing that the frame represents a small amount of surface area. But it’s also an area for thermal bridging, and how that will impact comfort when near the windows. On the other hand, it’s also a higher level of detailing, which can change the risk-reward balance.
Our wall stack, inside to outside:
3/8″ finished decorative interior cedar
5/8″ sheetrock
2×6 framing, R21 blown in insulation
1/2″ OSB shear ply
Grip0Rite ProWrap -SA
1-1/2″ R6 Rockwool Comfortboard
1/2″ roughsawn ply (“batten”)
3/4″ 1×8 red cedar boards (for reverse board and batten)
I’ve seen the thermal maps, and read the Passive House cases, but I’m wondering if anyone here on GBA has done this, and felt it was worth it?
Attached is a sketch of the proposed wall stack with window.
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Replies
Having lived in CZ6 (7000 FT) for over 26 years and having designed built and
lived in several houses during that period we (wife & I) came to a window
solution. We installed insulated shutters - interior - and closed them at night.
I know this isn't for everyone. You have to like the aesthetics. The shutters can,
of course, be built in a wide variety of styles but you have to like the look. They
provided a strong sense of security both at night and when we weren't home
and, of course, upped the R-Value of the glass considerably. I know this isn't
exactly the answer to your question. Have Fun!
I would not trust butyl flashing tape in a reverse lap as you have drawn.
@freyr_design - Can you point me to a better detail? Absolutely open to suggestions.
zetetic,
I would mount the window trim on the rain-screen strapping, and use a metal head-flashing directly above the window frame. That way any water that gets into the cavity above the window has a good path to the outside. Below the window this would allow the sloped sill-pan to drain into the cavity too.
I'd also consider moving your WRB outside the foam, which would make the flashing easier.
What looks like foam is actually Rockwool Comfortboard 1.5". We're applying a layer of plywood (the batten for a reverse board-and-batten) directly on top of the Rockwool, and then cedar boards to create the reverse board-and-batten.
@malcolm - Can you put Henry Blueskin VP100 directly on Comfortboard? We can't put the WRB on the plywood, because then you'd see it between the boards. So it seemed that the OSB was the most reasonable place.
We don't have a rainscreen in this wall design, but we do have an insect screen below the Comfortboard.
zetetic,
My mistake - I assumed it was foam.
I doubt the WRB would adhere, and it would be really finicky at penetrations. Best leave it where you show it.
I'd still use a metal head-flashing. Mount it on the 2"x4" blocking and have it run above the window trim, then run a piece of peel & stick over the blocking and up the sheathing.
Below the window I'd cut grooves in the trim to allow water to drain from the sill-pan into the space behind the siding.
I would trust a high quality euro acrylic tape, though over another company wrb it might be susceptible to peeling depending on the facer. A liquid flash could be used. Lastly you could install another sheet of wrb over that butyl to make an appropriate lap. Certain SA membranes allow reverse lap but often require further treatment at the seam. For example Henry requires termination sealant at their reverse lap on vp100.
@gstan - Do you have any problems with condensation?
We're thinking rolling blinds because of limited interior wall space around the windows.
In general no, but we lived in Colorado (very dry climate) and had
carefully installed weather stripping.