Energy and Glass - How It Works
To understand energy transfer in windows and glass - and the value and savings that can be gained from high performace types - it is essential to understand the language involved, and how the relationship between glass and the outside environment.
There are three factors to consider when choosing the glass for your new build or retrofit:
1. U-Value (Uw) measures how readily a window conducts heat. It is a measure of the rate of non-solar heat loss or gain through the assembly.
The rate of heat flow is indicated in the terms of the U-Value of a window assembly which includes the effect of the frame, glass, seals and any spacers.
The lower the U-Value, the greater a window's resistance to heat flow and the better its insulating value.
A low U-Value is ideal for all climates as it stops unwanted heat gain in summer and unwanted heat loss in winter.
|2. Solar Heat Gain Coefficient (SHGCw) measures how readily heat caused by sunlight flows through a window.
The SHGCw is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward.
SHGCw is expressed as a decimal between 0 and 1. The lower a window's SHGCw, the less solar heat it transmits.
In cooler climates, a high SHGCw (as illustrated) is beneficial for north facing windows during winter, however shading will be required in summer to prevent unwanted solar heat gain. In hot climates, a low SHGCw is always ideal.
3. Visible transmittance (Tvw) measures how much light comes in through a window.
It is an optical property that indicates the amount of visible light transmitted.
Tvw is expressed as a decimal between 0 and 1.
The higher the number, the more light is transmitted. Ideal for all climates, a high Tvw maximises natural light.