Film is applied as a vapor barrier in the steel-framed prototype.

CARB is part of the Building America Program, an initiative sponsored by the U.S. Department of Energy with field support from the National Renewable Energy Laboratory (NREL). The CARB team consists of a select group of home builders, designers, and producers led by Steven Winter Associates (SWA), a building systems research and consulting firm in Norwalk, Connecticut.

Ryan Homes, a CARB team member, is one of the largest-volume builders in the United States. The CARB team used the Ryan production system to create two prototype high-performance town houses; one prototype was steel framed and the other used engineered wood with a structural insulated panel (SIP) envelope. CARB chose both steel and wood framing to demonstrate the use of different materials in the design. The two prototypes were constructed as internal units (not end units) in a row of seven town houses in the Dearbought development in Frederick, Maryland (see "What's inside the CARB Prototypes?"). 

What's Inside the CARB Prototypes?  The CARB high-performance town houses have approximately 1,400 ft2 of living space on two floors plus a full basement. The prototypes contain several key energy-saving features:  Windows and patio doors have glazing appropriate to their respective exposures; we used low-e glass and heat-reflective glazing on southern and western exposures to cut solar gain. Window frames and sills are made of Fibrex, a structural composite material made from reclaimed wood and vinyl.  We used round HVAC ducts instead of rectangular ones; air flow in round ducts is more efficient, and this enabled us to use smaller ducts with less material and lower costs. Duct runs are also short and straight; they have few turns, and are totally inside the thermal envelope.  We provided a vapor barrier against the steel studs in the steel-framed prototype. Since we couldn't staple a conventional vapor barrier into the studs, we used wide sheets of self-adhering film to obtain a tight barrier without punctures. For the SIP prototype, we used 4-inch exterior wall panels, with foam sealant at all joints to cut air infiltration.  In both prototypes, we installed advanced hydronic coil space heating, cooling with a heat pump, and mechanical ventilation with a heat recovery unit.  We located mechanical spaces, duct chases, bathrooms, and plumbing at the center of the floor plan in the joist space. This allowed us to eliminate the dropped mechanical chases in the ceiling.  The SIP/engineered wood unit had greater zone control. We achieved this by using three programmable thermostats and motorized zone dampers in the ductwork.

Testing the CARB Prototypes

We tested the prototypes' mechanical system for general performance, such as combustion efficiencies, duct leakage, system flow rates, and comfort measurements. The units also underwent Short Term Energy Monitoring (STEM) for three days to determine performance, such as heat loss and air infiltration. While several of these performance tests have been completed, other tests will continue for 12 months.

So far, the prototype units have shown a marked difference when compared with the control units. The STEM tests revealed that the SIP/engineered wood unit performed the best in terms of the annual heating load, peak heating load, and building load coefficient. In terms of ACH (measured at 15°F and 30°F outside temperatures), the SIP/ engineered wood unit performed better than either the control unit or the steel-framed unit (see Table 1). The slightly higher infiltration in the steel unit was believed to be due to the prepunched holes in the studs and particular construction details. We were able to identify cold spots in the walls around these holes using an infrared scanner.

Getting All the Ducts in a Row

These duct-sealing strategies produced dramatic results (see Table 2). We found much lower leakage in the prototypes than in the control. We also found less leakage in the steel than in the SIP prototype. We attributed this latter difference to the fact that different installation crews were used on the two prototypes.

Table 1. Comparison of Building Thermal Load Coefficient (UA) and Net Air Exchange for Prototypes and Control* Unit Total UA at 15°F Total UA at 30°F ACH at 15°F ACH at 30°F UA Conduction Control 351 322 0.44 0.33 230 Steel 389 332 0.58 0.39 227 SIP 288 269 0.32 0.25 200 *Infiltration at 15°F and 30°F outside temperature. Conduction UA = Total UA - Infiltration UA. Effective Volume = 16,750 ft3

Table 2. Comparison of Supply and Return Air Leakage (CFM) Unit Supply Air Leakage Return Air Leakage Control 126 141 Steel 9 89 SIP 55 122

Future CARB Prototypes

The completed units: from the left, SIP/engineered wood unit is second, steel unit is fourth, control unit (with blower door) sixth.

--Michael J. Crosbie

Michael J. Crosbie is a senior architect at Steven Winter Associates in Norwalk, CT.

Publication of this article was supported by the U.S. Department of Energy's Office of Building Technology, State and Community Programs, Energy Efficiency and Renewable Energy.

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