Self Build & Improve Your Home - September 2013

If You plan to build an extension, at the top of  your list is probably the desire for more light and therefore lots of windows. So what can you do if you want to introduce as much glass as possible into your design?

First off, know that a fully-fledged glazed extension is referred to as a conservatory,
meaning everything is made of glass, including the roof. A sunroom, meanwhile, has a solid roof and this is probably what you will end up building due to energy efficient requirements in the Building Regulations (solid roofs and walls are much better at providing insulation and achieving the required u-values than panes of glass). See our Winter 2012 issue for more on Building Regulations and glass box extensions, available on

The verandah is an optional extra. popular in the last century, this semi-sheltered space can be
used to relax in when the sun is shining but the air temperature is cold or it’s windy. The
overhang can also act as a shade to keep the room behind from overheating, blocking the high
summer sun when the solar gain is not needed and conversely in the winter when the sun is at a
lower angle, allowing the solar gain to heat the room when it is most useful. Shading can be provided by other forms of solid construction, e.g. ‘brise soleil’ (see Symphony of Light article on page 104), or by retractable alternatives (canopies), which sometimes have the disadvantage of appearing like an after-thought. note that shading works best on south facing windows; those facing east or west will not benefit as much from it as the sun is shining at a lower angle from these directions.

Another solution to overheating is the use of blinds to block glare and the transference of heat
into the building. There are now highly sophisticated systems on the market which may
be automatically controlled; the blinds can also be housed within the window frames, making
them wind, weather and dirt-proof. They also have the advantage of giving instant privacy to a
room and where the design look is minimalist, they provide a very attractive alternative to
traditional blinds or curtains. If your extension windows are south facing and of ‘passive house‘ standard (triple glazed, thermally broken frame, u-value of unit 0.8 W.m2K or better) then the net heat gain for the window throughout the year is a positive one. put another way, the window is acting as a large radiator when calculating energy loss throughout the year. (See Systems and Services on next page). These high quality windows also have the advantage of not causing draughts within the room (airtight construction) and the ‘cold sheeting’ effect is minimised, that is, when the temperature outside falls to -10°C, the surface temperature of a high quality triple glazed window will be able to be maintained at 17°C as opposed to 12°C with a double glazed unit. 

The cost of triple glazing is now roughly just 10% more expensive than double glazed and is rapidly becoming the most popular option. Just remember to compare windows on the basis of the u-value of the entire unit (and other technical details such as the G-value) rather than base your decision on the number of panes – a high spec double glazed unit could theoretically perform as well as a low spec triple glazed unit, or near to it. note that with a large amount of glazing comes structural considerations – windows are heavy and require support, especially when triple glazed. A wall of glass will require the use of steel lintels, for instance.