Superinsulation For the Masses
(Page 4 of 5)
September/October 1986
By the Mother Earth News editors
Staggered double wall: The amount of framing lumber for a double wall can be cut by staggering the 2 X 4 studs to the inside and outside, also shown m Fig. 1. All of the members are structural, but the plates — the only conductive paths from inside to outside — must be as wide as the walls.
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Larsen Truss: This is a trademarked system of nonstructural built-up plywood braces that support the outer wall and a thick layer of insulation. (See Fig. 2.) It is often less expensive to use than double-wall construction, particularly in two-story superinsulated houses.
Thick conventional wall: 2 X 6 or 2 X 8 fully insulated studs can be used in conjunction with an inch or more of rigid foam insulation on the outside to form walls with R-values of greater than 30. The exterior foam should cover all wooden members, including the bottom plates, sills, rim joists, etc., as shown in Fig. 3. Materials are more expensive than are those for conventional construction, but the labor is about the same. Because contractors are familiar with the techniques necessary, this is the most commonly used approach to achieving high R-value walls.
It is currently believed that the vapor barrier formed on the outside by the impermeable foam insulation doesn't cause condensation problems inside the wall, as long as the inner vapor barrier is effective. In fact, it's now recognized that the vapor barrier should go on the side of the wall where humidity is highest, and in many climates that will change with the seasons. Therefore, a double vapor barrier may actually be the right choice in temperate climates.
Superinsulation techniques can be used in new or retrofit construction.
Panelization: One energy-efficient construction approach that's gaining a lot of popularity is called panelization — a system by which walls are built in a factory and trucked to the building site. Though it's not exactly an owner-builder technique, panelization has significant advantages for energy-efficient construction.
The basic principle is to assemble walls in sections at least four-feet wide and full height. Foam insulation is sandwiched between the interior finish (drywall, perhaps) and the exterior siding or sheathing. All wiring and electrical boxes are installed at the plant, and the air/vapor barrier is carefully integrated. The panels are bonded with glue and held in a press during setting so that the interior and exterior layers add to overall strength.
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