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The Science of Building Performance

Author: David Leff
November, 2009 Issue
Building green is supposed to be the way in which each of us can do our part to reduce carbon emissions and slow global warming. However, in many ways, we’ve been distracted by “green” materials and are doing less than we think to solve the problem. Many products and materials used in home building are called green, and many services and trades related to home building now have green components. Often, these products are low in volatile organic compounds (VOCs) and contribute to better air quality in the home, or have a high-recycled content and reduce the amount of waste going into landfills. Solar hot water and photovoltaic (PV) systems reduce the use of fossil fuels and help slow global warming.

All of these goods and practices do make a difference. But the fact is, no house is really green unless it’s designed and built (or remodeled) to reduce the amount of energy it uses. The use of green materials and installation of solar energy systems are important and necessary, but unless a building is properly insulated—with as few air leaks as possible, correctly sized and oriented high-quality windows, and a properly sized and installed heating and cooling system—it will continue to unnecessarily contribute to the overheating of the planet. The fairly new field of building science, which studies the energy efficiency and environmental quality of buildings, is emerging from university classrooms and U.S. Department of Energy testing facilities into the mainstream world of home building and remodeling.

Building science has developed procedures and testing equipment that let a trained energy auditor go through a home and, in about four hours, locate air leaks in the building envelope, find uninsulated or poorly insulated surfaces, test for duct leaks in the heating and cooling system, test the air quality for excessive moisture and carbon monoxide, look for moisture problems around the home, and produce a report with recommendations for improving energy efficiency and environmental quality. An audit will also usually include a “blower door test” (in which the interior of the home is depressurized and smoke sticks or infrared cameras are used to locate air leaks and inadequately insulated walls and ceilings) and a duct blaster to test for leakage in the heating ducts.

In many cases, especially with older homes, the amount of air leaking through poorly insulated walls, ceilings and tiny penetrations around windows, doors, light fixtures, electrical outlets and pipes can be equivalent to a more than one-square-foot hole in an exterior wall. The duct system that carries heated or cooled air through a house often leaks at least 25 percent of its air. Not only does such a leak waste energy, but it can create an air pressure imbalance that may cause unheated and unfiltered air to be sucked into the house from a crawlspace or attic.

The good news is, other than the installation of PV or solar hot water systems, what improves the energy efficiency of an existing home the most is the “low-hanging fruit,” meaning those things that are the easiest and least expensive to do. Sealing air leaks in the building envelope has the greatest impact for the lowest cost. Every electrical or plumbing penetration through the floor, into the crawlspace or through the ceiling into the attic is a potential air leak. Sealing the often-oversized holes around the wires and pipes with caulk or expanding foam is an important first step.

If the insulation in the attic is old or poorly installed, it’s often easier to remove the old insulation (which creates easier access for air sealing) and install new, blown-in cellulose insulation. Older recessed light fixtures and ceiling fans weren’t designed to be covered with insulation, so the ceiling insulation is held back from the fixtures, causing a large hole in their surrounding insulation envelope. When the attic insulation is replaced or upgraded, it’s a good time to replace these old fixtures with airtight, insulation contact (AT-IC) fixtures.

Uninsulated floors can be insulated from the crawlspace, and uninsulated walls can be filled with cellulose insulation, which is blown into the wall cavities through small holes in the siding or through the interior drywall. Installing a vapor barrier over the dirt in the crawlspace is a cost effective and simple way to keep ground moisture from entering the house through leaks in the subfloor. Replacing old, single-glazed windows will help reduce heat loss in the winter and will keep the house cooler in the summer, but the return on that initial investment usually isn’t as high as with air sealing or insulation upgrades. The last piece of low-hanging fruit—the replacement of leaking heating and cooling ducts—isn’t a project for the do-it-yourselfer, but it is very important.

Tackle these problems first, and then, if it’s time to replace an old furnace, the new one won’t need to be as large, or if a PV or solar hot water system is installed, fewer panels will be needed, because the home’s energy loads will be so much less. If you’re planning to remodel your home anyway, be sure to include energy upgrades in your plans, and be sure your contractor is aware of this building-performance approach to remodeling.

There’s more good news: The recently established Sonoma County Energy Independence Program distributes funding to homeowners in Sonoma County who want to improve the energy efficiency of their homes [See “Financing a Greener World,” Aug. 2009]. This program will finance many energy upgrades, including the ones listed in this article, either alone or as part of a larger remodel, over a five- to 20-year term with an assessment (including interest) added to the property tax bill, making the entire payment tax deductible. In addition, tax credits in the 2009 federal economic stimulus package emphasize energy upgrades to existing buildings. As these programs are implemented, and as builders and homeowners become educated about the benefits of a building performance approach to home improvement, there’s potential for a 25 percent reduction in the amount of energy used in our homes, taking a significant step toward reducing the 21 percent of total carbon emissions produced by residential buildings in the United States.
David Leff is president of Leff Construction (, a Sonoma County design/build remodeling company. You can reach him at (707) 823-4899 or


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