In Perspective - Windows
Someone made a comment about building efficiency last week stating that their landlord had replaced the windows at their complex but it did not seem to make a difference in energy savings. I couldn’t debate the comment because just replacing windows is like taking a statement out of context. What I can say is that there must be other major problems that have been overlooked. Buildings are a system and must be looked at as a whole. I have always suggested that you start addressing the issues of efficiency with the easiest and least expensive and work your way up and window replacement is one of the last steps due to costs.
But since windows were the issue, let’s take a closer look at windows. Windows are not as simple as you may think. Nowadays there are a number of parameters you should be aware of when talking windows. A few of the major parameters are usually attached to the replacement window or given in a window catalog and they are: U-factor, Visible Transmission, Solar Heat Gain Coefficient.
The U-factor is actually the mathematical inverse of the R-Value. The R-value of 4 is equal to the U-factor of 0.25. So where a higher R-value is better, the lower the U-factor the better. Here in New York, with the exception of 8 southern most counties, a U-factor of 0.35 or better is required based on the 2006 International Energy Conservation Code.
Visible Transmittance (VT) is a measure of how easy it is to see through and the windows’ ability to admit visible light. Don’t be surprised at clear glass having a 90% VT and as the window tints are added, the number drops. Around 50% VT the window starts looking dark or reflective.
Solar Heat Gain Coefficient (SHGC) is the fraction of available solar heat that passes through the window. Here in New York, southern facing windows would likely have a high SHGC while in northern facing windows this is not as important. That means a house will use different windows based on their location for optimal performance.
Another factor that you need consider is Air Leakage. Air Leakage (AL) is expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.
If you've lived in a cold climate, you've seen condensation and even frost on windows. Pay attention to the Condensation Resistance (CR) which is a measure of the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation.
You will see options like Low-E coatings, low conductance gas fillings and multi-glazed windows all of which change the U-factor, SHGC and VT of the windows. There is also a wide variety of window frames from wood, vinyl and aluminum to composites and fiberglass. Here there are a couple of things to keep in mind, maintenance and thermal expansion. Thermal expansion is important here in New York because of the range of temperatures which can be as high as 120 degrees.
Windows are important to the overall building system but keep in mind that they are expensive. Before you replace your windows make sure you address other issues such as door seals and insulation.