Underground Buildings

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There are many variants, both ancient and modern, of earth sheltered or fully underground buildings. An underground building can be defined as a house having its roof and at least two walls covered by layers of earth more than 50 cm deep.The insulation value of earth is about one-twenty-fifth of the value of mineral wool, so if the roof is thinner than 2-3 m, extra insulation is needed. By planting trees or bushesonthe roof, heat loss is reduced. The building should preferably be on a south-facing slope to take advantage of solar radiation and the floor must be higher than the water table. The loading on the roof can be more than ten times that of a normal building and the pressure on the walls slightly greater than on a normal basement wall. It is important

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A traditional subterranean dwelling in atrium style in North-western China. These houses have proved to combine low energy use with a high quality indoor climate (Chen et al. 1997). Source: Anne Sigrid Nordby.

14.12

A traditional subterranean dwelling in atrium style in North-western China. These houses have proved to combine low energy use with a high quality indoor climate (Chen et al. 1997). Source: Anne Sigrid Nordby.

to have good drainagefrom the roof, which should also be laid on a well-drained material with a high friction coefficient.

These days, houses are generally built above-ground.There are probably cultural reasonsfor this move to the surface of the Earth, because, practically speaking, nothing is as sheltered as an earth-sheltered house. In the USA, over 600 underground buildings were erected between 1978 and 1980, including homes, libraries, schools and offce buildings.This happened even though the cost of an underground building has been calculated at about 10-20% more than that of a conventional building (Winquist,1980).

One main aim of an underground house is to save energy, and it is symptomatic that the sudden rise in popularity came after the energy crisis of the early seventies, only to fall again once oil prices began to fall.Temperatures at a certain depth underground are stable over the year: they are warmer in winter and less cold in

14.13

The temperature at different depths of the Earth throughout the year in southern Scandinavia. Source: Lag, 1979.

14.13

The temperature at different depths of the Earth throughout the year in southern Scandinavia. Source: Lag, 1979.

summer (Figure 14.13). At 20 cm below the surface, the variations in temperature over a 24 hour period are hardly noticeable. This means smaller temperature changes in the fabric of the building and thereby fewer maintenance problems and a longer life span. These houses cannot, of course, be built where there is radon in the ground.

Naturally traditions of underground building are most prevalent in regions wherethe soil or rock is easy to excavate.The American experience is that underground houses are best suited to sunny climates with cold nights. In such locations energy consumption is reduced by 20-80%.

Structures built in on the surface of the earth also have interesting climatic aspects, particularly with respect to temperature equalization and moisture buffering.

Only about the outer 2 cm of the material will be active as a humidity regulator during a 24 hour cycle, but the moisture buffer value is probably better than for both timber and aerated concrete (Minke, 2006). In Northern Europe there are indoor swimming pools and moisture-sensitive libraries built with clay as the main material. Also, a whole series of earth-based plasters has been developed for concrete and hard fired brick in order to reach a more stable humidity within the building (Figure 14.14).

To achieve reasonable thermal insulation a porous substance can be added. This can be expanded perlite, crushed pumice, foamed glass granules or plant matter such as sawdust, wood shavings, cork or straw, see later sections in this chapter. Earth is also wind-proof when compressed.

Water will not usually penetrate a horizontal layer of earth more than 50 cm deep. Thick traditional earth roofs found in Iceland are therefore relatively safe from leakage. Earth containing a large quantity of clay is waterproof, even in thinner layers. The optimal clay is bentonite (see 'Montmorillonite' page 262), which is waterproof at only 0.5 cm thickness.

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