What is a G-Value for Windows & Glazing?

What Are G-values?

G-value, also known as total solar energy transmittance, is a coefficient used to measure the transmittance of solar gain through glazing. Or, how much heat is transmitted through a window from the sun's rays.

The G-value is a scale between 0-1:

• A high G-value of 1 represents the full transmittance of solar energy.
• A low G-value of 0 means that all solar energy is blocked by the glass.

G-value Calculation

G-value = total solar gain + incident solar radiation

G-value is calculated from the direct energy transmitted through the glass plus the energy absorbed by the material and radiated internally into an enclosed space.

What Is a Good G-value?

Most double glazing has a G-value between 0 – 1.0, with solar control glazing will be at the lower end of the scale.

A good G-value depends on the direction your windows face and the climate you live in.

• For houses in colder climates, a high G-value is more important for south facing windows to allow maximum solar gain and can capture free heat from the sun.
• For houses in warm climates, south facing windows need a low G-value to reduce solar gain and overheating.

What Is Solar Gain?

G-value represents the solar gain effect in an enclosed space.

Sunlight produces shortwave infrared radiation – this is what warms objects in the sunshine. The shortwaves are absorbed by glass from the outside and the energy becomes re-radiated from the inside of the glass as longwave infrared.

Shortwaves can pass through glass but the longwaves can't, so the thermal energy of the infrared radiation becomes trapped bumping against the glass unable to pass through. This process is known as the greenhouse effect.

The rise in temperature is known as solar gain – we measure this by G-value.

Why G-value and Solar Gain Matter When Choosing Windows

Solar gain can be an advantage or disadvantage, depending on the direction your windows are facing, the time of year and where you live.

Solar gain effectively provides free heating and rooms on the south side of a building are always much warmer than on the north side.

However, in summer this can be a problem when rooms can suffer from overheating. Anyone who has sat in a conservatory on a sunny day can tell you this.

To use solar gain effectively, you need to consider the direction your windows face and if you want a high or a low G-value for that window. One size does not fit all when it comes to the glazing installed in your windows.

For this reason, at Everest, we recommend windows on a per-room basis with different glazing for different needs throughout the house.

Cold Climates

In cold climates, having a high G-value for south facing windows will allow your home to collect more energy from the sun and benefit from free heating. If this is used in combination with shutters that close during the day in summer months, you can benefit from the warmth in winter and avoid overheating in summer. Some progressive housing designs in Europe use this feature.

UV Bleaching in South Facing Rooms

For south and south west facing windows and rooms prone to furniture damage through UV bleaching, a specialist solar control coating on the glass reduces the transmission of UV and solar gain. Windows with an internal laminate layer also reduce UV rays.

North Facing Cold Rooms

For north and east facing windows, we recommend a Low-iron combined with a Low-e glass that allows the maximum light through whilst also reflecting any heat back into the room.

More panes of glass reduce the transmittance of solar energy, so triple glazing has a lower G-value than double glazing. It's for this reason that triple glazing can have a lower WER energy rating, even though it is more efficient at retaining heat in a room.

G-values and Building Regulations

An update to Building Regulations was introduced in 2022 to reduce overheating in buildings with large glazed areas.

As part of a drive towards reducing emissions, the regulations are intended to reduce overheating in houses by limiting unwanted solar gains in summer and providing adequate means of removing excess heat from a building.

For new builds, the size of a glazed area will be limited within a room. For replacement windows, the window must have any trickle vents replaced with the same size vent.

Read more: Planning Permission and Building Regulations for windows...