An issue with g-values

Inkling have recently become aware of a growing trend for specifying the g-value (solar transmittance) of glazing for windows as whole window values, which include the shading effect of the frame, rather than centre pane values where the solar control is purely a property of the glazing.    

We don’t think it is well understood within the modelling community that there is this significant difference in how g-values can be specified. We hope this blog will help draw attention to this issue and reduce the risk of inadvertent errors creeping into TM59 and other overheating risk assessments.

The type of g-value used in a model can make a  significant difference to modelling results for TM59/Part O overheating risk assessments (or any other analysis). This is because whole window g-values are lower than centre pane values. As an example, with a frame factor of 70% a typical whole window g-value for a window might be 0.48, while the centre pane value, which excludes the framing, is 0.68.

If 0.48 is unintentionally used as a centre pane g-value, then the solar gains calculated      in the model will be significantly (in this case 30%) lower than for the case where the actual centre pane value of 0.68 is used. The consequent overheating risk predicted by the model will therefore also be significantly lower than in the actual design.

BS EN 410 defines g-values as the total solar transmittance of the glazing at normal incidence under specified conditions (i.e. centre pane value), and this is the definition modellers are accustomed to using. This applies purely to the glass. Models then include a frame factor or frame dimensions, with frame elements assumed to be opaque. This is also the convention used by Building Regulations Part O (see Figure 1).

However under the conventions of the British Fenestration Rating Council, the  whole window g-value is defined as:

g-value_(BFRC) = 0.9 * (1 – frame fraction) * g-value_{(BS EN 410)}

BRFC certificates for windows (see Figure 2) quote whole window U- and g-values.

Modelling software tools may allow g-values to be applied using either, or both, of these conventions so it is imperative that modellers understand and are mindful of the difference.    

Assuming that the software expects centre pane g-values, workarounds can be used when only whole window g-values are available. For example:

• windows could be modelled with zero frames and the whole window U- and g-values applied.      

• if the window frame factor is known, the BS EN 410 centre pane value can be calculated using the equation above and the correct g-value and frame factor entered in the software.

If the g-values supplied by the design team are centre pane values, then windows should be modelled with the proposed frame dimensions/frame factor and the centre pane U- and g-values applied.

As this issue potentially presents a high risk to the design, we recommend that modellers always ask for confirmation of which type of g-values have been supplied to them, and state clearly in their report whether centre pane or whole window g-values have been used within the modelling assessments.