Energy-Efficient Windows 101
By Ray Kamada
Window technology is complex and advancing rapidly, but the key word is tradeoffs. Better energy features, looks, construction, and durability all add to the cost. Fiberglass frames with low-E glazings look like the wave of the future. But learn as much as you can before buying and check the National Fenestration Rating Council (NFRC) ratings.
It's About Tradeoffs
There's a lot to learn here because window technology is complex and has improved rapidly. For now, it's about tradeoffs: cost vs. energy efficiency vs. maintenance vs. longevity vs. beauty. Generally speaking, the more glazes, inert gas, and low-E coatings, the better the energy efficiency. However, you also have to consider the framing materials. Each improved component adds to the cost.
Starting at the high-end, triple or quadruple-glazed, krypton-filled, multiple low-E coated, wood-framed windows look beautiful, conserve lots of energy, and are costly. People love the look, but wood requires maintenance. It's subject to chipping, cracks, and both wet and dry rot, unless it's clad in aluminium, vinyl, or fiberglass. Less expensive, low-E windows are usually vinyl-framed. Like wood, vinyl passes heat slowly. However, most people find the look less desirable, and vinyl may dent, degrade from UV exposure, and possibly sag in extreme heat. Titanium dioxide coatings, honeycomb construction, and steel-reinforcement are said to address the latter issues, at more cost, of course.
Meanwhile, aluminum frames are strong, durable, and need little maintenance. However, they conduct heat rapidly, and costs vary widely. Fiberglass windows are fairly new and therefore expensive. They are strong, durable, won't warp, resist heat losses better than even wood or vinyl, and require little maintenance. Like vinyl, initial color options are limited, unless they're veneered. However, you can paint fiberglass, and paint for vinyl is now also available. Wood interior/fiberglass exterior windows make a great, but expensive, composite. If you're opting for durability and extreme energy efficiency, fiberglass frames seem like the way to go.
Smaller windows may lose more heat per window area, because the ratio of frame to pane area goes up. So, French doors and windows, with lots of framing and little panes, can be lovely. But if they're aluminum-framed, you lose the value of low-E coatings and multiple glazes. On the other hand, a faux grill adds the French effect without incurring extra heat losses. At the other end, some very high-end frames in fiberglass or composites are packed with insulation for extreme energy efficiency.
Air Infiltration and Overpressure
Regarding other heat losses, air infiltration is often overlooked. Casement and awning windows that close onto compression gaskets are generally tighter than double-hung or slider windows. Though poor installation can make a mockery of this figure, look for infiltration rates below 0.06 ft.3 per minute. There are also some issues with inert gas infills. Between ambient conditions and sealing technology, it's not clear how long low heat-conducting, inert, argon gas will last. Some say 5 to 20 years, unless the seal breaks. If your window fogs now, the seal's broken and your argon's gone. Check the seal warranty. If you're building or retrofitting at high elevation, consider this: Significant overpressures and leakage may occur, if the windows are installed at an elevation much different from where they were made. Extreme differences may be hazardous. In these cases, install windows with pressure-equalizing valves.
Bang for the Buck
With a typical house wall insulated to R-19, wall studs and insulation breaks drop the effective value to maybe R-14. Suppose the wall includes some single pane windows that account for 25% of the wall area. Almost 5 times more heat passes through the windows as the rest of the wall. With typical low-E windows at U=0.35, this figure drops from 5 to about 1.6. On cold nights, you can feel the difference. For a high-end window with U=0.2, this number only drops to 1.2. You could easily pay more than double for the extra insulation. So, unless you're also super-insulating the walls to R-30 or better, really high-end windows won't be cost-effective for you.
But let's get real a moment. Unless it's custom construction, most of us don't get to pick our windows. Sun angles and exposures are whatever the developer left us with. And we're not likely to spend thousands casually for a complete retrofit, unless our old windows are rotting and need replacement anyway. If new windows are beyond your budget, less costly options are available. For example, if you feel a cold draft even when your windows are closed, some careful caulking, sealing, and weather stripping provide a great return on your dollar. You can also add stick-on plastic, low-E films and insulated drapes to your old, single pane windows for less than the cost of window replacements. Use curved-end valances to enclose the whole frame. To conserve more heat, extend the drapes all the way to the floor.
Here's my own low-budget idea. I keep wanting to try do-it-yourself panels of 1- or 2-in. thick styrofoam, covered in tasteful, decorative fabric, cut to insert snugly, and velcroed to each other and the window frames. For winter nights, I think the idea would work wonderfully, but my designer wife absolutely refuses to hear it. Like I said, there are many tradeoffs. Some are more equal than others.
Window Buzz Words
Triple-glazed, low-E, argon, U-factor. If you really understand these buzz words from window technology, you're a passive solar expert, a window salesman, or you've been reading the brochures.
Glazing refers to a smooth, transparent sheet of glass or other material. Coatings on a thicker sheet of another material aren't usually called glazing. So, a triple-glazed window means 3 panes of glass, or maybe 2 panes and a sandwiched plastic sheet. Space between the glazes may be filled with air, argon, or krypton. The latter are inert, heavy gases that conduct less heat and sound than air.
Low-E refers to a very thin, tin or silver oxide coating, bonded onto at least the inside surface of the outer pane. This coating reflects infrared radiation. So, it keeps heat in during winter. When applied to the exterior surface, a low-E coating also blocks the sun's infrared rays but not visible light. These coatings are usually tuned to the regional climate where they're sold. If you've got the money, electro-chromic windows are the ultimate. They can be programmed to change their heat reflectance features to suit the time of day and season of the year.
U-factor measures how fast radiant heat passes through a closed window. U-factors are the inverse of R-values that rate how well wall insulation stops heat gains and losses. So, you want high R-values for walls, low-U factors for windows. Typical low-E windows have U-factors around 0.35. The best ratings are as low as 0.18, which correspond to an R-value of 5.5, or about 1 in. of good wall insulation. Don't ask me why we need 2 opposing measures for one idea. It never made sense to me either. The non-profit National Fenestration Rating Council (NFRC) publishes a rating guide. However, note that the U-factor refers to heat transmission through the center of the window, not the whole window. So, this neglects frame effects entirely. Also, the NFRC uses separate ratings for solar heating through windows and air infiltration. They're called the Solar Heat Gain Coefficient (SHGC) and Air Leakage ratings. Using all 3 ratings gives you a better idea of how a specific window model performs.
Now let's recap your options. With old and drafty windows, start with caulk, weatherstripping, heavy drapes, and glue-on, low-E film. If you're on a budget but really need to retrofit now, then for a cool climate, go with double pane, low-E vinyl; use reinforced vinyl in a warmer climate. If you can wait, prices on fiberglass frames are likely to fall. Wood interior/fiberglass exterior is an attractive, though more costly, option. If money's no concern, then quadruple glazing with insulated fiberglass frames, krypton infill, and electro-chromic coatings yield the ultimate. However, ultra-energy efficient windows only make sense with super-insulated walls.