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OPTIONAL DESIGN METHOD • <br /> 'At an option to completing the Exterior Envolope Average "U" Computation where <br /> "U" values have to be calculated, the following method may be used. <br /> NOTE: All of the following six items must be accomplished in order to <br /> utilize this method. <br /> �—;• 1 . Ceilings which meet one of the following criteria satisfy this requirement: <br /> A. R-38 throughout the entire ceiling. Indicate on plan section drawing how <br /> this will be accomplished. Complete appropriate diagram F, G, or H. <br /> B.' If a portion of the ceiling is less than R-38, the insulation in the <br /> remainder of the ceiling must be increased to yield an overall average <br /> thermal resistance of not less than R-38. <br /> C. Where the roof at the perimeter of the ceiling prevents installation of <br /> insulation to full depth, the insulation in the remainder of the ceiling <br /> must be increased to reduce the overall ceiling heat loss to no more <br /> than if R-38 had been installed throughout the entire ceiling. <br /> (Submit calculations if this method is used). <br /> 2. For the insulated cavity of opaque wall and rim joists, but not foundation <br /> walls, a minimum R-20 is required. Complete diagrams B and C. <br /> 3. . For the insulated cavity of floors of heated spaces over unheated spaces, <br /> a minimum of R-20 is required. <br /> 4. Maximum glass area may not exceed 12% of the area of exterior walls not <br /> including foundation walls. All windows shall be double glazed or have <br /> storm windows. Complete the following equation using data from the <br /> first page: <br /> 3(a) - 3(t) _ <br /> N <br /> J. <br /> I (x) must be less than .12- to meet requirements of this item. x <br /> 5. Maximum glass area may not exceed 10% of the area of exterior walls, not <br /> including foundation walls, when a sliding glass door is installed. All <br /> glass shall be double glazed or have storm windows. If sliding glass <br /> doors are to be installed, complete the following equation using data from <br /> the first page: <br /> E3(a) + 3(c� . 3(t) _ <br /> y <br /> Y <br /> must be less than .1 m requirements <br /> (Y) 0 to eet of this item. <br /> 6. A 1 3/4" metal faced door system with an insulated core providing an R value <br /> equal to or greater than 3.0 or a conventional door and storm. All primary <br /> doors must have durable weatherstripping. <br /> I <br /> Page 6 <br />