Double Stud Meets Swedish Platform Framing
CZ6A, Ottawa ON
So this is the proposed design I’ve come up with after lots of deliberation. A big thanks to those who have offered their advice to my many questions here!
I wanted a double stud wall right from the start, settling on a 12″ depth filled with dense pack cellulose. What I didn’t want was a bunch of floor materials bridging through that insulating layer.
Starting from the bottom, I’ve shown an 8″ poured concrete foundation with the first floor resting on a 2×4 plate. A 2×6 load bearing wall sits atop the floor system made of 14″ tall joists and 3/4″ sheet. The exterior non load bearing 2×4 wall sits on a 2×6 plate cantilevered out 2″, effectively leaving 1.5″ of the 2×4 studs supported. Steel angle can be added to support this detail should my Engineer see it necessary. A 1″ thick strip of foam can be used between the 2×6 and 2×4 plates, caulked in place as a spacer and extra insurance against moisture from the foundation.
At the second floor I’ve shown the floor system supported by a ledger, keeping the whole system out of the insulated wall cavity. This also allows a flat plane through which the vapor retarder can pass uninterrupted (installed before the ledger). A single sheet of 3/4″ plywood joins the outer and inner walls at the plates. A double plate on the outer wall allows a slight trimming of a 10′ 2×4 to fit with minimal waste while providing a 9′ ceiling inside. A 4′ awning truss projects to provide shading to first floor windows and break up the appearance of the straight wall.
At the roof I’ve shown a raised heel truss supported by the inner 2×6 wall, a 3/4″ plywood bridge to the outer 2×4 wall, and a vertical 2×3 extension wall that allows the R80 blown in cellulose to completely cover the exterior insulated wall cavity and eliminate thermal bridging from the roof truss to the outer wall. The 3/4″ plywood bridge extends inboard slightly to allow the 7/16″ OSB attic floor to be taped securely to it. The truss overhangs 4′ to shade second story windows and, combined with an 8/12 slope, allows for a full depth raised heel to accommodate very deep attic insulation while extending high enough to keep wind washing of the insulation a non-issue (30″ as shown). An interior ledger board to support the trusses over window openings is to be discussed with my Engineer.
Not shown in these sections are the 3/4″ plywood window boxes or the 2.5″ interior service cavity to be framed with 2x3s. The service cavity will be stuffed with 3.5″ R12 fiberglass batts, compressed to give roughly R10. The service cavity allows the interior mounted ledger boards without disrupting the drywall mounting plane, as well as a space for wiring and plumbing pipe to co-exist with an uninterrupted vapor retarder. The compressed fiberglass batts will keep some pressure on the cellulose pack and vapor retarder, keeping any movement suppressed during building pressure changes.
Center of cavity wall insulation will be R54-R55 plus whatever R value I get from drywall, OSB sheething and air gaps to siding etc. I’m guessing clear wall R value should be about R50, maybe a little more.
The primary air barrier is the continuous layer of exterior OSB sheething from the foundation to the attic, bridged through the wall by the upper 3/4″ plywood bridge. The interior vapor retarder will be Intello Plus in a straight shot from the attic floor to the first floor sheething, only needing to be detailed out and around the first floor rim joist to allow sealing to the inside of the foundation wall.
Looking forward to your comments!
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