Greening Your Home: Windows

If you are looking at replacing or updating your windows there are some important things to know. Windows have grades and numbers just like other appliances but if you saw the information on your window would you know what it meant? Here is a simple guide:



U-Factor: This is a measure of the non-solar heat flow through all parts of the window (glass, frame, sash). Lower is better for this number. Values generally range from 0.14 (super insulating suspended film window) to 0.5 (standard double pane window).



Solar Heat Gain Coefficient (SHGC): This is the percentage of the sun’s solar heat that passes through the window. The higher the number, the more passive solar heating potential.



Visible Transmittance(VT): This is a measure of the amount of visible light that passes through a window. Values range from 0 to 1. Most rating are between .3 and .8 because they take into account the light blocked by the frame. High VT maximizes daylight passing through.



Air Leakage(AL): This is a measure of the amount of air passing through the window assembly which is a major source of heat gain and loss. This rating is OPTIONAL and is measured in cubic feet per minute through a square foot of window. For this rating, you want to stick to something below 0.3. Lower is better.



Window Frames come in Aluminum, Wood, Clad, Vinyl, and Fiberglass. Wood is pretty but requires a lot of upkeep. Vinyl is low maintenance. Aluminum is low maintenance but highly conductive so you will lose a lot of heat. Clad offers the low maintenance of Vinyl, Aluminum or Fiberglass with the insulation and appeal of wood, but these are also generally the most expensive. The real key to choosing a window frame is durability. There is no rating method for durability of a window frame. A well constructed frame of any material is always better than a poorly constructed frame of another material. Read everything you can about the windows you are considering. Notice how the corners are joined, how well the sashes seal and how rigid the unit is.



What about the glass? Is it worth it to spend more on triple glazed aluminum clad wood versus double glazed insulated fiberglass? Don’t be afraid, it’s not that bad. Glazing simply means glass. Double Glazed means there are two glass plates, triple glazed means there are three. A window is “insulated” if it has space between the panes and reduces the transfer of heat. Some windows replace the air between the windows with gas for improved insulation – mainly Argon and Krypton. Krypton is best because it allows smaller spaces between the panes which puts less stress on the sashes. If you think you are now so smart about windows you can start referring to them as fenestrations, someone may slap you, besides you still have a lot to learn.



Is your window Low-e? Low-e means there is an energy efficient coating on the window. Most windows manufactured after the 70s are Low-e. Low-e coating reflects short wave radiation (less heat gain), filters UV rays (less fading), reflects long wave radiation (less heat loss), and if they are tinted, they temper visible light.



Films are another way to control heat flow through a window. There are high solar gain and low solar gain films. One lightweight film can be the equivalent efficiency of a 5 pane window without the weight and cost. Most windows are R1-R3 rating, but films can increase that value as high as R-11 which can rival the insulation in walls in many parts of the country.



Some other features are impact resistant glass, privacy glass and self cleaning windows. If you live in a hurricane or tornado prone area, you may even be required to have windows that are made from tempered glass to keep the window from shattering when it is broken. Frames are also rated as impact zone 2, 3, and 4 – at level 4, the window can withstand a strike from 2-8 foot 2 x 4s travelling at 50 feet per second followed by 9000 cycles of negative and positive pressure wind similar to that of a hurricane. If you are putting a window in the bathroom, you may want to look at privacy glass such as pebbled, frosted or wavy glass that lets light in, but not peeping toms. These forms of glass also inhibit sound transference if you happen to be a loud bather :P. Finally, if you don’t do windows, technology has finally given you a nod with windows that have been coated with a special finish to keep dirt from adhering and rain from collecting.



Different parts of the country have different window requirements. For example, if you live in Florida, you want glass that is impact resistant, has a U-Factor of <0.65 and an SHGC of < 0.4. If you live in Kentucky or North Carolina, you probably don’t care so much about impact resistance, would want a U-Factor of <0.4 and an SHGC of < 0.55. It's important to pick a window that is appropriate for your area.



Now that you know about the technical stuff, you can choose the other stuff like Double Hung (traditional), Awning (opens at the bottom – seen above doors), Tilt and Turn (tilts in for easy cleaning), Casement (side hinged to open outwards), Horizontal Slider (slides left to right instead of up and down), Fixed (doesn’t open at all) or Hopper (upside down version of awning – seen in basements).



Have you started bragging about your fenestrations yet?