Civil Engineering Reference
In-Depth Information
Wind breaking membranes should not let through more air than 0.1 m
3
/m
2
with
a pressure of 10 Pa. In extremely windy conditions such as heavy storms or hur-
ricanes it is very difficult to prevent wind penetrating the building. In exposed
locations it would be best to use heat insulation materials with good wind-proof-
ing properties as well, e.g. well-compressed cellulose fibre.
Diffusion of gas and breathing walls
Internal climate usually needs a flow of fresh air equivalent to half to three
changes of the whole air volume per hour, depending on the room's function.
In buildings which have airtight vapour barrier membranes in their walls, the
flow of fresh air depends upon specific openings for ventilation such as win-
dows. In a building with dynamic insulation, the flow of fresh air enters through
the external surfaces. At the same time, contaminated gases in the internal air
will be drawn out through the surfaces by gas diffusion. Gases have the particu-
lar property of always wanting to spread themselves evenly in the surroundings.
The flow through the walls will therefore travel in both directions, and is per-
manent, though the pressure and the particular gas and molecular weight decide
the speed. This also depends upon the material's capacity for letting through the
different gases, i.e. the resistance to gas diffusion.
In principle there will also be substantial gas diffusion through materials that are
initially far too dense to be used for dynamic insulation. For example, a 20 cm thick
brick wall with an area of 10 m
2
lets about 90 litres of oxygen through each hour
under normal pressure. This is the equivalent of one person's use in the same peri-
od. An equivalent calculation for concrete gives about 11.25 l/hour. The conditions
for this calculation are that the oxygen content of outside air is 20 per cent and for
inside air it is 15 per cent. It also assumes that conditions are ideal without com-
plicated variations in pressure around the walls, ventilation intakes, etc.
Little is known about how walls breathe in practice. Researcher Lars
Möllehave at the Hygienic Institute in Århus in Denmark has measured the dif-
fusion of freon gas through material in walls in rooms with no cracks, which
clearly shows that the process exists and is very active.
Snow as a climatic material
The thermal insulation of dry snow is equivalent to that of rockwool. This is
reduced with increased water content.
Over large areas in Northern Europe, dry snow settles every winter and
remains for six months, helping with insulation just when it is most needed. So
it is quite clear that this snow should be conserved. There are six ways of retain-
ing snow on a building:
