Balancing DFEE, Overheating & Natural Daylight

We have been undertaking extensive facade optimisation work both at the planning and detailed design stage. This involves balancing the competing objectives of fabric energy efficiency, overheating and natural daylight. This is particularly relevant to the most sensitive dwellings such as penthouses and single aspect apartments.

Energy efficiency, overheating and natural daylight are commonly under conflict when selecting the most appropriate glazing type (solar / light transmittance properties), external shading device (overhangs, louvres, brise soleil) and natural / mechanical ventilation strategy.

Figure 1 below clearly illustrates the strong correlation between glazing g-value and light transmittance value for a sample of glazing types. A glazing product with low g-value for minimising overheating risk and high light transmittance for maximising natural daylight can be said to be most optimal. However, such glazing products are generally the most costly. These costs must be balanced with the benefits.

Figure 1: Sample of Glazing Types: LT value Vs G-value

Although a low g-value is advantageous for managing excess summertime solar gain, it also restricts useful wintertime solar gain. This can impact dramatically on the DFEE (dwelling fabric energy efficiency).

Our facade optimisation work has helped effectively guide design to achieve the most cost effective solution.

In order to maximise wintertime solar gain, as well as natural daylight for penthouses, a computer optimisation analysis was carried out instead of specifying low transmittance glass, we were able to recommend an optimal overhang design. This serves to shade penthouse apartments from high sun angles in peak summer months, but enable penetration of useful solar gain from low winter sun angles.