Improve Your Home with New Replacement Windows
New windows can be one of your homes most attractive features. New windows can provide you with convenient ventilation, ease of use, superior natural light, and cleaning convenience not found in older windows. If you have a older home, or inefficient windows you could be throwing up 30% of your heating and cooling dollars out the window, literally. On average for the year 2001 the owner could expect to recoup approximately 69% upon the sale of the home when replacing their older windows. Today there are so many manufacturers of windows, it can be difficult to choose a quality product. Of course every manufacturer or window salesperson claims to have the best product. There are several design elements and options to look for when choosing your new windows. Some of these are listed below:
- IG Units – is the number of panes of insulated glass
- Low-E – is “low emittance” coatings, which minimize thermal radiation (damaging rays)
- Low Conductive Gas Filled – Argon or Krypton
- U-Factor- heat loss rating
- SHGC Factor – amount of solar radiation (heat) admitted through a window
- VT – Visible Light Transmission
The following information will take some of the “mystery” out of these terms.
IG Units- What is “IG” unit?
Today, quality windows are built around what’s called an insulated glass (IG) unit. An insulated glass unit starts out as two or three individual panes of glass that are chemically bonded by a sealant to spacer bars. When comparing vinyl replacement windows, always ask about the IG unit. Because not all windows or glass are alike, you should require the highest quality and performance standards.
Low-E – is it for me?
Low-emittance (Low-E) coating are thin, metal or metallic oxide layers deposited on a window surface primarily to reduce the U-factor by suppressing the heat flow through the glazed window unit. Coating the inner glass surface with a low-emittance material between the glass layers, blocks a significant amount of this radiant heat transfer, which lowers the heat flow through the window. Low-E coatings are transparent to visible light. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain. We usually only recommend Low-E for the south side of homes if any in our region where the sun shines throughout the day. One thing to remember is the windows have a slightly tinted appearance.
Low Conductive Gas Filled Units
An improvement that has been made to the insulating performance by the use of argon or krypton gas between the panes of glass. Normally, the space is filled with air or dry nitrogen prior to sealing the window panes together. In a sealed glass insulated window unit, air currents between the two panes of glazing carry heat to the top of the unit and settle into cold pools at the bottom. Filling the space with a less conductive gas minimizes this and the overall transfer of heat between the inside and outside is reduced.
Argon is inexpensive, nontoxic, non-reactive, clear, and odorless. The optimal spacing for an argon- filled unit is the same as for air, about 1/2 -1 inch.
Krypton has better thermal performance, but is more expensive to produce. Krypton is particularly useful when the space between glazings must be thinner than normally desired, for example, 1/4-inch. A mixture of krypton and argon gases is also used as a compromise between thermal performance and cost.
The use of conductive gas filled units has significant insulating benefits but comes at higher initial cost.
U-Factor – What exactly is it?
The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. The lower the U-value, the greater a window’s resistance to heat flow and the better its insulating value. Winter nighttime U-values are calculated using an outdoor air temperature of 0 degrees F and outdoor air velocity of 15 mph, indoor air temperature of 70 degrees F, and air velocity of 0 mph. This is similar to the R-value that you think of when choosing insulation except a low U-factor is better, unlike R- value which the larger the number the better.
It is calculated as 1 divided by the R-value = U-factor. If you used this calculation R-13 insulation would have a U-factor of 0.077.
To convert U-factor to R-value you take 1 divide by the U-factor. If you used this calculation for the average aluminum framed window with a U-factor of 1.125, your R-value would be 0.89 .
Good replacement windows depending upon the glazing options and low conductive gas fills range between 0.5 and .1 for the U-factor.
SHGC Factor – What is it?
Solar Heat Gain indicates the total amount of solar energy that passes through glass relative to a 1/8″ (3mm) thick clear glass under the same design conditions. It includes both solar energy transmitted directly plus any absorbed solar energy subsequently re-radiated into the room. Lower values indicate better performance in reducing summer heat gain and, in turn, air conditioning loads. Your average single paned wood framed window has a SHGC of 0.98 whereas a quality replacement window will have ratings from 0.91 – 0.39 depending upon glazing options.
VT- Visible Light Transmission
The visible transmittance (VT) is an optical property that indicates the amount of visible light transmitted. The VT is a whole window rating and includes the impact of the frame which does not transmit any visible light. While VT theoretically varies between 0 and 1, most values are between 0.3 and 0.8. The higher the VT, the more light is transmitted. A high VT is desirable to maximize daylight.
I hope this article helped demystify the terms used when you are contemplating your new vinyl replacement window purchase. Purchasing a quality window is only a part of the equation. The installation is the largest factor of how your windows will perform. Take great care installing and insulating around your existing openings and new windows to assure you receive the maximum performance possible from your new windows.