Builders on Duct Location
It is common practice in California to put ducts outside the conditioned space, in attics and crawlspaces. These duct systems are typically 50%-70% efficient due mostly to air leakage and heat transfer losses, while sealed conditioned-space duct installations have efficiencies over 90%. Our group at Lawrence Berkeley National Laboratory (LBNL) wanted to demonstrate that builders can market homes with increased energy efficiency, while incurring no increased first costs. We offered incentives to tract home builders in California to install forced-air distribution systems inside the conditioned space of a few houses and allow us to monitor energy consumption. We reasoned that builders should be able to cut costs by downsizing the installed heating and cooling equipment in houses with conditioned-space ducts. However, the realities of the market presented several roadblocks for builders who wanted to participate.
We proposed two designs for installing sealed conditioned-space ducts in new California housing developments, the majority of which have single- or two-story houses with concrete slab-on-grade foundations. In single-story houses, duct systems inside conditioned spaces would run above false ceilings--which are enclosed, sealed, and insulated from the attic--or inside soffits running along the upper part of walls (see Figure 1). In two-story houses, webbed floor joists between stories provide a natural space to install conditioned space ducts (see Figure 2). Many builders already install ducts between stories but do not seal the space from the outside, so the ducts are not truly in conditioned space.
In both designs, the ducts must be sealed, or some rooms may be inadequately conditioned. Also, in cases where supply and return ducts are in the same space, conditioned air from supply leaks can be drawn into return leaks without circulating through the house.
We contacted 12 California builders, who represent a substantial portion of 1995 California housing starts. This group included low income home builders, builders in rapidly expanding markets, and builders developing upscale tracts. The builders were generally interested in the project but believed that there would be a net cost increase after they established the practice. They cited several issues, including placement and timing of duct installation during construction, initial cost and interest rates, marketability and aesthetics, compliance with state energy efficiency standards, and verification of energy efficiency measures.
Builders are concerned about extra labor costs from sealing and insulating the space containing ducts, as well as the timing of duct installation. The current practice is to install the forced air system during the last phases of construction. The advantage is that construction workers are less likely to damage the bulky duct systems. One builder said that, in two-story houses, the need to install ducts before the second story is floored complicates the timing of duct installation by HVAC contractors. In addition, most open-webbed trusses are made of metal and have sharp edges, so the potential for tearing or puncturing flexible plastic ducts is high.
With current interest rates around 9% in California, housing sales are down, along with enthusiasm for investing in an unproven construction practice. The builders we interviewed were reluctant to increase their initial cost. This was particularly true of builders producing houses for the first-time buyer. (Builders producing more upscale housing were more receptive to our proposal.) We offered to offset extra design and construction costs for the first few houses, but builders were still concerned that the initial cost would remain high in the future.
Most builders felt that placing the ducts in the conditioned space would detract from their houses' aesthetic value, particularly in single-story slab-on-grade houses. Builders we contacted were reluctant to use soffits because of their perceived unsightliness and the extra labor (and thus cost) involved in sealing the soffits from unconditioned spaces. Sealed false ceilings over a central hallway were considered a more workable solution, because the enclosed duct space could easily be sealed and insulated from the attic. However, this option would work only for ducts servicing rooms off the hallway, not necessarily for all rooms in the house.
Because much depends upon the house floor plan, viable solutions could be determined only on a case-by-case basis. Most of the aesthetic issues in single-story houses are automatically addressed in two-story houses, because the ducts are installed in the space between the floors.
Figure1.Proposed dropped ceiling or soffit system for single-story houses. The figure shows a sealed-combustion furnace located in a closet indoors and ducts in a dropped ceiling in the hallway and bathroom. The dropped ceiling is sealed and insulated to prevent leakage and heat loss to the attic.
Figure 2.Proposed webbed floor joist system for two-story houses. As in Figure 1, the sealed combustion furnace is located in an inside closet, but supply ducts are located in the space between the floors. This space is sealed from the wall cavities, to prevent conditioned air from leaking out through the attic and crawlspace.
State Efficiency Standards
Under California's Title 24 Efficiency Standards, an efficiency credit can be claimed for conditioned space ducts if the entire system--the ducts and the heating and cooling equipment--is placed inside the conditioned space. This presents problems for the majority of builders we contacted, who install gas furnaces in vented closets or garages. Title 24 does not award conditioned space duct credit if the furnace closet is vented to the outside. This allows only sealed-combustion furnaces (those with combustion supply air and exhaust gases vented directly to outside, without using draft diverters). However, sealed combustion is only available on high-efficiency (>90% Annual Fuel Utilization Efficiency) furnaces, which cost approximately $500 more than standard furnaces. In addition, the credit offered is small compared to those awarded for other, less expensive measures, such as upgrading single-glazed windows to double-glazed, or doubling the attic insulation. If the credit were higher for conditioned-space ducts, builders would have more incentive to put ducts inside. On the other hand, no insulation is required for ducts in conditioned spaces under Title 24, which means that there are potential savings in materials and labor costs.
Verification of energy efficiency measures in new construction was a problem mentioned by a builder of upscale houses. He places a premium on building energy-efficient houses and wants this reflected in a standard verification process. For example, the local utility gave a seal of approval to new houses that contained certain energy efficiency measures. A house earned the seal after inspection by utility technicians. His main complaint about this process was that there was no verification of energy efficiency measures involved, and competitors who he believed were doing low-quality work were having no trouble getting the seal for their houses. Regarding our proposed project, he indicated that he could market his houses more effectively if there were some verifiable form of efficiency certification not only for ducts, but for any efficiency measure he would consider installing.
Breaking Down Barriers
Installing conditioned-space ducts may require significant changes in design and construction techniques and have an uncertain impact on marketability. The issues that came up in our discussions with builders demonstrate that measures that require architectural integration face significantly larger market barriers than those that are easily installed (such as efficient lighting or added insulation). However, to overcome these barriers, two elements are already in place. First, two-story houses are already being built with ducts between floors. Second, California's Title 24 recognizes conditioned-space ducts as an energy efficiency measure.
Our strategy now focuses on two-story houses. Of the 12 builders we originally contacted, two remain interested. Also, we may have found a willing participant through contacts with an association of home builders.
David A. Jump is a visiting staff scientist at Lawrence Berkeley National Laboratory.