How to Understand Window Efficiency Ratings – know the U, S, V, A & C!
When looking at different models of windows you want to be aware of the manufacturer’s rating. Most of this sounds technical at first – but understanding these ratings can help you in your purchasing process.
“U Factor” – A low number is better than a high one. This measures windows on a scale of 0 to 1 on resistance to heat flow – think, “how much of my heat is going out that old window?” A good double pane window can have a “U Factor” around 0.30. However be sure that what’s being measured is the entire window and not just “COG” (Center of Glass) which will produce a better number. You want a reflection of the whole window.
“Solar Heat Gain Co-efficient” – This factor is more toward how much heat the window lets in. Obviously this is a concern in warmer climates and in warmer months. Many homes can spend far more in cooling costs than heating costs. Again, a lower number is better and the scale is from 0 to 1. Also, as with “U Factor,” you want the SHGC measured for the entire window not just the COG.
“Visible Transmittance” – This factor measures the amount of light the window allows in – not heat like with the SHGC number but just light. Here a higher number is better. Light = energy = money, so you don’t want a window that doesn’t let any light in! You want as much light to get in as possible; the scale is between 0 and 1 with values around 0.50 being acceptable.
“Air Leakage” – Lower number is better here – this measures the amount of air that can pass through the window. For most of us that’s what we know best, feeling cold air come blowing through an old window a really cold day – “boy, I wish I had windows that were nice and tight!” You want to see an “AL” number in the 0.1 – 0.30 range. Air leakage is an optional rating, so manufacturers can choose not to include it on their labels.
“Condensation Resistance” – Another factor those with old windows know all to well – moisture forming on the inside of your window. This is measured as relative humidity, which is like the “dew point” as referenced by your weather man. Bottom line – the higher the number the better (resistance to moisture) and the range used is 1 to 100.
Here is an example of the ratings as they appear by window manufacturers:
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