High-Performance Walls: Page 3 of 6

Structural Insulated Panels
Fast Assembly, Tight Construction

Structural insulated panels (SIPs) are sandwiches of insulating foam faced on both sides with a thin skin, often oriented strand board. They combine the framing, insulation, and sheathing in a single package. SIP houses can be built much faster than conventionally framed homes.

Panels can be up to 8 feet wide by 24 feet tall, with standard thicknesses ranging from 4 1/2 to 12 1/4 inches, according to the Structural Insulated Panel Association. Small panels can be moved around and set by hand. Large wall panels and roof panels require a crane to help set them in place.

R-values vary considerably, depending on how thick the panel is and what type of foam is used. For example, a 4 1/2-inch EPS panel has an R-value of 14, while a 10 1/4-inch XPS panel has an R-value of 48. Today, the most common core by far is expanded polystyrene (EPS). Less frequently, panels are made with extruded polystyrene (XPS) or urethane foam. Both of those alternatives have higher R-values than EPS, but they also have a few drawbacks, such as higher cost; a higher melting point, which makes field alterations with heated cutting tools more difficult; and, in the case of XPS, limitations in panel thickness.

For those who are concerned about using plastic foam, Agriboard Industries makes a panel with a core of compressed wheat straw. Tests at the Oak Ridge National Laboratory found the R-value of a 7 7/8-inch-thick panel was 16.47, although the manufacturer has claimed higher effective R-values because of the wall’s mass (see the discussion on thermal mass and R-values in the “Insulated Concrete Forms” section).

SIP manufacturers can take construction drawings and turn them into precut panels with door and window openings already cut. There is a learning curve to assembly, but seasoned carpentry crews should be able to pick up what they need to know fairly quickly, and some manufacturers offer job site assistance.

Speed of assembly, low rates of air infiltration, and dramatically reduced thermal bridging all are energy-saving advantages with SIPs. Although material costs are higher than for conventional frame construction, costs usually even out when lower labor costs for SIP assembly and less job site waste are taken into account. In addition, improved energy performance may allow the installation of smaller, less-expensive heating and cooling equipment.

Straw Bales
Simple Techniques & Materials

Straw bale construction in the United States dates to the late 19th century in a part of Nebraska where trees were few and far between. Some of those straw bale houses are still standing.

Advocates point to a long list of advantages of straw bale construction, including durability, fire- and insect-resistance, the renewable nature of the building material, low material cost, high R-values, and relative ease of construction, making houses well suited for owner-builders on a tight budget.

Straw (not to be confused with hay) is an abundant resource—it is what’s left after the harvest of grain crops such as rice, wheat, oats, barley, or rye. Bales can be two-string, which weigh about 50 pounds each and are up to 40 inches long, and three-string, which may weigh 100 pounds and measure up to 47 inches long. Straw bale houses can be planned around these dimensions to make construction simpler, just as a conventionally framed house can be planned around the 4-foot increments that panel products come in. According to one online calculator, it takes 240 bales to make a 30-by-40-foot one-story house (see ironstraw.org).