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L&y ? 5045) �3 y .7Jtb �(15`�1 96 WALLS -7�1U7P a 6-mil polyethylene sheet installed on the outer surface J, R-60 INSULATION of the inside wall. INSULATION STOP One of the most noticeable, and sometimes objection- k able, features of this wall design is the cantilevered outer wall.An alternative is to set the outer wall back so that it is either flush with the platform (Figure 7.20a) or cantile- AIR'VAPOR BARRIER, vered only a few inches to accept exterior rigid foam PLYWOOD foundation insulation (Figure 7.20b). Another alternative NON-STRUCTURAis to use balloon-type framing for the outer wall, resting STRUCTURAL L the studs on the foundation outside the latform Figure INNER WALL OUTER WALL p ( g t 7.20C). BUILDING PAPER AND SIDING The Embedded Air/Vapor Barrier The primary rule SHEATHING for or air/vap barriers is that the must be installed on the AIR/VAPOR BARRIER y R-11 FIBERGLASS INSULATION worm side of the insulation. R-19 FIBERGLASS INSULATION However, the Saskatchewan double-stud walla ears R-11 FIBERGLASS INSULATION pp to violate this rule: the air/vapor barrier is near the middle of the insulation (Figure 7.19). Despite this apparent de- parture from recommended practice, the Canadians have x� 3/8"PLYWOOD not reported any problems. Why?According to Canadian TREATED PLYW ODNDATION WALL researchers,the embedded air/vapor barrier will cause no AIR/VAPOR BARRIER moisture problems as long as the insulation on the Cold R-11 F.G.INSULATION ii side of the barrier has twice the R-value of the insulation R-11 FIBERGLASS INSULATION on the warm side. In other words, the air/vapor or barrier R-19 FIBERGLASS INSULATION p 4 t d z"RIGID INSULATION may be safely placed one-third of the way into the insula- ;•F 1 tion from the warm (heated) side. , _ Does this technique—called the !,-1 rule—actually MOISTURE /z"RIGID FIBERGLASS work? It would seem to in theory. Consider the following BARRIER DRAINAGE LAYER interesting parallel between a Saskatchewan-type double wall with embedded air/vapor barrier and a double- 2"RIGID lazed window (Figure 7.21). ' INSULATION g ( g The wall has three layers of 3.5-inch (R-11) fiberglass Figure 7.19 Section of the "Saskatchewan"-type double batts with the vapor barrier installed between the middle wall. and inner batts. 66 percent of the R-value is outside the vapor barrier. J r—7771 157 L y„ aQ Q _ 19 A. OUTER WALL FLUSH WITH PLATFORM B.OUTER WALL CANTILEVERED TO ACCEPT C. MODIFIED BALLOON FRAMING OF EXTERIOR FOUNDATION INSULATION OUTER WALL Figure 7.20 Double-wall variations at first floor joist area.