We’re asked a lot about “high performance” or “super insulated” wall systems.
I’ll save you the 25 minute rant, but I will tell you where we are and where we’re not.
Where we’re NOT. Foam Wraps.
I’m a big fan of Fine Homebuilding Magazine and the Journal of Light Construction. They’re both a good source of building information. However it seems that every issue they’re changing their details on how / where / what type foam / flashing details. Further they tell you that if you screw it up, you’ll rot the walls from the inside out. . . you will, in fact, build a composting wall.
However this lack of consensus is not my biggest problem. My biggest problem with Foam Wraps is the execution (or lack there of) of the flashing details. This is where the “rubber meets the road”.
I have no doubt that most Architects, Energy Raters, etc understand the science and details that go along with foam wraps, but in the field I’ve seen atrocious execution. See below for a picture I recently took of a house using an exterior polyiso foam wrap.
I’m not singling out a particular builder, but I see this all the time. The reality is that the Building Scientist nor Architect are the person installing the flashing. . . . It’s the low bidder. While I don’t think that all flashing details look like this, seeing just this one scares the heck out of me. What good is an R60 wall if water can pour around the window openings.
Where we ARE. Double Wall Systems.
There are many versions / degrees of a double wall system. Taken to it’s extreme (no thermal bridging) is the Larson Truss (see picture below). This particular house was built by Vermont builder Robert Riversong. See here for a description of the details of his Larson Truss.
Scaled back to a simpler form a double wall system can be built using 2×8 plates and 2×4 studs both inside and outside the wall. See below. This picture was taken in our shop today. We’ll install dense pack cellulose into the wall cavities making sure we “stuff” it between the 2×4’s.
Our simple version of the system allows for an R28 wall and reduces significantly the thermal bridging that happens in the field of the wall. The wall R-value can be increased simply by increasing the plate depth. A 2×12 plate would yield an R43 wall.
This system isn’t perfect (there is still thermal bridging at the top / bottom plates, window / doors RO’s, and floor assemblies), but it’s better then a sharp stick in the eye. While the plates do thermal bridge to the exterior, that thermal bridge (roughly an R6) is less then the 15sf R4 window hole you have next to it.
BEST PART about our version of the double wall system is that flashing details are STRAIGHT FORWARD. You’re using conventional methods against conventional materials.
And, I sleep well knowing that your window isn’t leaking water.