Figure 1: Annual Average Ventilation Costs of the Current U.S. Single-Family Housing Stock ($/year/house).
Infiltration and ventilation in dwellings is conventionally believed to account for one-third to one-half of space conditioning energy. Unfortunately, there is not a great deal of measurement data or analysis to substantiate this assumption. As energy conservation improvements to the thermal envelope continue, the fraction of energy consumed by the conditioning of air may increase. Air-tightening programs, while decreasing energy requirements, have the tendency to decrease ventilation and its associated energy penalty at the possible expense of adequate indoor air quality. Therefore, more energy may be spent on conditioning air.
In a recent report*, we used existing databases to estimate the energy and indoor air quality liabilities associated with residential ventilation in the U.S. housing stock, and how scenarios of energy conservation and ventilation strategies changed those liabilities. ASHRAE standards 62, 119, and 136 guided us in determining acceptable ventilation levels and energy requirements.
A straightforward modeling procedure, described in the report's appendix, can be used to estimate a house's heating and cooling demand, as well as its air-change rate. Rather than applying this to each of the 75 million single-family households in the U.S., we have combined data from a variety of sources using database management tools to develop average and aggregate estimates from each data source. The 1990 U.S. Census provided the number of houses for each county; the type and size of the houses were derived from DOE's Residential Energy Consumption Survey; their leakage properties were from the LBNL Leakage Database; and weather data came from standard databases of 240 sites and representative years for each site.
Characteristics of Current Stock
The ventilation rate of the stock is dominated by infiltration due to envelope leakage and is calculated from leakage distribution and weather data. These rates were calculated only to determine acceptable indoor air quality, not for energy calculations. We estimate that the average effective air-change rate is 1.1 air changes per hour (ACH) for the U.S., and that about 95 percent of current stock meets the intent of ASHRAE standard 62. The standard requires a minimum ventilation rate of 0.35 ACH.
Energy impacts associated with the high infiltration rates are large. The heating load attributable to infiltration and ventilation in the current stock is 3.4 EJ (exajoules), and the cooling load is 0.8 EJ. Using our air leakage and other databases, the estimated national annual cost to provide this much ventilation is $6 billion per year. The average annual cost per house would thus be $820/year, with costs ranging from $170 to $2,100 per house. The national cost distribution is shown in Figure 1.
Alternative Scenarios
The high cost associated with residential ventilation suggested that there may be cost-effective ways to reduce the infiltration rate. We developed a base case that was similar to the U.S. existing stock described above, and two alternative scenarios to test this hypothesis. In the ASHRAE scenario, the goal is to meet ASHRAE air tightness standard 119. The envelope is tightened as needed, and mechanical ventilation is supplied if necessary. The Scandinavian scenario is similar, except the tightness level is increased by a factor of two, based on Northern European trends.
In the ASHRAE scenario, the effective air-change rates range from 0.48 ACH to 1.18 ACH, with a national average of 0.52 ACH. The total energy load for the U.S. is about 1.8 EJ. The national annual cost is $3.6 billion, a reduction of $2.4 billion over the base case. The annualized cost of ventilation is $490/year for the average house, with a range from $130/yr to $1,000/yr. The annualized cost reduction is not as large as energy reduction due to purchase and operating cost of a mechanical ventilation system.
The Scandinavian scenario is modeled after the northern European shift toward tighter building envelopes and a small number of operable inlets. The optimal system configuration uses 1.6 EJ and has a national annual operating cost of $4 billion, a reduction of $2 billion over the base case. The annualized cost is about $550/yr for the average house, ranging from $210 to $860/yr per house.
For the country as a whole, the average cost saving is $330/house for the ASHRAE case and $270/house for the Scandinavian case. Assuming that, on average, house air-tightening costs $1,000/house and that the ventilation system operating cost savings are applied to this effort, a homeowner could expect a payback of under five years for air-tightening in either scenario.
--Max Sherman and Nance Matson
* "Residential Ventilation and Energy Characteristics," LBNL-39036, available from the authors; to be published in ASHRAE Transactions.
Max Sherman
Indoor Environment Program
(510) 486-4022; (510) 486-6658 fax
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