Energy Tips: What to Look For When Buying Windows
By Andy Mazal
Filed Under Green Living | Leave a Comment
Seemingly, windows are simple devices designed for very simple purposes: to let light inside and to let you see outside. However, if you’ve ever purchased windows before you know that the illusion of simplicity disappears quickly once you begin to be confronted with all the choices you have to make, from the material the frame is made out of, to the “U-Factor”, Solar Heat Gain Coefficient, number of panes and films, and more. Making matters more complicated is that valid advice for one region might not apply to another region due to differences in climate. For example, a window in the Rocky Mountains needs to perform a very different job to a window in a hot humid climate.
First, let’s get something straight: no matter how much you spend on a window, it is very likely to be by far the weakest link in your home’s insulated building envelope. Even the best, most expensive and highest tech window will not even come close to insulating as well as the wall surrounding it. All the same, the choices you make in selecting a window can dramatically affect the energy consumption of your home and, just as important, your comfort. Here we’ll attempt to de-mystify the window buying process a bit.
Number of panes: This refers to the number of sheets of glass that make up a window. Old windows were single-paned, with only one thin piece of glass between the interior of a building and the outside world. These windows were horribly inefficient, with their main benefit being that they kept wind out, but did little to keep heat inside (or outside). The advent of dual pane windows improved the efficiency of a window dramatically by sandwiching a layer of air (or an inert gas) between the two panes. Today there are also windows with three panes as well which are even more efficient, but are also usually much more expensive. When buying windows today you’ll almost always be dealing with dual- or triple-paned windows; the only reason you would ever buy a single pane window today would be because it is required by zoning, most commonly in historic districts where the main concern is maintaining the historic character of the area.
Frame Materials: The material that the frame of a window is made of can have dramatic effects on performance and price. At the low-end, but very common, is vinyl. This is a great “bang for your buck” material as it’s cheap, easy to work with and does not transmit heat very well (which is good). However, it has a few disadvantages as well. First, you’re very limited in color: white or almond, in most cases. Secondly, vinyl is somewhat unstable and has a tendency to warp a little over time when exposed to extremes of heat and cold, which results in the window leaking air around the cracks more over time as well. All the same, millions and millions of vinyl-framed windows are installed every year, and they can be a great choice as a replacement window when replacing old, leaky inefficient windows. Even the cheapest off-the-shelf vinyl window will be far more efficient than a leaky 1960s or 1970s-era aluminum frame window, and especially an even older a single-pane window. Other materials run the gamut from wood to fiberglass to composite materials. All work well, and often the decision will be made based on cost and maintenance issues. For example, wood-framed windows look great and are quite efficient, but often cost more and require more maintenance — they have to be painted every few years. Another option we’re starting to see more and more of is “clad” windows which are sort of a hybrid: wood on the inside and clad with some other material such as aluminum or fiberglass on the outside. These exterior materials are often pre-finished from the factory and might never have to be painted at all.
However, even after you’ve decided on style, manufacturer and material, your job is not done yet. When you look at a new window you’ll see a large label with lots of strange numbers and statistics printed on it. At the least, you’ll see two Energy Performance Ratings: U-Factor and Solar Heat Gain Coefficient (SHGC). You might also see numbers for Visible Transmittance (VT) and Air Leakage (AL). Here’s what all that means:
U-Factor: The U-Factor is a measure of a window’s resistance to heat flow expressed as a number between 0 and 1. The lower the number, the more effective it is at resisting heat flow, and thus, the better it insulates. In almost all cases you want to get that U-Factor down as low as you can, because in either hot or cold climates you want the window to insulate as well as it can to either keep heat outside or to keep heat inside. An extremely efficient window will have a U-Factor of around 0.15 but even if you aim for a U-Factor of 0.40 or 0.35, you’ll be doing well.
Solar Heat Gain Coefficient (SHGC): This is a measure of how much solar radiation (i.e., heat) a window allows to pass through it. It is also expressed as a number between 0 and 1, with 0 being no solar heat is transmitted through the window and 1 being 100% of solar heat transmitted through it, although in most cases it will vary between 0.2 and 0.8. In this case, whether you want a lot or a little solar heat transmitted through the window will depend very much on where you live and what direction the window faces. In the cooler northern climates you will generally want your south-facing windows to have a higher SHGC (above 0.7) so you can take advantage of the passive heating effects of the sun during the cold winter months. In the warm southern latitudes you’ll want to choose windows with a much lower SHGC (below 0.4) since your biggest concern is usually keeping cool during the hot summer months. In fact, SHGC is even more important than U-Factor in hot southern climates, so if you live in a hot climate and have to choose based on cost, opt for a lower SHGC at the expense of a higher U-Factor if you must.
Visible Transmittance (VT): VT is the measure of the amount of visible light transmitted through the window. This number will vary between 0 (no light transmitted at all) to 1 (100% of light is transmitted) but because the VT rating includes the frame of the window, which obviously transmits no light through it, the number will generally be between 0.3 and 0.8. In most cases you want this number to be as high as it can be, since much of the point of having windows in the first place is to let in light, however there are certainly situations where you’d want to have less light transmitted, such as when glare is an issue.
Air Leakage (AL): This is a measure of the amount of air passing through the cracks in the window assembly, expressed as a cubic feet of air passing through a square foot of window. Although all windows that open will have some air leakage, it should be pretty obvious that this is a measure you’ll want to keep down as much as possible. Aim for 0.3 or less.
Low-E: This stands for “Low-Emittance Coating,” and is a microscopically thin and virtually invisible coating of metallic layers deposited on the glass of a window or skylight. The purpose of a Low-E coating is to reduce the U-Factor of the window (and thus make it more resistant to heat flow) by reflecting long-wave infrared radiation, otherwise known as “heat.” Most windows today have some sort of Low-E coating on them as a way to push that U-Factor down.
Hopefully this has helped to shed some light on what you should be looking for when buying windows, whether replacing old ones or when building a new house. In almost all cases you’ll have a professional installer involved during the ordering and installation process, but now that you’re an informed window consumer you can double-check to make sure that this person is recommending an appropriate product. Ask them to explain why they are recommending what they are recommending and not just ordering the “default” because it’s easier. The characteristics of the windows you end up with can make a very large difference in your energy consumption and comfort, so make sure you’re getting the correct product for your climate and for the orientation of each window you’re installing. At the very least, ask your window professional “Why is this window here and why is that window there?” — and make sure they have a good answer!
Andy Mazal
Inhabit - Green Building Consulting
andy@inhabitconsulting.com
www.inhabitconsulting.com
Posted on November 11, 2008 |
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