Solarity Extension in Oxfordshire Nears Completion

The extension faces just off South, and its large roof overhang is intended to reduce solar passive heat gains from overheating the extension. Inside, rooflights bring overhead light deep into the plan. Rooflights are useful in that they can be adequately sun-shaded quite easily with blinds, as well as being useful in promoting natural cooling if they can be opened.

As a simple analogy, the roof overhang on the South elevation acts like the brim of a hat. It shades out the high, hotter sun of summer while admitting the lower more useful heat of the winter sun.

There is a rather more subtle aspect of a large roof overhang. This is the sense of shelter and protection it gives to the inside. The famous American architect Frank Lloyd Wright called it ‘an edifying sense of shelter’. Apart from the ‘edifying sense of shelter’ aspect, there is another aspect of the roof overhang to this extension that is subtle. It allows the full-height glazed doors to be left open even in deluging rain. There are few things that a building can offer that are more exciting than watching and smelling the rain outside from a place of shelter. 

All solar passive buildings need to balance the desire to admit sunlight for its free heating, against stopping the sun from making a structure so hot inside as to be uncomfortable, or even uninhabitable. And in the foreseeable future cooling is going to be more important. Hence in this extension the use of the broad roof overhang to shade the high hot summer sun out of the house while letting in the useful rays of low winter sun. 

Another aspect of the roof design of this extension is interesting. The broad roof overhang necessitates extra attention being paid to holding DOWN the roof against wind lift. Here this was achieved by the use of steel straps at every other roof joist passing down the outside line of the masonry leaf and fixed to the masonry with resin fixings at approx 400mm centres. The extension of the straps down the wall is of the order of 2 metres. We are assured that the roof will not be parting company with the walls anytime soon.

The structure of this extension is unusual in that, contrary to the way British builders build houses, the internal leaf of the wall elements is designed to be secondhand brick or block, with the outside skin being timber framing with a minimum infill of 200mm of insulation. With this form of construction – completely the opposite of how houses are normally built in the UK – the material of high thermal mass is INSIDE, where it can help keep temperatures warm in winter (by holding the heat in) and cool in summer (by holding the cool in), with the insulation OUTSIDE of it.

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