With a consultation anticipated soon on building regulations for energy-efficiency, Phil Brown, technical advisory service manager at Pilkington United Kingdom Limited, part of the NSG Group, 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, the key set of building regulations in England for the conservation of fuel and power, 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?
Calculating the right balance
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, g value and light transmittance.
The U value is the rate of heat loss per square metre through the glass by a temperature difference of one Kelvin, between the outside and inside environment. In layman’s terms, the lower the U value is, the better the glass is at keeping heat inside the house during the winter.
The g value is the total amount of the sun’s energy transmitted through the glass. This is made up of the direct transmittance and any heat absorbed by the glass that is then re-radiated into the interior.
Lastly, light transmittance, as the name suggests, is the amount of visible light (or daylight) that passes through the glass, usually measured at near normal incidence.
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 air-tight 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, as such, windows on this elevation can be large without risk of excessive overheating in the summer - but they should still be of 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 (i.e. better solar control performance) may also be appropriate here, particularly if the windows are large.
We’re all increasingly aware of the health, wellbeing and productivity benefits of exposure to natural daylight, but it is not always adequately addressed in regulations. 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.
For the time being, we can only wait to see what the new proposals for Part L will bring and evaluate the opportunities for housebuilders and property developers to fully utilise glass products to deliver comfortable and healthy low-energy dwellings.