Bridging the gap

Alternative envelopes
I’ve frequently written about the importance of the building envelope in reducing energy end-use in homes. The building envelope consists of walls, windows, roof, floor and foundation and has the greatest impact on heating and cooling loads. Let’s take a look at some “alternative” wall systems that offer improved energy efficiency and greater sustainability when compared to conventional construction. These systems are “technically mature”—meaning they have been around for many years and are affordable for residential construction in the North State region.

Most folks are aware that the most common residential wall system in the United States is “stick-frame” or “stud-wall” construction often insulated with fiberglass batts. This construction has been the primary choice of builders for more than 100 years, popular due to its structural integrity and low materials and labor costs. A wood-framed wall also provides a modest degree of heat-flow resistance, but thermal performance is its weakest attribute in this era of rapidly rising energy costs.

Conventional stick-frame walls suffer from an effect known as thermal bridging. Wood studs, typically made of fir, transfer three to fives times more heat per unit area than insulation due to its higher thermal conductivity. In other words, wood studs act as a “bridge” that short circuits the thermal integrity of the wall. The effect is significant because the solid-wood area comprises about 25 percent of a typical stick-frame wall when one considers the framing around windows, doors, corners, and the top/bottom wall plates. This effect reduces the nominal R-19 insulation for 2x6 walls to about R-14 for the whole wall.

What’s in a wall
Optimum-Value Engineered (OVE) walls represent an advanced wood-framing system that uses much less solid wood and reduces labor time while offering improved thermal performance over conventional stick-frame walls. OVE walls typically have wood studs spaced at 24 inches (rather than the typical 16 inches), which are aligned with the usual 24-inch truss or rafter spacing. In addition to providing more space for insulation, this technique reduces much of the solid wood normally needed to frame windows and doors without loss of structural integrity. OVE construction also includes improved methods of sealing gaps and joints to reduce air infiltration.

Structural Insulated Panel (SIP) walls consist of expanded polystyrene (EPS) sandwiched and glued to two structural skins of oriented strand board (OSB). The result is an engineered panel that provides structural framing, insulation and exterior sheathing in a solid, one-piece component. SIP-wall homes have very significant advantages over stick-frame homes in terms of construction time, thermal efficiency, sound proofing, shear strength and air-tightness. Because thermal bridging is eliminated altogether and EPS is a superior insulator, the resulting whole-wall R-value of a 2x6-equivalent SIP wall is R-22!

Insulating-concrete walls offers the ultimate in strength and longevity. There are many variations in their construction; the most common are Insulating Concrete Form (ICF) walls. ICFs are EPS forms that hold concrete in place during curing and remain in place to provide thermal insulation for the wall. The forms come in many different shapes unique to the manufacturer, but all tend to sandwich or surround the concrete and produce a wall with a monolithic concrete core. ICFs do not suffer from thermal bridging since the insulating form creates a continuous wall layer. ICF walls also provide thermal mass that can reduce cooling loads in hot climates.

Because OVE, SIP and insulating-concrete walls produce significantly higher whole-wall R-values than conventional stick-frame walls, a homeowner’s energy savings will more than offset any additional up-front costs over a relatively short period of time. Check back in with Sustainable Space in the coming weeks, as I present a detailed look at the construction, performance, costs and sustainable benefits of these wall systems.