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��1 i�!�i o►,� ��7�u5y�T�o� ( int ' f-F®��r C7 C()��t�G.U� C�� Ems•) �� THE 1/3-2/3 RULE Some of the wall systems developed and tested in Canada use two layers of insulation (strapped wall and double wall) ith the air/vapor barrier positioned between the two layers (Figure 3-6b). This is OK as long as at least two thirds of the insulation (R-value) is outside the air7vapor barrier. With twice as much R-value outside the air vapor barrier as inside, the barrier should never gel cold enough to cause moisture condensation. This technique has been used in climates with 12,000 degree days without pr blems. One exception to the 1/3-2/3 rule would be in spaces with very. high indoor relative humidity, such as swimming pool enclosures or hot-tub rooms. In those applications, one should position the air/vapor barrier on the inner surface of the wall. The air/vapor barrier should never be installed on the-outside of the insulation as in Figure 3-6c. (A possible exception is in very hot humid climates.) Double Vapor Barriers There has been considerable concern re{ ently about the inadvertent installation of a double vapor barrier on the inside and outside of walls'.in superinsulated houses. Exterior foam sheat' ing, for example; forms a vapor- barrier on the outside of a wall. Theoreti ally, this should cause moisture condensation problems, but field investiga ions have shown no evidence of damage or degradation of insulation. Although.fur her research is necessary in this area, current thinking is that exterior fo m sheathing is an acceptable practice. The best insurance against any mQisture problems is a carefully installed, well sealed air/vapor barrier on the interior surface of the insulation. THE JOIST HEADER PROBLEM The most troublesome spot when desigi�g and installing the air/vapor - barrier ir/vaporbarrier is at the floor joists. How does one make an effective airtight seal between floors. One approach is to terminate the air/vapor barriers at the joists, then install panels made of rigid foam insulation between all joists, sealing the panels to the joists and floor above. When properly done, this is a satisfactory method, but it is very time consuming. Each panel must be precut, set in place and sealed. T3-7 - THE FLOOR HEADER PROBLEM - NEVER! The best method for continuing the air/vapor barrier between floors is to wrap it around the floor headers, but it is very important to avoid runnin .the air/vapor barrier outside the insulation. figure 3-7 shows one of the worst mistakes that can be made in a superinsula ed house. Unfortunately, this error, or some variation of it, is quite common and has even been shown in superinsulation construction guides. Never do it. If the air/vapor barrier is to run around the joists, then it must be insulated on the outside. T3-8 - A PROPER AIR/VAPOR BARRIER INSTALLATION AT JOISTS Figure 3-6 shows a good air/vapor design. This house has 2 x 6 walls on the first floor and 2 x 4 standoff walls in the basement. The air/vapor barrier is run around the joist header and is sealed tD the barrier of the walls above and 3-4