Anatomy of a Double-Wall System

If you conduct a web search for “double wall” framing, you’re bound to come across many ways to create a super-insulated wall system from dimensional lumber.  (See for instance,, or search out “Larsen Truss” for more information)  Our wall system is a little more complicated, however, because the north portion of our house is built into a small hill. You cannot (or at least should not) frame below grade. As a result, about 4.5 feet of the outer portion of the north wall is comprised of concrete foundation.  The above-grade portion of the outer wall, (again 4.5 feet), is framed on top of the foundation.  If this isn’t complicated enough,  the entire inner wall – above and below grade but on the interior of the house – is framed entirely with lumber!  A picture says a thousand words, so here it is:
The lower portion is the ICF foundation with an extra 2″ of exp on the outside.  (the ICF is mislabled – the blue portions are actually 2.5″)  The 2×4 frame and 3/4″ gap are filled with dense-packed cellulose.  The end result is R48.25.The upper portion is a 13″ wall cavity from the outer faces of the 2x4s.  From right to left, though, the wall is as follows:  1.125″ BP “R4″ sheathing board (R4), 2×4 cavity filled with roxul (R14); 5.25″ of esp type I foam (R19.75); and a 4.25″ 2×4 + small 0.75” gap filled with the dense-packed cellulose (R14.9) for a nominal R52.65 wall.And it should be extremely air-tight.Tomorrow I will be laying the main plumbing drain – should be fun …
~ John
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3 Responses to Anatomy of a Double-Wall System

  1. hoserman says:

    Neat stuff – hope the plumbing went well – it certainly was a cold enough day for it!

  2. John Scime says:

    Plumbing didn’t happen! One of the issues with building this time of the year is that you are at the mercy of winter. The foundation is currently tarped over and there is a 165,000 btu diesel heater inside attempting to melt the frozen ground. The air quality in there is terrible so I could not work. The upside is the Jonathan, our builder, is going to install the drains!

    I was there today and the frost is almost gone.

  3. HI John,

    It’s Matthew Salkeld from Fibertec Windows. We spoke last week but unfortunately I didn’t get your phone number. Mine is 613 606 1546.

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