One of the hallmarks of the contemporary home is a large area of south-facing glazing. Everybody seems to want it, whether you are building traditional or modern, new build or extension. Architects love it, self-builders love it, it photographs well, it looks good. And of course, it’s something of a techno-marvel: big glazed screens weren’t realistically possible until fairly recently and our attraction for them has grown as our mastery of their installation has matured. We’ve gone from French doors to patio doors, to folding sliding doors, to glazed gables, right through to frameless glass expanses.
The attraction is obvious; not only does it blur the distinction between inside and outside – or appear to bring the garden right into the house – but on sunny days it can literally bathe the interior in warm sunshine.
It’s this latter phenomenon – sometimes known as passive solar gain – which is where it all starts to get a little difficult and uncontrollable. In theory, passive solar gain is a very good thing because you are getting loads of free heat in winter. If your house is of heavy masonry construction – and therefore has high thermal mass – it can store this heat in the walls and floors. They then act like a radiator, gently seeping heat back into the living spaces long after the sun has set. Back in the 70s, people built houses designed to heat themselves using nothing more than this effect and many thought we had cracked the problem of space heating using no energy at all. Unfortunately, they only ever seemed to work in places like Arizona. In the UK, it was just too cloudy and damp and the winter days were just too short.
Instead, while the large glazed screens provided little comfort in winter, people found that they were increasingly uncomfortable on hot summer days, as the area behind the glass acted like a conservatory. There are other effects as well: the ultra-violet (UV) in the sun’s rays fades the colours it exposes and can even cause floor adhesives and some flooring types to break down, leading to a number of re-decorating issues.
Exactly the same issues occur with conservatories, but a conservatory can conventionally be shut off from the rest of the house – a thermal break – so that the particular problem of too much sunshine and too much heat can be overcome. When the glazing forms part of the fabric of the house, the problem cannot be shut off so easily.
In fact, our Building Regulations now address the issue, which they refer to as ‘limiting the effects of solar gain in summer’. The regulations themselves refer to another document, SAP Appendix P, which includes methods for calculating solar gain, and indicates some of the solutions we can use to minimise some of the less welcome effects.
The further away from due south you face the glazing, the smaller the problem of summer overheating. UV rays in particular are also much stronger when the sun is high in the sky. Once you get away from the idea of capturing as much winter sun as possible (for which south-facing glazing is essential), you are free (neighbours permitting) to open out the house in any direction you choose. West, towards the setting sun, is probably best.
You can introduce an element of shading from a roof overhang, designed to take account of the fact that the summer sun tracks across the sky at a very high angle. A correctly positioned overhang has the potential to shade much or all of a south-facing window in summer, yet absorb all of the low winter sun.
This translates as ‘sun breaker’, and seeks to do the work of a roof overhang but with a permanent structure built adjacent to the window. Ideally, a brise soleil needs to look attractive in its own right, provide shading from the high summer sun and yet not block the view from the interiors. They are widely used on glazed office buildings, but are not so common on homes.
Blinds and Curtains
Whereas a brise soleil is a fixed element, blinds can – by their very nature – be removed or adjusted. They can be fitted internally, externally or sandwiched within the glazing units. Curtains are, of course, the low-tech, old-school solution but generally they don’t fit well aesthetically with large glazed panels. Blinds are the preferred option but be warned some can be very expensive, often costing as much as the glazing units themselves.
Visit the Mediterranean and you can’t fail to notice the use of shutters on windows. Shutters here serve a dual purpose: they keep the sun out of the house, but they also allow cross ventilation, using a concept known as night time purging. This is where the windows are left open all night whilst the shutters are closed but open vented which allows a throughput of air across the house, removing much of the heat stored within the walls.
Whilst shutters won’t work with a large glazed screen, the importance of ventilation is worth noting. A well-planned ventilation system will be capable of removing much of the unwanted heat and make the house much more comfortable to live in during the summer months.
Specially coated glass is available which works to reduce solar gain. Products like Pilkington’s Suncool and St Gobain’s Cool-Lite work to reduce solar transmittance whilst maintaining excellent low-emissivity — necessary to meet today’s low U value standards. These techniques are improvements on what we all know as glass tinting — unlike car glass, tinted glass for the home has to be clear to be acceptable.
Unless you can be absolutely certain that the summer sun will never find a way through your façade, you should take action to avoid some of the worst side effects (the faded shop window look you often see on high streets). Avoid natural flooring materials like linoleum, cork and sisal which tend to break down or, at least, lose colour under UV. Furniture will also tend to fade rapidly, so go for pale colours. Keep artwork away from direct sunlight, and rugs should be rotated frequently. If you are laying a floor which requires adhesives, check to see if the adhesives are UV resistant — rubber-based ones are best avoided.