A Complete Guide To Replacement Windows Ratings

When you are purchasing replacement windows for your home, one of the most important things you should be taking into account, next to window replacement cost, is the efficiency of any windows you buy. There are several reasons efficiency is important, the main ones being that:

  • Firstly, the more efficient your window then the lower your energy bills are likely to be
  • Secondly, if you want to save money on your replacement windows with window tax credits then usually there is a stipulation on the efficiency of the windows you purchase
  • Lastly, being energy efficient is extremely positive for the environment

So what are the factors you need to be thinking about when choosing your replacement windows? There are essentially 4 main things to take into consideration, as our “Ratings At A Glance Table” shows below.

Table 1: Replacement Window Ratings At A Glance

Rating Value Description Average Ranges Rating Value To Aim For
U-Factor A measure of the rate of heat transfer. Tells you how well a window insulates. 0.25 – 1.25 Low
Solar Heat Gain Coefficient (SHGC) A measure of the amount of solar energy transmitted through the window. 0.25 – 0.80 Low
Visible Transmittance (VT) A measure of the amount of light allowed through by the window. 0.20 – 0.80 High
Air Leakage (AL) A measure of the rate at which air passes through the window joints and gaps. 0.3 (Required) 0.3
Condensation Resistance A measure of the window’s resistance to water build-up 0 – 100 High

The U-Value

The U-Value, sometimes also referred to as the U-Factor, is a measure of the heat flow through the window material or the measure of heat loss. This is calculated using the entire window system including the frame and the glass and will generally provide a value between 0.20 and 1.25 Btu/h·ft²·°F. The U-Value is actually calculated by examining the rate of heat transfer through a specific window over a given period of time.

It’s more preferable, where possible, to have windows with a lower U-Value as this will mean the windows will insulate better than those with a higher U-Value.

The U-Value is something which can be largely effected by the types of window replacements you buy, our guide to choosing window replacement materials covers some of the most common material types along with their relative efficiency.

Air Leakage

Air leakage refers to the amount of heat loss which is able to occur through the passage of air through gaps or cracks in the window assembly. Air leakage is something which is usually tested by measuring the volume of air, in cubic feet, which is able to pass through the window system over 1 minute.

Therefore, as you would expect, the lower the air leakage the better. The currently building regulations require an AL value of 0.3 cf·m/ft².

Solar Heat Gain Coefficient (SHGC) & Visible Transmittance (VT)

Windows have the capability to allow heat gain through solar radiation, this is true regardless of the outside temperature. The solar heat gain coefficient, which is also known as the shading coefficient (SC) will be represented by a numerical value between 0 and 1, the lower the SHGC number the better.

The visible transmittance or the VT represents the amount of visible light which can be transmitted through the glass panes. If a particular window carries a higher VT then is basically means that the window will allow more light into the room. Thus, it’s preferable to get a window system with a high VT value.

Condensation Resistance

The value for condensation resistance will provide a measure of how well the window resists against water build-up. This value is provided by score of between 0 and 100, the higher the number means the window will have a higher resistance to condensation formation and is therefore better.

Ideally, any replacement window system you by should have a low U-Value, a low SHGC value, a high VT value and a high condensation resistance factor.

The Energy Star Windows Program & The NFRC

The replacement window ratings which have been mentioned above must be backed up by ratings from the NFRC or the Energy Star Windows program, sales people who claim certain ratings without recognition from this organisation may be making invalid claims. Opting for a window system which has been approved by Energy Star will usually ensure that the units have all of the things mentioned above and you can be 100% confident you are getting what you think you’re getting. Going for an Energy Star approved window will also mean that you maybe eligible to apply for a windows tax credit to reduce the overall cost of your replacement windows.

The exact same thing is true with the NFRC, or National Fenestration Research Council, which rigorously tests windows for their energy values and provides the company selling the window with a true and accurate rating value to advertise with their product. If a company is claiming rating values without the approval of the NFRC or the Energy Star program then those values could be highly inaccurate and misleading.