Thermal ratings for windows
Thermal ratings for windows are now a fundamental part of building legislation. For window manufacturers, understanding what the rating system means and how a rating is obtained is an important part of the program.
WADIC members and building designers are at the forefront of a revolution in the way we use timber windows. With thermal ratings now a key part of window design and manufacture, the industry is able to offer consumers tangible proof of window performance.
In Australia, window thermal ratings are obtained by having a National Fenestration Rating Council-based (NFRC) rating for:
- the window frame;
- rating for the glazing; and
- having these two measures combined in a manner that is approved by the NFRC.
There are a number of ways to describe thermal ratings. The Australian Government has adopted a format that requires only the U value and solar heat gain co-efficient (SHGC) of the window.
Other measures, such as visible light transmittance and air infiltration can be argued to be important but current Australian regulations do not include those particular measures.
Therefore, obtaining the U value and SHGC for your window is very important.
It is equally important that any rating is based on NFRC conditions, not the ANAC conditions that have been used for a large number of the ratings now seen in the marketplace. Further, the rating must be for the whole of the window, not just the glazing or the frame in isolation.
The options for obtaining the required measures (U value and SHGC) include:
- using the industry generics as listed in AccuRate;
- using one of the independent NFRC-qualified simulators to manually calculate the thermal properties of the window.
It is possible that others may be conceived in the future, and the NFRC protocols allow for (and encourage) that, provided that the required protocols are followed.
In time, the Australian Fenestration Rating Council (AFRC) may become a local equivalent of the NFRC. However, the AFRC is a very new organisation and has yet to put in place all the required procedures to properly manage the production and auditing of window thermal ratings.