Attic areas typically should be filled with loose-fill or blanket insulation between and over ceiling joists in order to achieve a thermal resistance value of R-38. Cathedral ceilings and slanted ceiling areas are especially problematic due to the restricted space for insulation installation. As a minimal requirement for most climates, a total value of R-30 in any combination of building materials and insulation should be achieved with preferably 2" of clearance between the insulation and the underside of the roof sheathing. Standard framing practices typically allow for only Iм of clearance. This should be considered the absolute minimum for proper ventilation.
As mentioned earlier, finished attic areas with knee walls should not be overlooked. Each of these assemblies is analogous to those already mentioned, with the walls receiving a total wall value of R-19 in the knee walls, R-30 between the sloped roof joists, and R-38 between the horizontal ceiling joists (Fig. 5.1).
Figure 5.1 Areas to insulate. (.NA1MA)
Around 1980, a new approach to fuel and energy conservation came to the attention of architects and builders. Called superinsulation, this approach provided a high degree of comfort in winter and summer and reduced fuel consumption by 75 to 95 percent relative to conventional houses.3 The method was announced and explained in talks at building construction conferences, in magazine articles, and in books. Enthusiasm spread rapidly, with the result that by 1987 there were several tens of thousands of superinsulated houses in routine use in the United States and Canada. The origins of superinsulated design actually date back to the 1940s, when a group of individuals at the University of Illinois began analyzing heat losses from houses.3 In 1976, researchers were able to prove that superinsulated design concepts would need only as much as one-third as much auxiliary heat as the designs being promoted by the U. S. Department of Housing and Urban Development (HUD).3 A new moniker for superinsulation being used today is the airtight drywall approach (ADA).
In some very cold regions, this new method now dominates home construction.3 It is important to note that a universal standard is not possible because a design that is optimal for Boston will not be optimal for sunny Colorado, cloudy Rochester, or bitterly cold Anchorage. Because its main reliance is on heat conservation (excellent insulation, excellent retention of intrinsic heat) rather than on solar radiation, the superinsulated house is tolerant of less favorable sites and orientations. It is permissible to locate such a house in a moderately wooded area and to employ a far from southfacing direction (orientation).
The main distinguishing characteristic, or hallmark, of a superinsulated house is thick and widely applied insulation. An actual effective R-value of R-25 in all walls, ceilings, and floors is a minimum standard.4 Even at the sills, headers, eaves, window frames, door frames, and electric outlet boxes, a moderate amount of insulation is provided. The construction must be airtight but still introduce a steady and controlled supply of fresh air. The occupants benefit from the absence of drafts, cold floors, and cold spots near windows. A superinsulated house, throughout most of the winter, is kept warm almost entirely by the modest amount of solar energy received through the windows and by intrinsic heat. Interior heat is generated by systems and appliances typically functioning within the home. These sources include stoves for cooking, domestic hot water systems, human bodies, clothes washers, clothes dryers, dish washers, electric lights, television and radio sets, refrigerators, and other electric appliances.
A conventional furnace typically is not used. Most homes rely on “borrowing” from the domestic hot water system, or on a minimal amount of electrical heating. A wood stove or portable electric space heaters may be used during extreme temperature dips to keep such a house comfortable. There is not a significant added expense for superinsulated housing mainly due to the elimination of a furnace or a big heat distribution system.4 In fact, what little auxiliary heat is needed is less than 15 percent of that required for typical houses of comparable size built before 1974.3 In summer, the house stays cool because south-facing heat gain has been minimized due to the reduction of window area as well as the use of roof eaves.4
The extra cost of superinsulating a house is usually only 3 to 6 percent of the total construction cost.3 Experts suggest that the best insulations for superinsulated home design are those with highest R-value per inch, lowest cost per R-value, durability, and great resistance to settling. Fiberglass blanket products are especially popular owing to their low cost, ease of installation, and good fire resistance. Blown-in fiberglass when installed in dense quantities is gaining popularity and because of its ability to be installed in hard-to-reach places; however, settling can be problematic. Some insulation types are not recommended. These include vermiculite and perlite. Urea formaldehyde foam insulation (UFFI) is discouraged due to shrinkage problems and an errant controversy over formaldehyde emission. Balloon framing is preferred over platform framing because platform framing, although more popular, easier to construct, and less expensive, is inherently problematical from a thermal standpoint, owing to areas around floor joists, band joists, and headers that act as thermal bridges.4