May 12, 2014

Methodology for Estimated Energy Savings from Cost-Effective Air Sealing and Insulating

EPA estimates that homeowners can typically save up to 20% of heating and cooling costs (or up to 10% of total energy costs) by air sealing their homes and adding insulation in attics, floors over crawl spaces, and accessible basement rim joists. This estimate is based on energy modeling (using REM/Rate version 11.0) of cost-effective improvements made to ‘typical’ existing U.S. homes with a weighted composite of characteristics. The modeled results are corroborated by the field experience of professional building science contractors who have done air sealing and insulation work for more than 20 years.

Establishing a ‘Typical’ U.S. Existing House

The Residential Energy Consumption Survey (RECS) indicates that a large block of existing U.S. housing stock was constructed between 1975 and 1985, just after the 1973 oil embargo, when there was a new increased awareness of energy use in homes. As a result, EPA based its modeling around the common construction characteristics of homes built in this era as a proxy for a ‘typical’ existing U.S. home.

Construction characteristics for the 1975–85 era were determined based on a review of RECS data from the U.S. Department of Energy, 1997 EDS (Energy Data Sourcebook for the U.S. Residential Sector and earlier versions) data from Lawrence Berkeley National Laboratory, and other supporting data, including anecdotal experience of ENERGY STAR staff and stakeholders. Based on these sources, EPA assumed the following characteristics for a house from the 1975–85 era:

  • 1,500 square feet of conditioned floor area;
  • 14% window-to-floor-area ratio;
  • 20% duct leakage to the outside;
  • three bedrooms; and
  • “stick” construction (wooden studs, joists and rafters), with batt insulation in walls and blown insulation in attics.

Geographic climate factors, regional construction styles (e.g., basement, crawl space or slab-on-grade), and fuel type characteristics (e.g., natural gas or electricity) were then proportionally weighted; and estimated energy use calculated for “typical” composite houses in two climates that represented a weighted average for a Northern and a Southern home.

Estimating Energy Savings from Improvements Made to the ‘Typical’ Home

For the purpose of energy estimating savings, EPA assumed that a knowledgeable homeowner or contractor could cost-effectively:

  • Seal air leaks throughout the house, focusing on leaks to the attic space, through the foundation, and around windows and doors. An average documented baseline value of 0.91 ACHNAT (natural air changes per hour) was used for Northern homes and 0.94 ACHNAT was used for Southern homes. Both Northern and Southern homes were estimated to be improved to a leakage level of 0.50 ACHNAT.
  • Add insulation to improve R-values from the average documented attic insulation values of R-15 in the North and R-13 in the South to R-38; improve basement rim joists from R-0 to R-11; and improve floors over crawl spaces from R-0 to R-11.

Note: In estimating savings opportunities, EPA considered that the 1975–85 construction era coincided with the period after the 1973 oil-embargo when early residential energy conservation measures were first becoming widespread (e.g., storm windows over single-pane/clear glass windows, some caulking & sealing to reduce air leaks, increased attic insulation, etc.). EPA also assumed that original, as-built HVAC and water heating equipment was replaced in the 1990s by 1993–2000 MEC/NAECA-era equipment.

Based on these projected cost-effective improvements, EPA estimates the following potential energy and utility bill savings:

LocationSite MMBTU¹ SavingsUtility Bill Savings (2007 data²)
NorthTotal House14%12%
Heating and cooling only20%19%
SouthTotal House13%11%
Heating and cooling only23%20%