By Carl Seville, CR
As we enter the hottest season of the year, we naturally start thinking about ways to keep heat out of the house to save energy and cut our utility bills. But heat gain and loss is a problem all year round; after all, the problem in winter isn't that cold air is entering our homes. It's that the air that we've paid to heat to a comfortable level is escaping. It makes sense that if we can solve the problem of heat gain for our customers, we've fixed heat loss at the same time. Let's take a look at killing two unnecessarily expensive birds with one stone.
The first step to making a house energy-efficient is to manage heat entering or leaving the building. That happens through conduction, convection, and radiation. The relative importance of conduction, convection, and radiation depends on your local climate. Convection is an issue in almost all regions; conduction is most important in very extreme climates; and radiation has the most effect on buildings where the sun shines the brightest.
- Convection creates what is called the "stack effect"—the result of cool air entering a house at lower levels and rising to replace hot air that vents out at higher levels. When properly controlled, this natural ventilation can keep a house comfortable during mild months. But when the stack effect occurs from unintentional leaks in the building envelope during the hottest and coldest months of the year, it wastes energy and reduces comfort and even health by bringing dirty, unconditioned air inside. Sealing air leaks and open chases from floor to floor will reduce heat loss through convection.
- Conduction, the movement of heat through solids, is most apparent in cold climates, where the temperature outside can be more than 80 degrees colder than the interior. These extreme differences can cause a house to lose much of its heat directly through the ceilings, floors and walls, in that order. In all climates, ceiling or roof insulation is very important. As the temperature becomes milder, floor and wall insulation become less important; heat moves more slowly with less difference between the inside and outside temperatures.
No matter what the climate, though, insulation must be installed in direct contact with an air barrier to work effectively. It cannot be compressed or have voids in it, either. No matter how much insulation you install, if it allows air to leak, it will won't do much good.
- Radiation affects a house primarily through solar gain—the addition of heat to a house from direct contact with the sun's rays. In warm weather, the goal is to keep the sun from overheating a house. On cooler days, though, the sun can be used to heat a house, either partially or completely, depending on the design. Using the natural heat of the sun to condition a house is called passive solar heating. Proper orientation of a house and its windows will help by allowing the sun to provide heat when desired and block the sun's rays when not needed.
When you're adding or changing windows in a house, think about how the sun hits each wall. North walls receive almost no direct sunlight, while south walls get full sun at different angle, depending on the season. East walls get direct sun in the morning, as west walls do in the afternoon.
East and west sun is the most difficult to control and the most prone to overheating a house. It is best to limit the windows on the East and West faces of a home. If you must put windows on these walls, use exterior shades, low-e glass, or large porches to reduce heat gain. On the south side or the house, relatively shallow overhangs above windows will provide complete shade in the heat of the summer while allowing the sun to shine in during the colder winter months. Proper orientation, shading, and window placement can reduce air conditioning costs by as much as 30% at essentially no additional cost to the builder or owner.
Once you have controlled the heat flow in and out of a house, you are well on your way to making it a very efficient, healthy, and green home.
Carl Seville, CR, is an Atlanta-based consultant focusing on sustainable building and remodeling. He is a former partner in Sawhorse Construction and has won several awards for his work in green remodeling, including the 2005 NAHB Green Advocate of the Year: Remodeling. For more information, visit www.SevilleConsulting.com.
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