A hot topic: preventing homes from overheating

A hot topic: preventing homes from overheating

Featured Article
04 Jan 2019

As Featured in GGP Magazine - 01/18

Phil Brown, European regulatory marketing manager at Pilkington United Kingdom Limited, part of the NSG Group, explains the opportunities and challenges for supplying solar control glass into the residential building sector.

Glass is playing an ever more important role in the design of buildings. Many new commercial developments feature glass facades, glass boxes or even entirely glazed envelopes. Solar control glass is typically specified in these applications to help prevent large glazed areas from creating a greenhouse effect. 

In homes, we see a lot less solar control glass being specified. But, as the UK’s residential stock becomes better insulated and airtight in order to achieve carbon emission goals, the industry will need to consider its specification more to stop homes becoming stuffy and uncomfortable. But, what do building designers and specifiers need to know when choosing this type of glass for their projects?

Potential overheating issues have been examined in a new guide for designers that highlights the different interactions and trade-offs that are important when selecting windows for homes.

Energy, daylight and thermal comfort

The NHBC Foundation’s recent publication Windows – making it clear: energy, daylight and thermal comfort, summarises the latest research into how window design can influence the energy performance of dwellings. 

To compile the report, the NHBC Foundation used the methodology on which Passivhaus is based. This is the world's leading ‘fabric-first’ approach to energy efficiency, where the building itself does the work, rather than using energy-renewable devices like solar panels. A computer simulation modelled four home types, and compared four properties related to the windows: the glass type, frame factor, total window area and orientation. The risk of overheating in each home was evaluated by calculating the number of hours in the year when the room temperature exceeded 25oC.

The study identified several parameters that housing designers need to consider relating to windows. This included minimising heating demand during the colder months; avoiding excessive overheating during warmer periods and maximising daylight admittance. 

It’s possible to find a balance between all three to optimise performance, but care must be taken to ensure that improving one parameter does not result in a detrimental effect on another.

Frame factor, orientation and type of glass

The frame factor, which is the proportion of the glass-to-frame area, was found to be very influential in achieving the right performance balance. Windows with slimmer profiles and fewer frame members will generally lose less heat and admit more daylight than those with wider profiles and more frame members.

The window's orientation in terms of solar gains was also examined. The findings may encourage designers to give more thought to the glazing type in each window. While it might be convenient to have the same glass specification on all elevations, this approach does not optimise performance. This is because windows that are orientated differently, or are on different walls, will all have different levels of sun exposure.

Preventing overheating is technically challenging as it involves the thermal mass of the building, ventilation levels and the type of glass, among other considerations too. The report acknowledges the importance of the g value; the total amount of the sun's energy passing through the glazing, when selecting a type of glass. A higher g value can usefully heat homes in winter, whereas a lower g value may be desirable for solar control purposes during the summer months. It’s a delicate balance!

For an apartment that’s surrounded by apartments above, below and around, for example, triple glazing was the best option for a southern orientation. It resulted in the lowest heating demand in the winter, but also offered the least risk of overheating in the summer. Perhaps surprisingly to some, increasing the window area in the mid-apartment actually led to a reduced space heating demand.

Triple glazing and solar control products

As triple glazing generally has a lower g value than double glazing, due to the extra pane of glass, it’s more effective at combating solar gain in the hotter months. 

As specifiers become better informed on how to specify and install solar control solutions we should see this type of glass given more consideration in housing in the next few years. Mid-range products such as Pilkington Suncool™ One 60/40 that offer solar control without compromising too much on daylight, might be key in keeping homes at optimum temperature.

Another interesting recommendation from the report was to avoid product substitutions as these could affect energy performance and compromise the original design. Windows that are outwardly similar may have very different U values, g values or frame factors, all of which could potentially reduce the energy performance of the house. In other words, stick to spec!

Improving thermal comfort

Quantifying the effects of different glass and window specifications on the energy performance of a home is not simple and can depend upon many factors, including the dwelling type and specification, its environment and location. In particular, the solar gains and heat losses in dwellings interact in a complex way.  

This latest research from NHBC can only help to inform the debate and help the glass industry offer products to optimise the performance of buildings and, as the effects of climate change become more apparent, alleviate overheating and improve thermal comfort for householders.