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7 Steps to an Energy-Efficient House
by Betsy Pettit
 Editor's introduction: With energy prices rising again, many homeowners are planning energy-efficiency improvements to their homes. But most people are unsure of where to begin, and even seasoned builders don’t always know which priorities should rise to the top of the list. Betsy Pettit, an architect at Building Science Corporation, recommends starting where you can get the most bang for the buck.
If you can only afford one step, replacing an aging heating appliance may be the best investment. According to Pettit, "An old furnace or boiler is often the worst energy user in an old house. Many houses built prior to 1920 still have old coal-fired boilers that were converted to gas or oil. These units are workhorses, but use a lot of energy. A new furnace or boiler can save energy dollars right away. Replacing window air conditioners with a central system also can save energy right away, as long as the ductwork has been placed in the conditioned space."
If you're planning a major retrofit involving multiple steps, however, it's probably best to start in the basement.
Step 1: Bring the basement or crawl space inside the home's thermal envelope
Warm, dry basements and crawlspaces can extend living and storage space. In an old house, a wet basement is frequently the source of high levels of indoor humidity. They can also harbor mold growth that gets distributed around the house.
Closed-cell spray polyurethane foam — installed around the rim joist areas and on the basement walls — is a fast, effective way to bring these areas into the conditioned space while sealing cracks between the foundation and floor framing that let air into the house. Most building inspectors require foam installed on basement walls to be protected with a layer of gypsum drywall as an ignition barrier.
Step 2: To stop air leaks and reduce heat loss, seal up your home's cap
If air leaks in at the bottom of a house, it also leaks out at the top, making the house cold and drafty in winter. A poorly insulated roof also can make a house hot in summer.
Air-sealing work should always accompany any insulation improvements. Using spray foam under a roof can eliminate the need for roof venting, which is tricky in complicated roofs. Other insulation options include cellulose on the attic floor or rigid insulation on top of the roof sheathing.
Step 3: Insulate your walls
Filling empty wall cavities with cellulose is a cheap, easy, effective way to warm up an old house. Blowing insulation into existing wall cavities is an art, to be sure, but many contractors have been doing it for years. By checking the quality of the job with an infrared camera, an insulation contractor can verify that all voids have been filled.
If the home's existing siding is nearing the end of its useful life, it's possible to install a thick layer of rigid foam on top of the existing sheathing at the same time that the siding is replaced.
Step 4: Replace your windows
With the bottom and top of the house sealed and insulated, as well as the walls, windows represent the next opportunity. Old windows often leak both air and water into the house. They might not open and close properly, and are sometimes be obscured by unattractive storm windows and screens that reduce the amount of light that can enter. Moreover, they rarely have low-e glazing.
Properly installed Energy Star (or better) windows seal the holes in the walls to keep out water and weather extremes. If high-quality glazing is specified, the windows can admit useful solar gain during the winter, while still preventing excessive heat loss at night. To prevent summer overheating, the best glazing for east and west windows is usually low-solar-gain glazing.
Properly specified new replacement windows should reduce your energy bills. Since replacement windows are expensive, though, don't expect a fast payback on your investment. But if you decide to replace your windows, at least you can enjoy higher comfort from Day One.
Step 5: Replace your furnace, boiler, or water heater
An old furnace or boiler is often the worst energy user in an old house. Many houses built prior to 1920 still have old coal-fired boilers that were converted to gas or oil. These units are workhorses, but use a lot of energy. A new furnace or boiler can save energy dollars right away.
Replacing window air conditioners with a central system also can save energy right away, as long as the ductwork is located in the conditioned space. Solar water heating is a good option to add here if you can afford it, but at the very least, upgrade the efficiency of hot-water production by coupling the tank to the boiler.
Step 6: Buy Energy Star (or better) fixtures, appliances, and lighting
Once you have reduced your space-conditioning and water-heating loads, the lighting, appliance, and plug load will be your next big energy item. A new Energy Star refrigerator will use 15% less energy than a standard model. Replacing old light fixtures with pin-based compact-fluorescent fixtures ensures your electric
bill will stay low.
Step 7: Install renewable-energy equipment
Once your energy consumption has been reduced significantly, it may be reasonable to produce your own energy with a photovoltaic system, wind turbine, or even a small hydro system (if you happen to have a stream nearby). Energy-efficiency improvements yield a faster return on your investment than renewable energy equipment, though — so until you slash your energy use, it makes no sense to invest in photovoltaics or wind. Conservation is still the cheapest game in town.