Civil Engineering Reference
In-Depth Information
screened building. In a standard house, 30 per cent of the heat loss happens through infil-
tration. But the air around a building should not be completely still; 1-2 m/s is optimal.
Heat radiation has a greater effect when there is no wind.
There are three main methods of reducing the infiltration of wind into the main
body of a building:
• Windbreak
• Turbulence membrane
• Airtight membrane
Windbreak
A windbreak is perforated and should preferably be on battens at a good dis-
tance from the outer wall, so that a useful storage area is available in the space
between. By using about 30 per cent perforation a minimal difference of pres-
sures between the front and the back of the screen is achieved. The formation
of eddies is thus reduced, and wind and rain are effectively slowed down.
Suitable materials include climbing plants, trellis work, timber battens or metal
ribbing.
Turbulence membrane
A turbulence layer is mounted directly on the main wall, and is usually made of
different types of roughly-structured surfaces which cause innumerable small air
movements in the material - a sort of air cushion. The wind is stopped dead
instead of penetrating further into the wall. Materials suitable for this are rough-
ly-structured render, cladding made of branches or a living surface of plants.
None of the methods are 100 per cent efficient; there is always the possibility of
weak points, and some wind will force its way through. The turbulence layer has
no effect on infiltration as a result of suction, and usually needs to be comple-
mented with an airtight layer.
Airtight membrane
Suitable materials for an airtight layer include sheeting, boards, paper sheeting
and mastic, as well as external cladding. Holes and gaps in the structure, e.g.
around windows and other building components, should be closed.
An airtight membrane for a whole wall often consists of paper sheeting, wood
fibreboard or plasterboard sheeting, which can be improved by waterproofing.
This is placed behind the external cladding and is well ventilated.
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