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DESI N AND CONSTRUCTION DETAILS 97 The double-glazed window has a total R-value of 1.84, Dou le-Wall Assembly Sequence The double composed of the inner and outer air films, the air space wall i prefabricated into a single unit on the platform, between the panes, and the glass itself. The inner surface then )ted into place and fastened as with conventional of the window is a vapor barrier. The inner air film (R- single stud wall construction. .68) is inside the vapor barrier, while the rest of the win- Th sequence is as follows (Figure 7.22): dow (R-1.16) is outside the vapor barrier. As with the 1. he inner wall is framed on the platform. 1 double wall,about two-thirds of the R-value is outside the 2. he air/vapor barrier is stapled to the outer surface vapor barrier. f the inner wall. Theoretically, the temperature of the inner glass win- 3. Structural sheathing is applied over the air/vapor dow surface should be almost exactly the same as the barrier. temperature of the air/vapor barrier in the wall. More- 4. he outer wall is framed on top of the inner wall. over, moisture condensation on the window should be 5. Temporary spacers (either 3.5- or 5.5-inch) are just about identical to moisture condensation on the air/ laced between the walls. vapor barrier in the wall. Therefore, by observing mois- 6. -inch plywood top and bottom plates are installed. ture condensation on the window, we can deduce the 7. a insulation can now be installed in the central amount of condensation in the wall. Ovity and the outer stud cavities. Or it can be in- Common experience has shown that double-glazed stalled from the outside, later, after the wall is windows do not exhibit severe moisture condensation erected. problems unless the indoor humidity is excessively high. 8. The wall is tipped up into place. The 3-1 rule would, by extension, work for double-stud Note:The assembled wall is heavy, so it should be fabri- walls as well, cated ip position for tilting into place. Once up, it is very One of the reasons the H embedded air/vapor barrier stable and strong. rule can be relied on is that moisture problems do not Fortwo-story construction, the sequence continues as happen overnight;they take a while to develop. Consider follows,(Figure 7.23): a climate in which the 99 percent winter design tempera- 9. 3-foot wide strip of 6-mil polyethylene air/vapor ture is say, -1WE In an average winter,the temperature b mer is attached and sealed to the top of the first falls below -10°F. for only 22 to 54 hours, occurring as fl r wall air/vapor barrier. This strip is then placed 2-or 3-hour periods on 10 or 20 days during the winter. o er the top of the wall. The air/vapor barrier must This isn't enough time for moisture condensation to b protected here with roofing paper or other suit- cause a real problem. a le material to prevent damage when the second- There is one important exception to the J-3 mile. In hot fl or joists are slid up for installation. tub or pool enclosures, greenhouses,or any space where 10. a second floor joists and floor are then installed the relative humidity is constantly 70 or 80 percent, the o er the inner wall. air/vapor barrier should be installed on the winter-warm, 11. a air/vapor barrier is then brought up around the inner surface of the insulation. jo st header and onto the floor. R-VALUE R-VALUE EXTERIOR AIR FILM 0.17 EXTERIOR AIR FILM 0.17 SIDING 0.50 GLASS 0.04 31/z"FIBERGLASS BATT 11.00 AIRSPACE 0.91 31/2"FIBERGLASS BATT 11.00 GLASS 0.04 OUTSIDEV.B. SHEATHING 0.80 OUTSID V.B. ° AIR/VAPOR BARRIER -0- ' 31/z"FIBERGLASS BATT 11.00 VAPOR BARRIER -0- GYPSUM BOARD 0.45 INTERIOR AIR FILM 0.68 INSIDEV.B. INTERIOR AIR FILM 0.68 INSIDE B. % ILI TOTAL 3 - % TOTAL 1.84 0.68-37 DOUBLE WALL WITH EMBEDDED AIR/VAPOR BARRIER DOUBLE-GLAZED WINDOW Figure 7.21 Comparison of air/vapor barrier location in "Saskatchewan" double wall and double-glazed window. f ti