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)
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
UK minimum requirement for a window U-value = 1.8
Rationel standard window: U-value = 1.33 W/m2k (Aldus
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: Wall vs. Window
Wall: -20.6 kWh/m2/year
Standard window: -4.4 kWh/m2/year
Low-energy window: 23.9
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
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.
If you wish to read more about windows and energy - please click