How to... build a glass extension with a clear conscience

They might look amazing and let in lots of light, but can glazed extensions ever be good for your carbon footprint?


Everybody seems to be building them. From Glastonbury to Glasgow, glass extensions are protruding from the back of our homes. The appeal is obvious. Plenty of natural light, uninterrupted garden views and those sliding glass doors that, in estate agent speak, ‘bring the outside in’. But what is the impact of all this glass on the ecological performance of our homes?

Glass has some indisputable green credentials. A resilient, recyclable material that lasts for centuries, it lets natural light into our homes, reducing our dependence on artificial lighting. However, its thermal properties have incited much debate among the architectural community, and theories on how it should best be used abound.

‘It’s a very difficult subject to address in simple terms,’ says Chris Herring, a director of the Green Building Store (greenbuildingstore. ‘The main principle of glass is no matter how much you use – single, double or triple glazing – its thermal transition will never remotely approach that of a solid wall.’ This means that glass will allow a lot of heat to escape from a building – on the flip side, it will also allow a lot of it in. The fundamental of building with glass is well known: optimise winter solar gain by having as much of your glazing as possible facing south, and minimise solar gain with some form of shading. But this is not necessarily a green light for unlimited use of glass. ‘It’s about achieving a balance,’ says Herring. ‘You have to evenly distribute solar gain against heat loss and remember that at night you’ve got pure loss.’

MT640201Extensions that are separated from the rest of the house by an external wall or French doors are fairly straightforward. ‘The extension is outside of the thermal envelope of the building,’ adds Herring. ‘While it doesn’t really add to its performance, it won’t detract from it either.’ And temperature fluctuations (too hot in summer, too cold in winter) can be controlled using double or triple glazing. Yet, from a sustainable viewpoint, it’s crucial that you never heat this type of space. If you do, the heat will escape – a shameful waste of energy.

Most people, however, want to combine their glass extensions with the rest of the building, creating the popular open-plan, semi-transparent, kitchen-diner. This type of structure stirs debate.

Some architects argue that incorporating solar spaces with large glazed areas in houses is an effective means of creating low- energy buildings. Solar intake is maximised in south-facing glazed spaces, which heats up air that can then be drawn into the cooler parts of the building through passive stack ventilation (air moves as a result of temperature differences). Generous amounts of thermal mass, in either the floor or the building core, usually complement this approach, storing radiant heat and releasing it when the ambient temperature falls. More complicated schemes use technologies such as heat pumps. Architect John Lees ( has incorporated heat pumps into his designs, drawing cool air in at a building’s lower level and using the chimney effect of glass to pump it up to the top. A collector with a reverse air heat pump can then either provide electricity or cooling back into the building. ‘We should be asking how glass can work to our advantage rather than being negative,’ he says.

Lees also points to the flourishing market of high-performance glass. Companies such as DuPont ( and Pilkington ( sell a number of products that can do all manner of things, from reducing solar gain to trapping heat inside. The Green Building Store has just introduced a Passivhaus standard timber glazing unit that is triple glazed and has an insulated frame, while Eco Age ( has glazing units with built-in solar panels.

MT640228Yet, it’s impossible to deny that glass does not perform as well as a solid wall. ‘There’s actually very little evidence that extensive glazed areas lead to low-energy buildings,’ says Herring. ‘Many of the German architects who were championing solar architecture in the Eighties have developed their ideas around the Passivhaus approach: insulate well, keep the building airtight and get the optimum energy balance between glazing and opaque areas through modelling software. You can still have glass, but you can’t bolt a large glass extension onto your kitchen.’ Oliver Heath, of Blustin Heath Design ( agrees. ‘You can exploit passive solar gain, but it is a complex design challenge and requires a lot of fine tuning,’ he says. ‘The simplest solution is to avoid great panes of glass.’ So, it seems like with most things in life, if you want to build responsibly with glass, moderation is the key.

Words: Cathy Strongman

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