Engineered walls for energy-efficiency: building expert Marcus Renner explores the latest wall technologies.

Do you want to build a house that's more energy-efficient, but doesn't cost an arm and a leg? It may be easier than you think. Most homes today are built using a method called Western platform framing; you may know it as two-by-four or stick framing. Stick framing is a time- and cost-efficient way to build a home, but it is by no means the only one. During the last half century, other wall systems that have considerable advantages over our current form of building have been developed and tested. These systems are also accepted by the building code officials in most areas of the country. Let's explore a few ...

First, let's address stick framing and how it can be made more energy efficient. Engineers studying building science have created a system known as Optimal Value Engineering (OVE). This idea restructures the way a stick-framed wall is built. Studs are configured so that more insulation can be added in places that usually don't get insulated well. (In a regular stick-built house, inadequate insulation is common in many areas: at exterior corners, intersections of interior and exterior wails, the area where the roof meets the house, and around envelope penetrations and behind shower stalls, just to name a few.) In addition to allowing better insulation inside the stud wails, rigid insulation can also be used as the exterior sheathing, providing a continuous layer of insulation. OVE also addresses wood use, minimizing the amount used in a home by avoiding overbuilding and placing support wisely. If constructed using the OVE approach, a stick-framed home can achieve substantial energy savings.

Structural insulated panels (SIPS) are the most popular form of construction after stick framing. They are composed of foam insulation sandwiched between a "skin" of oriented strand board (OSB). Together the foam and wood create a strong wall section that provides a continuous layer of insulation. Few if any structural members divide the insulation. The layer of OSB on the two sides also provides a continuous nailing surface for drywall and siding.

There are two different types of foam insulation that are used between the layers of wood; the most popular and least expensive is extruded polystyrene (EPS). You know it as bead board or by the brand name Styrofoam. When we look at all types of insulation, we use the R-value measurement. The "R" of a material is its Resistance to heat transfer. The higher the R-value, the better a material will be at insulating the home from the heat or cold. EPS foam, which is glued to the OSB, has an R-value of about 3.5 per inch. Individual panels can be factory cut to the specifications of the plans or can be cut on site. An average home can be erected within a few days, which is an advantage in our rainy climate.

Polyurethane foam SIPS have insulating foam that is injected into a mold, adhering the OSB to the foam. The foam hardens and has an R-value of about 7 per inch. Although their insulating value diminishes slightly over time, polyurethane panels are better insulators. Being stronger, there is no need for structural wood in the panel like what is needed in EPS SIPS. This creates an unbroken layer of insulation around the envelope of the home.

Insulated concrete form (ICF) construction uses foam blocks to create a form into which concrete is poured. Unlike conventional concrete work, the form stays in place, providing a layer of insulation on either side of the wall. This interesting configuration creates a layer of thermal mass that essentially becomes energy storage since it is insulated on both sides. ICF homes can be fifty percent more energy efficient during the life of the home and much quieter than a common stick framed home. Concrete walls also make the home stronger, allowing it to better survive severe storms and earthquakes

There are as many as fifty manufactures of ICF form systems. Most use EPS foam to create the form walls and plastic to separate the foam, creating space for the concrete. The blocks are stacked like Legos[R], reinforced with rebar and then filled with concrete. Some ICFs are made with recycled materials such as mineralized wood chips and recycled EPS foam and cement.

Another engineered wall system that is gaining in popularity is a pre-cast concrete wall. Pre-cast concrete is mostly used for earth-bermed basements and lower floors, although they can be stacked three stories high.

The concrete wall is usually two to three inches thick and the interior of the wall is insulated with rigid insulation that has an R-value of 12.5. Pre-cast walls typically don't need a concrete footer as most below-grade walls do. All that is needed is a gravel trench that allows water to drain away. A slab floor is poured and drywall can easily be attached to special ribs on the interior. The exterior comes finished to look somewhat like stucco and can be painted. This wall system can be installed in a less than a day and provides an insulated concrete wall with a small amount of concrete use.

Autoclaved aerated concrete (AAC) is a product that uses cement to create a lightweight material that is filled with tiny air bubbles. The material comes pre-cut into blocks or panels. For residential construction, blocks that are from eight to twelve inches thick are used for the walls. AAC provides both structure and insulation; one product that does both! Any type of interior and exterior finish can be used, but plaster and stucco are the easiest and most popular.

AAC construction is fast and easy. Common carpentry tools can be used and the process is quickly learned. The material is easy to sculpt and architectural details can be adhered anywhere with the glue-like mortar. AAC walls are also very soundproof, since the tiny air bubbles act as thermal and sound insulation. AAC has been found to perform best in climates that require more cooling than heating, however; your location may affect the wall's performance.

These are a few of the most popular forms of engineered wall systems. Each has advantages and disadvantages, and each needs to be studied and understood by the builder before the decision is made to use them. Keep in mind that a house is a system made up of many other systems. A holistic approach needs to be taken to understand how all these systems interact. Engineered wall systems inherently provide a tighter building, and we have to allow the home to breathe through a well-designed passive or mechanical ventilation scheme to avoid moisture build-up on the interior of the building.

Today's engineered wall systems are usually designed to provide more insulation, a tighter building envelope, and a stronger wall than a stick-framed building. By utilizing these products and methods, we can save money and the environment.

More Information on Engineered Walls www.icfweb.com www.sips.org www.new-technologies.org/ECT/Civil/autoclaved.htm

Marcus Renner works with Appropriate Building Solutions, Inc a sustainable construction company in Western NC. He also teaches Sustainable Building classes at Appalachian State University. He can be reached at [e-mail]