Glass Properties
The properties of the glass used in your glazing can also impact your home’s passive energy performance. This guide will explain the main characteristics of the glass in your glazing and what this means for your home design. You can find these properties in most glazing manufacturer product guides.
Visible light transmittance (VLT)
The visible light transmittance, or VLT, is the amount of visible light passing through the glass. The less VLT of the glass, the less visible light will pass through the window. A low VLT may reduce the amount of natural light coming into the home, however, will also reduce the amount of heat gain in the home by the sun. The desirability of a lower VLT will depend on your environment.
Conduction (U value)
Conduction relates to the heat conductibility of the glass pane, often denoted by U or Uw for windows. Higher conduction means it is easier for heat energy to transfer through the material. Conductibility tells us how good or bad the material is for insulation. A lower U value suggests the glass is more resistant to heat transfer through the glass and has better insulative performance.
Solar heat gain (SHGC)
The solar heat gain coefficient (SHGC) is a measure between 0 and 1 that explains how well heat directly from the sun can get through a window (frame and glass). The higher the number the more heat can be transmitted into the house.
The SHGC is a figure quoted by glazing manufacturing companies for each product, however, the actual performance can be reduced depending on the angle sunlight hits. The positioning of the window itself affects this, as does the position of the sun. As discussed in “The Seasons and Your Home’s Orientation”, this positioning can vary throughout the day as it can throughout the year.
If sunlight hits the window perpendicularly then the quoted SHGC is accurate, and this allows the most sunlight through the window. This can happen with skylights or windows facing the sun during sunset and sunrise. A higher angle of incidence (less directly into the window) will result in more of the light being reflected instead of transmitted. This means the true solar heat gain will be less than the quoted coefficient.
Again, the angle of incidence can change throughout the day and throughout the year.
- At sunrise and sunset, the sunlight can be more direct into east and west-facing windows, resulting in more solar heat gain at these times. Meanwhile, at midday, the sunlight angle is sharper to the window, minimising solar heat gain.
- In summer the sun is generally at a higher angle of incidence than in winter. This means in winter the sunlight is more direct through the window, resulting in more solar heat gain through the window in winter than in summer.
Summary
Your glazing options are extensive and a smart choice can help assist the passive heating and cooling functionality of your home. Your choices for glazing types are dependent on your environment and what you want for your home. There are some standard properties you want to optimise no matter the environment.
- A lower VLT glass may be darker, increasing your need for artificial lighting and consequently increasing your energy use.
- You want to minimise the U value in both hot and cool environments. This means your windows do a good job of keeping the outside temperature out, and the inside temperature in. If you ever want your home to be the same temperature as outside your home, you can simply open the window. See more on casement types in our Glazing Frames article.