In order to fully understand the energy efficiency factors relating to home replacement windows you will first need to have a basic understanding of replacement window ratings. In this section of the Window Price Guide resource, we examine the multiple factors associated with energy efficiency in window ratings, helping you to better understand your options when looking at window prices and buying replacement windows.
What Are Window Ratings Important?
Window ratings are directly related to the energy efficiency of a replacement window, they are the main factors which are measurable when assessing the efficiency of any window product. But why is that important? The main reasons good window ratings are important are simply because:
- You’ll lower your overall heating and cooling costs throughout the year, saving you money
- If you use a window tax credit, rebate or discount then there is usually a stipulation that the product you buy meets certain energy efficiency ratings
- Good energy efficiency is kind to the environment
What Are The Main Energy Rating Factors?
There are 5 main window ratings which are assessed when looking at the energy efficiency of any window product, these factors are outlined in brief in the table below:
Table 1: Window Ratings For Energy Efficient Windows
|Value||Rating Description||Expected Ranges|
|Solar Heat Gain Coefficient (SHGC)||The quantity of solar heat transferred through the window.||0.25 – 0.80|
|Visible Transmittance (VT)||The amount of light which is allowed to pass through the window into the inside space||0.20 – 0.80|
|U-Factor||How well a window insulates the room in which it’s installed||0.25 – 1.25|
|Air Leakage (AL)||The amount of air lost through gaps and seal losses around the window unit||0.3 (Required)|
|Condensation Resistance||The amount of condensation resistance provided by the window||0 – 100|
The U-Factor Or U-Value
Looking at the U-factor measurement of a window means measuring the heat which is able to flow through the window material from the environment inside to the outside, or in simpler terms a measure of heat loss.
The U-factor will be measured by assessing heat loss for the complete window product, this includes the glazing and the frame. The expected and required value will vary depending on the state in which you live, in general though a value of between 0.25 and 1.25 Btu/h·ft²·°F is expected. In simple terms the lower the U-factor or U-value then the more energy efficient the window product is.
The Air Leakage Measurement
The air leakage value (AL) of any window product is similar to the U-factor, the main difference though is that the AL measures the air and heat loss through gaps around the window frame or in the window seal. AL is usually measured in cubic feet or in volume of air transferred in a given period of time, usually 1 minute.
An AL value of 0.3 cf·m/ft² or less is expected from an energy efficient product.
The Visible Transmittance & Solar Heat Gain Coefficient
The Solar Heat Gain Coefficient, otherwise known as the SHGC or Shading Coefficient (SC), measures the heat gained through the window unit by the influx of solar radiation. The construction of a good replacement window should reduce the amount of solar radiation passed through the window unit. Therefore a SHGC value of between 0 and 1 would be expected as an industry standard.
Secondly, the Visible Transmittance, also known as the VT, is a measurement of the total visible light be can pass through the pane of glass in the window unit. In a real world situation the VT gives you a measure of how good the window will be at allowing light into the inside space, particularly helpful when you’re trying to make the most of a dark space in your home. It’s for the above reason that one would expect a good quality and high energy efficient window product to have a high VT rating value.
The Condensation Resistance
One of the worst things a window unit can do is allow condensation formation on the inside of the window, in a standard window condensation formation indicates a fault in the seal.
The measure of a windows ability to prevent condensation formation is known as the condensation resistance value. This value is usually between 0 ad 100, ideally the number should be as low as possible.
Putting It All Together
This guide has explained above what each of the individual window ratings means in relation to energy efficiency, but in the real world what do you need to be looking for to get the ideal replacement window? The best window product will have an energy rating criteria of the following values:
- A low U-factor or U-value measurement which at least below 1.25 Btu/h·ft²·°F
- A low Solar Heat Gain Coefficient which is somewhere between 0 and 1
- A high Visible Transmittance which is 0.20 at it’s lowest
- A high Condensation Resistance
Finding Energy Efficient Windows Easily
Having an understanding of what the energy efficiency ratings for windows actually mean is a great start, but how do you go about finding energy efficient windows?:
- Look for Energy Star rated window products (Use our tool to get instant window prices)
- Look for NFRC (National Fenestration Rating Council) rated products
It’s not enough to simply look for energy efficiency claims from the window manufacturer any more, you need to have evidence and proof to back those claims up and ensure they’re being 100% honest about the values they’re claiming. By looking for the Energy Star or NFRC energy rating label (shown left) you can be sure that the window product offered has been rigorously tested and has met standards outlined by the U.S. Department Of Energy and the NFRC. The main values, as well as the NFRC rating, are always shown on the product certificate.