How to Measure Window Performance Ratings
Window Energy Performance Ratings
In the world of replacement windows in Baltimore, there are a lot of things to consider. They range from energy efficient replacement windows, to how to making existing windows more energy efficient and all things in between.
Once one has made the decision to get new windows they may ask what energy performance ratings are. When they do, they will find a lot of information about it. Essentially they will find that the National Fenestration Rating Council tests, certifies and labels windows based on their energy efficiency. Their label provides a way to compare products on the market today. They test for a couple of different energy efficiency ratings. These are heat gain and loss and sunlight transmittance. Each one has a number of factors that go into its rating.
Heat Gain and Loss
Windows can gain or lose heat through:
· Direct conduction through the glass or glazing, frame, and/or door
· The radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls
· Air leakage through and around them.
The NFRC tests for these properties by measuring the U-Factor, the solar heat gain coefficient and the air leakage.
The U-Factor is the rate at which windows conduct non-solar heat flow. The NFRC measures the entire window and takes into consideration the frame. The lower the u-factor the more efficient the window is. So for people in warmer climates or people worried about the summer months taking this rating into consideration can make a big difference in cooling bills.
The SHGC is the fraction of solar radiation that penetrates a window. The lower the SHCG the less solar heat gets through. So for cooling the home a window with a lower SHCG is preferable. In the winter many homeowners in the Baltimore area want to capitalize on the sun’s heat. Perhaps for them a higher SHCG is preferable.
Air leakage is the rate at which the air around the window penetrates or escapes the window. Lower air leakage ratings mean less air is escaping. This is more important though for people who live in Baltimore and are searching for energy efficient windows. This is because in the winter air will escape from the heating system. Air will also get in from a windy day.
Sunlight Transmittance
Sunlight transmittance is the ability to transmit sunlight into your home and is measured two ways by the NFRC. These two ways are visible transmittance VT and light to solar gain LSG. Once again several factors are considered for each when making a rating.
Light to solar gain is the fraction of visible spectrum of sunlight transmitted through the window. A high VT rating means there is a high visible glare. The VT rating is measured in numbers between 0 and 1. When deciding what VT rating you want, you should consider how much sunlight you want in your home.
Light-to-solar-gain LSG is the ratio between the SHGC and the VT. The higher the number, the more light is transmitted without adding undo amounts of heat. One thing to keep in mind with the LSG is that its rating is not always reported.
So for people in Baltimore looking to understand window energy efficiency ratings, there is a lot to read and understand.
Once one has made the decision to get new windows they may ask what energy performance ratings are. When they do, they will find a lot of information about it. Essentially they will find that the National Fenestration Rating Council tests, certifies and labels windows based on their energy efficiency. Their label provides a way to compare products on the market today. They test for a couple of different energy efficiency ratings. These are heat gain and loss and sunlight transmittance. Each one has a number of factors that go into its rating.
Heat Gain and Loss
Windows can gain or lose heat through:
· Direct conduction through the glass or glazing, frame, and/or door
· The radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls
· Air leakage through and around them.
The NFRC tests for these properties by measuring the U-Factor, the solar heat gain coefficient and the air leakage.
The U-Factor is the rate at which windows conduct non-solar heat flow. The NFRC measures the entire window and takes into consideration the frame. The lower the u-factor the more efficient the window is. So for people in warmer climates or people worried about the summer months taking this rating into consideration can make a big difference in cooling bills.
The SHGC is the fraction of solar radiation that penetrates a window. The lower the SHCG the less solar heat gets through. So for cooling the home a window with a lower SHCG is preferable. In the winter many homeowners in the Baltimore area want to capitalize on the sun’s heat. Perhaps for them a higher SHCG is preferable.
Air leakage is the rate at which the air around the window penetrates or escapes the window. Lower air leakage ratings mean less air is escaping. This is more important though for people who live in Baltimore and are searching for energy efficient windows. This is because in the winter air will escape from the heating system. Air will also get in from a windy day.
Sunlight Transmittance
Sunlight transmittance is the ability to transmit sunlight into your home and is measured two ways by the NFRC. These two ways are visible transmittance VT and light to solar gain LSG. Once again several factors are considered for each when making a rating.
Light to solar gain is the fraction of visible spectrum of sunlight transmitted through the window. A high VT rating means there is a high visible glare. The VT rating is measured in numbers between 0 and 1. When deciding what VT rating you want, you should consider how much sunlight you want in your home.
Light-to-solar-gain LSG is the ratio between the SHGC and the VT. The higher the number, the more light is transmitted without adding undo amounts of heat. One thing to keep in mind with the LSG is that its rating is not always reported.
So for people in Baltimore looking to understand window energy efficiency ratings, there is a lot to read and understand.