Further thoughts on glazing

For most intermittently heated buildings, we can consider the daily usage of energy as divided into two parts during the heating season.

The first part is the energy requirement during the overnight unoccupied period. During this period, the heat input for pre-heat is a function of the installed capacity of the heating system. The influence of casual gains from occupants, lighting, solar etc is marginal. So, as the area and U-value of the glazing plays a direct role in the determination of the installed plant capacity, we must assume that the heating energy consumption is directly proportional to the glass area. There will also be an electrical energy requirement for any associated fans and pumps, and for any lighting. This will be independent of the glass area, since we can assume there will be no daylight to offset it. Therefore during the unoccupied period, the combined thermal and electrical energy will increase with glazing area.

During the daytime occupied period, for the majority of modern well-insulated buildings, the heating energy requirement is usually more than matched by the casual energy inputs from occupants, office equipment, lighting and solar gains. Therefore in any building with effective heating system controls, the demand on the actual heating system will be marginal. In fact, only on the coldest days should there be a demand for heating. The majority of simulations I have examined suggest that the available casual gains will tend to force the space temperatures above that required. This may lead to the occupants opening windows and a potential demand for mechanical ventilation or even air-conditioning. Therefore, the thermal demand during this period is likely to be independent of glazing area. The electrical demand for task lighting will, I suggest, remain effectively constant during the heating season. So during the heating season, the total combined energy demand will increase with glazing area.

During summer, there should be no demand for heating and the electrical demand for lighting during occupancy is the critical factor. In a well-designed lighting system this demand should decrease with increasing glazing area.

The resulting combined winter and summer demand can usually be shown to be independent of the glazing area and the designer has to look to other factors. The most crucial of these is the need to prevent overheating and avoid the resulting demand for air-conditioning.

Perhaps we should be looking elsewhere for criteria to judge the limits of glazing?

We need to produce suitable guidelines to arrive at buildings that will be energy efficient in practice as well as at the design stage. I have always thought that simply going through the design thought process looking at the various energy scenarios would produce a more efficient building than one designed by rote, conforming to every aspect of the Regulations.

John Harrington-Lynn, MCIBSE (retired)