Buildings
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Windows by Arnie Katz
Are you looking into windows? There's so much information — and misinformation — out there today that it’s really hard to sort out the fact from the hype. To listen to some ads, you’d think that buying a particular brand of windows will save you enough to put the kids through college, buy a boat, take a vacation cruise to the Bahamas and still have enough left over to treat your window salesman to a fabulous dinner. So first, let’s get rid of one of the big myths out there: In most existing homes, replacing the windows is not a high priority for saving energy. Typically, the highest priority items are:
In other words: Don’t replace your windows expecting the energy savings to pay for the project! If, however, you need to replace the windows — they are rotting, they don’t work, you are remodeling part of the house — then by all means spend the extra money to get energy-efficient windows. You will definitely save enough to cover the extra cost for the energy-efficient models in a reasonable period of time. When buying new windows, people typically look for certain features: cost, style (Will they fit in with the look of the house?), durability (Are they made from materials that will last?), ease of cleaning, reputation of the manufacturer and warranty. To these factors, be sure to add energy performance. In a typical house, about 20 percent of the heating and cooling costs are caused by heat gain or loss through the windows — that is, heat flow through the glass and frame materials. Additional heat is lost or gained by air leakage around the windows or through leaky window seals. Keep this in mind when you’re thinking about savings. If 20 percent of your heating and cooling costs are through the windows, and the window company advertises a 50-percent savings, that means you might save 50 percent of 20 percent, or 10 percent of the heating and cooling portion of your energy bill if the windows are installed correctly, which will typically be about 50 percent of the total utility bill. Let’s say that another way. If your total utility bill (gas and electric) is $2,000 a year, approximately $1,000 of that will be for heating and cooling. Approximately $200 of that is caused by heat loss or gain through the windows. Good, new energy efficient windows can save you 50 percent of that, or $100 a year. One hundred dollars a year is nothing to sneeze at, and if utility rates continue to go up, so will your savings. Typically, the cost of upgrading to energy efficient windows is about one dollar per square foot of window, or several hundred dollars for all the windows in the house. Your savings will pay for the upgrade very quickly, and will then put money in your pocket every year. But those savings probably won’t pay for the total cost of the windows for some time. So, what’s an “energy-efficient” window and how do you know if you’re getting one? Fortunately, many manufacturers now have their windows rated by an independent rating group (National Fenestration Rating Council, www.nfrc.org ). Each window has a label that gives a rating on three characteristics of the window: U-Factor (or how good a window prevents heat from escaping), solar heat gain coefficient (SHGC) (measures how well a window blocks heat caused by sunlight) and visible transmittance (measures how much light comes through the window). Some manufacturers also have ratings done for air leakage and condensation resistance, but these are not required to get a label. In North Carolina , we typically recommend double-pane windows with a “low e” coating, which reduces the U-Factor and the SHGC. For both of these, the lower the number, the better the window will perform in our hot, sunny climate. Look for U-Factors of 0.40 or less and SHGC of 0.48 or less. Windows meeting these specs are available throughout the state at reasonable prices, and you can often find windows with better (lower) ratings than these. If the window hasn’t been rated, you have to go on whatever the manufacturer says. As with most things, most people in the window business are honest and provide good information. But there are people out there who make unsubstantiated claims. If they don’t have an NFRC rating, ask a lot of questions. In addition to the glass, you need to consider the frame of the window. Most window frames are made of wood, vinyl, fiberglass or metal. Metal frames conduct heat very well, so they are not very energy efficient. If you decide on metal frames, be sure there is a “thermal break” (usually a strip of plastic) in the frame, so that the metal doesn’t go straight through from inside to outside. If you are building a new home, there are a number of additional things to think about. It’s financially silly to buy windows that are not energy efficient or spend extra money on high efficiency windows and wipe out any savings by installing a large number of windows, very large ones or placing them in undesirable sides of the house with reference to the sun. Here in North Carolina , probably the most important thing you can do is minimize the amount of window area on the west and east sides of your house. These are the sides where the summer sun is coming in at a low angle and will hit the windows and heat up the house. Too many windows on the east and west sides will not only add dollars to your air conditioning bills, but it will also cause you to need a larger air conditioner and could easily cause comfort problems, particularly in rooms with lots of west-facing glass. Go drive around some of the high-end developments, where walls of glass are popular, facing the best views of the mountains or the golf course, the lake or the greenway. Go late in the afternoon on a bright summer day, and you’ll see that if these walls of glass face west, the drapes are closed to keep the afternoon sun out of the house. So much for the “view.” If you do want windows on the east or west sides, make them as small as possible to get the view, natural light or cross-ventilation you want and then look at ways to shade them — with trees, awnings, trellises, out-buildings, etc. South-facing windows need special attention. If your building site is appropriate, consider passive solar design, where the windows act as solar collectors in the winter to let in heat that is stored in building materials. For this to work, the house has to face the right way, overhangs must be designed to block out the summer sun and the size of the windows must be calculated properly. In this case, you’ll want to use “clear” double-pane glass on the south side rather than low-e coated glass. Information on passive solar design is available from the N.C. Solar Center in Raleigh (www.ncsc.ncsu.edu). In addition to heat flow through the glass and through the frame, energy use is affected by air leaking in through the window unit (how tight do the parts seal to each other) and around the window unit (How well is the window sealed into the frame of the home?) Common practice is to stuff fiberglass insulation around the window unit when it’s installed. This stuffed insulation doesn’t stop air leakage, but merely filters the air that’s coming through. Be sure to tell your builder you want that space sealed with caulk or non-expanding foam or something that actually stops air leaks. This is the correct way to install windows. Finally, the style of window you select can have an impact on energy efficiency. Casement or awning windows with compression-type weather-stripping will tend to be tighter than double-hung and other “slider” type windows. Once you compare the ratings and the materials, you’ll be in a better position to judge where the best value is. A good way to get a handle on the potential savings is to ask your heating and air conditioning contractor to calculate the heating and cooling loads of the house with standard and high performance windows. Will this enable him to downsize the unit? If his response is a blank stare, you might want to consider another HVAC contractor. But that’s another story. |
