Window installation in a super-insulated wall

Designing a Super-insulated wall with good drying potential and good air sealing is easy. But the Devil is in the windows. With passive house level construction we want the window to be recessed toward the middle of the opening and the middle of the adjacent insulation layer. This is about thermal bridging and heat loss. An advantage that I’ve found is that it can potentially simplify the flashing which is certainly a welcome idea. Window and door installation has become increasingly complex over the past decade especially with exterior foam details. (I try to stay away from that) What I’m looking at in this sketch is how flashing for siding can be decoupled from flashing for the window to a great extent. Does the siding even need flashing anymore? Depends on how you do the siding. With some Rain screen siding details, perhaps not! Windows are now coming with really nice integrated sill flashing – you specify the depth when you order. This makes for a much more integrated and seamless system which should have long term durability advantages over what we have been doing up until now. What is missing from this drawing is an exterior layer of rock wool insulation to keep the sheathing warm (not rigid foam !) A more expensive detail and this may be less important when using cellulose than fiberglass insulation but is a detail I would certainly do on my own home. The other option is to use more of a Larsen truss system similar to what Chris Corson is doing in Maine with no exterior sheathing. It is A tough sell around here to leave the insulation protected only by a weather barrier such as Mento plus. The conversation on building science is terribly interesting and seems to be resolving itself towards simplicity as we gain knowledge and experience about what works and what doesn’t and develop products and materials based on our increasing knowledge base. Of course, 93% of builders and architects aren’t really paying much attention.

Join the discussion 5 Comments

  • Brad Morse says:

    Who is making windows that you can specify the sill width on?
    Thanks.

  • bob says:

    I believe that Schuco and Zola are currently Others too but those ones I’ve seen

  • Pete from La Crosse, WI here. I totally get why you Don’t want XPS or poly-iso sheathing on the outside of a house in cold-country. What about celo-tex with a Tyvek or Centos 1000 rain-barrier over that? While there is little strength to the celo-tex, it keeps the mice and insects out. And it breathes really well. It seems to me that steel diagonal straps + celo-tex would be a good exterior sheathing system. Pete

  • bob says:

    hmm. that could work. Some folks are completely eliminating the exterior sheathing in favor of straps for bracing or a zip wall over a 2×4 structural wall with all the insulation outboard of that in vertical I-joists or open web larsen trusses. Around here we are increasingly using rough pine from the local mills (cheap and local) for exterior sheathing. solid and very breathable.

  • Hi Bob:
    You commented on building a wall assy with no insulation within the structural part, then Zip sheathing and then insulation filled I-joists outside that. I sort of get it because of how busy some walls are inside, particularly in busy areas of the house like kitchens on an outside wall. What I don’t get is whether anybody bothered to read the specs on Zip sheathing. The maker claims a perm rating of 14-16, which isn’t worth anything in Vermont or Wisconsin. Perhaps the Zip sheathing is covered in a layer of 6mil poly?, but then why not just use OSB? People are obsessed with making an airtight home… from a wood box. Haven’t we already shown that it’s easier to make an airtight plastic bag? And where are those 100 year endurance tests on the tape that works with Zip sheathing? I couldn’t find them. There is nobody disputing many thousands of years of life for LDPE if kept out of the sun. So yes, by all means, Larsen trusses or equivalent on the outside of a skinny stick house with 6mil LDPE on the outside of same and then way breathable stuff out beyond that.
    I have questions about your window detail as I am planning a skinny Larsen retrofit for my house in the summer of 15. 4″ trusses– skinny I know. Before then, I plan to buy a used window from Habitat and build a wall around it and flash it. BTW I would flash the bottom under the sash to keep water that runs down the face of the sash from getting under the flashing. And the heat flux through the window is perpendicular to the glass. The insulation at the top and bottom of the sash only serves to make the rought opening for the window larger than it needs to be.
    More to follow on my mock-up wall. I suspect it will be based on assymetrical (z-shaped) Larsens around the windows and doors, to make the step-in/ step out to install the windows to.
    Pete Gruendeman, La Crosse, WI

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