Energy efficiency

6 factors that support low energy building performance

Windows play a crucial role in reducing a building's heating consumption to the lowest level possible. VELFAC windows can deliver U-values as low as 0.8W/m²k, however, a window is not like a brick wall, where the focus is simply on the ability of that wall to keep the cold out and the warmth in. As well as the window's basic energy performance, the primary factors to consider when specifying low energy buildings are:


Orientation in relation to the sun

Building orientation is a crucial factor when determining how much of the sun’s energy can be exploited to passively heat a building. Orientation considerations are largely concerned with optimizing the building’s total solar energy transmittance or solar gain (commonly referred to as the g-value) on the south/east/west facades while reducing the total thermal transmittance or rate of heat loss (the U-value) on the north facade.




Glass ratio

Windows with a high ratio of glass to frame (or a high Ff value) will allow a greater amount of solar heat and daylight to enter the building. Windows with the same overall dimensions, but with larger frame profiles have a lower percentage of glass and therefore restrict the passage of light and heat.




Sun protection and shading

Existing features such as neighbouring houses, hills and trees can affect the amount of sunlight that reaches the glazing. To maximise the energy performance of the windows it is therefore essential to take existing shading into consideration at the early design stages.


The addition of structural forms of shading, such as canopies and overhangs, can also help with solar control by preventing high summer sunlight from entering the building, while still allowing lower winter sunlight to reach the windows and help heat the interior.




Windows number and area

The number of windows and the extent to which they cover the building  envelope are both important factors in determining the amount of heat and daylight entering the building.


However, it is not as simple as covering the entire south-facing façade with windows in order to maximise solar gain. Instead the function of each window should be analysed in relation to the building as a whole. In buildings where screening is used to control the summer sun, for example, allowing light to enter from different directions will ensure good daylight levels throughout




Installation and thermal bridges

Heat will always find the path of least resistance. In low energy buildings, where opportunities for heat loss are removed or reduced, thermal bridges can account for as much as 15% of the building’s total heat loss. However, this source of heat loss can be virtually eliminated by using robust installation methods and perimeter sealing systems.


Those currently available can deliver a linear thermal transmittance (or y-value) that meet and exceed current Building Regulations.




Glass options and solar gain

The choice of glazing has a significant impact on the energy performance of a window. Triple glazing has better insulating qualities than double glazing, assuming that the other window parameters are consistent. Therefore, it is often advantageous to use three-layer glazing for windows not exposed to direct sunlight. Double glazing delivers more solar gain, so it is best used in south facing facade without shade.


Solar gain (g-value) is also dependent upon glass type. Glass with a low iron content, for example, allows more heat to reach the interior.  See more about the glass options here




Window energy performance index and terminology

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Glass ratio (Ff)

The glass ratio is  the proportion of glazing to opaque surface in a wall. Also called window-to-wall ratio, is expressed in percentage and it is a key variable in façade design affecting energy performance in buildings.

U-value (Uw – Ug)

A U-value value shows, in units of W/m²K, the ability of an element to transmit heat from a warm space to a cold space in a building, and vice versa. The lower the U-value, the better insulated the building element. For windows, the U-value is denoted U w, while the U-value of the glass is denoted U g


The window's U-value (U w) depends on, among other things of how much glass there is in relation to the frame.

Energy balance (Eref)

The energy balance of a window is the most accurate method to assess its energy performance. It is an equation that factors in the heat gains and heat losses and is weighted by the climatic conditions. In other words, Eref is calculated as the difference between the window's g-value (the window's ability to utilize solar heat) and U-value (the window's ability to keep the heat inside): Eref = g w - U w.


When Eref is positive, it means that the window lets more heat in than it lets out. In terms of energy, it is therefore best with windows that combine a high g-value with a low U-value, so that Eref becomes positive – the bigger the better. Eref is expressed in kWh/m2/year.

Solar heat gain (Gw, Gg)

G-value (sometimes also called a Solar Factor or Total Solar Energy Transmittance) is a measure of how much solar heat is being allowed in through the window. If the g-value is, for example, 0.65, this means that the window lets 65 per cent of the solar energy pass through into the building. The lower the figure, the less solar heat transferring from outside to in, whereas a higher figure will allow more in.

Daylight transmittance (Lt)

Daylight transmittance is specified in percent and is a measure of the amount of daylight that enters through the window. The higher the LT-value, the more visible light passes through to the building’s interior.

Edge region temperature

The internal surface temperature of windows is not uniform. The edge region temperature is the surface temperature at the edge of the windows which is commonly an area where the temperatures drop. This can lead to heat loss and condensation, so it’s an element to take in consideration to avoid heat loss.

Linear thermal transmittance (Ψ)

The linear thermal transmittance is a quantity describing the influence of a linear thermal bridge on the total heat flow. It indicates the heat loss through a window once installed and is measured in W/m²K




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More inspiration and knowledge

Fact sheet More daylight, less heat

Download our fact sheet and learn how to avoid overheating, regardless of fenestration.


"We wanted to make sure the building could stay comfortable without the need for mechanical cooling, even through the hottest summer. ‘This meant the placement of smaller windows on south facing facades, to manage solar gain, and the specification of multiple opening units across the building, including high level teleflex-operated lights."


- Corrie Rounding, Architect, Innes Associates


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