Why are site-specific window calculations needed under the Future Homes and Building Standard?

Under the FHS, a one-size-fits-all approach to window specification is no longer sufficient. The new Home Energy Model (HEM) uses half-hourly solar modelling, which means the orientation, size, and glazing specification of windows on each elevation of a home must be carefully assessed individually. The distribution of glazing across different orientations is significantly more important than total glazing area alone  as south and west facing windows behave very differently to north-facing ones in terms of heat gain and overheating risk. 

Compliance also requires satisfying Part O (overheating) alongside energy targets, and these two assessments are closely linked within HEM. Large areas of south- and west-facing glazing increase overheating risk, but external shading devices such as overhangs can be modelled and credited to help balance performance. This means window specifications — including U-value, solar heat gain coefficient (g-value), and size — should be calculated on a plot-by-plot, elevation-by-elevation basis rather than applied uniformly across a development.