October-November Special Focus: Energy Efficiency, Buildings, the Electric Grid
Throughout October and November, EETD presents a special series of articles, research highlights, and social media posts addressing some of its recent research on energy efficiency and other buildings-related topics, and the electric grid.
Tightening the envelope of homes should save energy by reducing the loss of heat when the exterior is cold, and the loss of cooled air when it is hot. Until now, it has not been clear for the current housing stock how much potential for energy savings exists in the U.S. stock of homes if all were brought up to codes requiring the tightening of the building envelope.
A research team in the Residential Building Systems group including Jennifer Logue, Brett Singer, Iain Walker and Max Sherman has now estimated the potential energy savings of implementing airtightness improvements or absolute standards, as well as adding mechanical ventilation throughout the U.S. housing stock. Logue, Singer, Walker and Sherman are scientists in the Lawrence Berkeley National Laboratory's (Berkeley Lab) Environmental Energy Technology (EETD).
The study predicts that tightening the envelope of all U.S. homes to the current average level of performance after an energy performance retrofit would reduce the residential sector's site energy demand by 0.72 quadrillion BTUs (0.76 exajoules) annually, or more than 10 percent of the energy required to heat and cools homes. But the study also points to differences in the relative savings of homes depending on what climate zone they are located in, and other factors such as building type. It provides information that can help improve the success of energy efficiency programs designed to save energy through weatherization and air sealing of homes.
Codes require adding mechanical ventilation to well sealed buildings to ensure that buildings have adequate air circulation, but this has the effect of slightly increasing the overall energy use of buildings. ASHRAE 62.2 required whole house mechanical ventilation in order to provide sufficient fresh air for occupant health. The standard also allows for credits that reduce the required mechanical ventilation if the home is sufficiently leaky (i.e. natural airflow through the home through cracks is sufficient to provide ventilation) The study found that, for the existing housing stock, adding ventilation to homes that are not leaky enough to provide sufficient ventilation would only increase annual site energy use in the stock by 0.05-0.07 quads.
The U.S. has about 113 million homes, and in total, they represent about 23 percent of energy use. Heating and cooling these homes requires about 5 quadrillion BTUs, or half of the residential sector's total energy use. Reducing the waste of heating and cooling energy could save quite a bit of energy and money, and various voluntary building codes, such as IECC building tightness standard, Canada's R2000 Standard and the Passive House Standard, are designed to provide guidance on expectable levels of building leakiness.
The results of the research are useful to policymakers—it points the way to optimizing the effectiveness of programs for improving the energy performance of homes and reducing energy costs by tailoring a program's approaches to factors such as the climate zone, household income, and a home's physical parameters. "It gives policymakers a wedge to improve programs by comparing incremental benefits of increasing air sealing effectiveness (or reaching more stringent air tightness targets) against the costs of achieving these higher levels of home performance" says Logue.
The research team developed a computer model to calculate the change in energy demand for each home in a national sample of more than 50,000 virtual homes developed using data from the Department of Energy's 2009 Residential Energy Consumption Survey. The sample's virtual homes model a variety of home types found in each climate zone of each state in the U.S. According to the RECS, the U.S. housing stock contains 63.2% detached houses, 24.8% multi-family homes, 5.9% attached homes, and 6.1% mobile homes.
Using this cohort of U.S. homes, they estimated the energy impact of tightening building envelopes and adding mechanical ventilation for a typical meteorological year. The research uses the ASHRAE 62.2 standard as the basis for adding ventilation to homes—all homes were assumed to have been provided with specified required levels of ventilation. Once all homes complied with ASHRAE 62.2, the research team studied the energy saving impacts of upgrading to a variety of different existing national and international standards on building envelope tightness.
In the first scenario, the study calculates how much energy would be required to bring all homes in the U.S. to only the ASHRAE 62.2 standard, providing sufficient ventilation for good indoor air quality. Four other scenarios provide estimates of energy savings resulting from bringing U.S. homes up to the 62.2 standard plus a tightness standard or constraint. For example, another scenario examines the result of improving the envelope airtightness of all homes at levels currently achieved by The Department of Energy's Weatherization Assistance Program (WAP) and non-WAP energy efficiency programs while complying with ASHRAE 62.2. The WAP is DOE's program aimed at improving the energy efficiency of low-income homeowners, and is significant as one of the largest federal programs to improve home energy performance in the U.S.
Another "advanced" scenario assumes the tightening of envelopes as necessary to ensure that each house reaches the current 90th percentile tightness for homes with similar key characteristics while complying with ASHRAE 62.2.
Other scenarios looked at the energy savings from a combination of ASHRAE 62.2 and three major international standards: IECC 2012 (International Energy Conservation Code), Canada's R2000, and the Passive House standard.
"Our study found that the annual energy impact of bringing the entire current stock of homes into compliance with ASHRAE 62.2 is relatively small; it would increase the annual site energy demand of the residential sector by less than one percent," says Logue.
The research also predicts that the average tightening would reduce the residential sector's site energy demand by 0.72 quadrillion BTUs (0.76 exajoules) annually, compared to the roughly 5 quadrillion BTUs annual energy use for heating and cooling all U.S. homes.
"We also estimated that using advanced methods of air sealing to get all homes to the of the tightest 10% would double the energy savings of tightening at current average improvement levels—representing about $22 billion in savings," she adds.
The researchers found that increasing the effectiveness of Weatherization Assistance Program and non-WAP retrofits to ensure that all homes reach 90th percentile air-tightness levels for homes of similar age and construction could double the energy impact of air sealing in these programs.
The value of this study to managers of energy efficiency programs at states, municipalities and utilities, Logue believes, is that it provides a considerable amount of data about how much savings can result from tightening the envelopes of homes by climate zone, and home characteristics. It provides tools to weigh the costs of these programs vs. their benefits according to such factors as where the homes are located, and what level of tightening is achieved.
This research was funded by the Department of Energy's Office of Energy Efficiency and Renewable Energy.
The paper, "Energy impacts of envelope tightening and mechanical ventilation for the U.S. residential sector," was written by Jennifer Logue, Max Sherman, Iain Walker, and Brett Singer of the Residential Building Systems Group.