Balancing act

With a consultation anticipated soon on building regulations for energy-efficiency, Phil Brown, technical advisory service manager at Pilkington UK, examines how smart use of glazing can help residential developers to create low-energy homes without compromising on daylight levels or thermal comfort.

It’s predicted that the next change to Part L will require housebuilders and developers to construct low-energy dwellings as standard. It will be yet another sign that energy efficiency no longer occupies a niche corner of building design.

As part of the changes, the supporting Approved Documents are expected to recognise that a building’s fabric, and in particular its windows, are crucial in the design of highly energy-efficient homes.

But making our homes better at holding in heat is often a delicate balancing act with other factors like daylight exposure and internal climate. So, what do building designers need to consider in terms of glazing specifications to optimise thermal insulation and conform with future regulations? And how can they fill homes with daylight while preventing them from overheating?

In the near future, homes will have to be designed so they require much less heating and cooling in order to maintain a comfortable climate inside. While designing smaller glazed areas may provide a quick win here, houses will also need to deliver reasonable levels of daylight to foster the health and wellbeing of occupants.

The right choice of glazing can help achieve this delicate balancing act but there are three important parameters to consider: its U-value (rate of heat loss), g-value (captured solar energy), and light transmittance.

These three values, combined with the size and orientation of the glazed areas, allow us to calculate the effect the glass will have in the internal climate.

Passive solar design, which involves using the sun’s energy to heat living spaces, has been an essential measure deployed in low-energy homes for many years. Passive solar gain can contribute up to a quarter of the heat needed to warm a home. Such a design helps achieve a comfortable indoor environment by combining the freely available energy from the sun with heat gains generated internally from people, electrical equipment and appliances.

However, to maintain thermal comfort there needs to be a balance between the summer and winter needs of a home. As buildings become increasingly airtight and thermal bridges minimised or eliminated, overheating will become more of challenge.

Allied with a good ventilation system – whether natural or mechanical – the glazing design can ensure high temperatures are avoided during the summer. Some of the experience gained in the commercial sector in controlling solar gains during warmer periods can be transferred to residential applications. This could foster greater take-up of solar control glass like Pilkington Suncool, which helps to reduce the amount of the sun’s energy entering a building, in the domestic sector.

Glazing orientation is an important factor in preventing homes from overheating, too, and it’s important to consider having separate specifications for windows on different elevations of the house.

For example, north-facing windows in the UK get little direct sunlight, which means they can be large without risk of excessive overheating in the summer, but they should still be high performance. Triple glazing would be appropriate here, offering a very low U-value combined with a high g value.

For south and west elevations shading may need to be used. For example, using overhangs, blinds and even trees and vegetation to block direct sunlight from entering the building. Glass with a lower g value may also be appropriate here, particularly if the windows are large.

If the focus is solely on U-value, it can be tempting for designers to make windows small, but this will inevitably reduce the amount of daylight enjoyed by occupants. Aiming for a total window area of a minimum 20% of the internal floor area can contribute towards good daylighting design.

Glass is already available to help alleviate overheating in homes. As glazing technology has advanced, large expanses of glass can be used in a building’s design while still being energy-efficient. But the next step is glazing that actively adapts to changing conditions daily and seasonally. There are a host of dynamic glazing solutions expected to be introduced to the market in the years ahead to make this possible, featuring technology such as thermochromic glass and electrically activated shading systems.