Windows and the energy code: what every Georgia homeowner should know: make a clear choices with efficiency expert Parker Snyder.
Homeowners in Georgia are benefiting from change in the Energy Code that requires builders to install high performance windows. Effective January 1, 2004, the Georgia Energy Code required windows to have a Solar Heat Gain Coefficient (SHGC) of less that 0.40. For most builders the code change meant switching to Low-E windows. For homeowners, the better windows meant less money spent on expensive air conditioning. Windows are just one component of whole house energy efficiency, so let's review the other factors of energy efficiency.
Energy efficient homes have a well-defined "envelope," analogous to the skin of an inflated balloon. Holes in the balloon cause it to deflate, in the same way that air enters or exits a house through holes, seams, and cracks. In the south, hot summer air infiltrates through an incomplete envelope and the air stream carries excess moisture, requiring the air conditioner to work harder. A tightly sealed envelope will prevent moisture from entering the home. After sealing the envelope, its important to correctly size heating and cooling systems.
Since the 1950's, central heating and cooling has been widely used in the south to control temperature and humidity. Most people are comfortable between sixty to eighty degrees Fahrenheit at a relative humidity (RH) between thirty and fifty percent. Most air conditioners are controlled by a thermostat, which reads temperature, even though excess moisture can be a source of discomfort as well. The air conditioner will turn on if the temperature is above the thermostat setting, and will turn off once the temperature setting has been reached, but it will not turn on if there are levels of high humidity.
The occupants may become uncomfortable if an air conditioner runs for only five or ten minutes an hour, because the system will fail to remove enough moisture. To condition and cool effectively, an air conditioner should be slightly undersized. It will run more consistently and control both temperature and humidity. In the summer, showers and cooking can add moisture to air that may already be high in humidity, especially if the home has excessive air infiltration or water is present in the basement or crawlspace. A properly sized air conditioner will remove the moisture that could create the conditions for mildew and mold growth.
All building materials have an R-value, which represents resistance to heat flow. As the R-value increases, resistance to heat flow increases. For example, the code minimum is R13 in exterior walls. R30 is the minimum in flat ceilings and R5 on foundation walls. An envelope must be sufficiently insulated to meet the requirements of the Georgia Energy Code.
To determine if a home meets the Georgia Energy Code, input each component of the envelope into a free residential software program called RESCheck[TM] available for download at www.energycodes.gov. RESCheck allows a builder to make a tradeoff. For example, a builder may increase the percentage of allowable window area by increasing the R-value of attic insulation.
In a typical home, windows account for ten to twenty percent of the surface area through which heat, moisture and light will transfer. Despite the small percentage of surface area, windows can cause serious comfort problems because of their relatively poor performance compared to other building components. Windows are assigned a U factor, which is simply the inverse of the R-value. A window with a U factor of 0.33 has an R-value of only R3. This means that the best windows allow as much as three to four times the heat transfer as average exterior wall. For this reason, high performance windows are an excellent, suggested investment.
Solar heat gain should be considerable on the east and west sides of a home because the midday summer sun is strongest, so architects provide daylight by utilizing the principles of passive solar design and intentionally minimize east and west-facing windows. South-facing windows are beneficial because they allow the sun to heat the home in the winter. It would be possible to reduce a home's cooling needs if a window allowed light to enter but reduced unwanted solar heat gain. Low emissivity (Low-E) windows provide that benefit--they allow light to enter but reduce solar heat gain. The transparent metallic coating works like aluminum foil and does not radiate heat, minimizing solar heat gain.
Low-E windows were adopted in Georgia to minimize solar heat gain. The code says all glazing must have an average solar heat gain coefficient (SHGC) of less than 0.40. Glazing includes glass windows, doors, sidelights and transoms--the transparent solid surfaces through which light travels. The average value of all glazing must be less than 0.40, so glass doors and skylights must be factored in as well. The Georgia Energy Code allows a five percent exemption to accommodate stained glass or a similar glass surface that would otherwise fail the requirement.
How do Low-E windows work? The surface of the sun radiates heat onto furniture, carpets and walls. The surfaces then heat up the room. If you stand in front of a window on a cold winter day its possible to feel the sun's heat as it radiates onto the surface of your body. When standing next to a Low-E window on a sunny day you won't feel nearly as much heat as you would standing by a regular window.
Remember, the SHGC is not the only value that describes a window's performance: the U factor, a measure of thermal conductivity, is a measure of a window's winter performance. Windows with low U factors allow less heat to conduct through the envelope to the outside. To be in compliance with the Georgia Energy Code, windows must have an average U factor of less than 0.65. A window with a dense, inert gas insert can reduce heat transfer even further. Argon seems to be window manufacturers' most common choice.
In new construction, a window with an SHGC and a U factor of around 0.35 is usually a good buy. Wood framed, double paned, Low-E, argon filled windows will have values at or below 0.35. As the values go below 0.35, the added expense is not typically justified because of the relatively small additional benefit. In renovations, adding new, high performance windows may initially seem cost-prohibitive, but they do save money in the long run.
All this technical talk about windows might leave you wondering, "Why put in new windows at all?" Common sense tells us that daylight is beneficial. It's dynamic, unobtrusive and lights a space for no cost at all. During the winter, solar heat can make a home more comfortable and reduce the cost of heating. Additionally, real estate agents have remarked that homes with new windows tend to sell for more than homes without them. For all of the above reasons, high performance windows are worth the investment.
Parker Snyder is a project manager with the EarthCraft House program, a residential green building program that is a partnership between Atlanta-based nonprofit Southface and the Greater Atlanta Home Builders Association. Interested in learn more about the program or finding a builder or renovator that meets EarthCraft House standards? Visit www.earthcrafthouse.com. Free fact sheets on a variety of home efficiency and resource conservation topics are available at www.southface.org.
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|Publication:||New Life Journal|
|Date:||Apr 1, 2005|
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