Facts about window energy terms
We have produced a short guide to the most important and well-used terms regarding windows and energy performance. The different definitions can be of help both when comparing windows from various suppliers and naturally, when doing energy calculations on building levels.
The following should help you to make an informed decision when looking for new windows with energy on top of your priority list.
Solar heat gain/Solar Factor (Gw)
The ability to let in solar heat through the window into the house called the g-value. The higher the value, the higher percentage of solar heat passes through the pane.
The g-value and daylight are interdependent to a certain degree. This means that high solar heat gain often comes with a high amount of daylight contribution, and vice versa. A well-lit room contributes to a convenient indoor climate and a lower electricity bill for artificial lighting - for everyone to enjoy.
Heat loss (Uw-value)
Heat loss through a building component in general is called the U-value. For windows it is called Uw. When comparing the heat loss from a standard window with the heat loss from a wall it looks like this:
UK minimum requirement for a wall U-value = 0.3 W/m2k
UK minimum requirement for a window U-value = 1.8 W/m2k
Rationel standard window: U-value = 1.33 W/m2k (Aldus 2-layer pane)
Rationel low-energy window: U-value = 0.79 W/m2k (Aldus 3-layer pane)
When looking at the U-value alone, it seems like the wall is better from an energy point of view. But opposite the window, the wall does not contribute heat or daylight.
Energy balance (WER)
The energy balance gives an overall idea of the window's energy performance. As shown on the picture above the energy balance expresses the balance between heat gain (in through the window) and heat loss (out through the window). In this calculation the heating season (Oct - May) and average window orientation are taken into account.
If the result is negative it means that the heat loss is greater than the heat gain. A positive value, on the other hand, means that the window contributes more heat in the heating season then it loses. The best results are obtained by combining a high g-value with a low U-value.
A south-oriented window can be looked upon as a radiator made by nature, depending on shading conditions, window choice, etc. - while a north-oriented window mainly causes energy loss, since there is no solar heat gain from the north.
It is therefore important that you are aware of the orientation when placing your windows. East- and west-orientated windows are close to energy neutral from an energy balance point of view.
Energy balance: Wall vs. Window
Wall: -20.6 kWh/m2/year
Standard window: -4.4 kWh/m2/year
Low-energy window: 23.9 kWh/m2/year