Passive Solar Design Principles - Floors

Floors are often the primary link between the structure of a building and its foundations (the ground upon which it sits).  Where floors are in direct contact with the ground, they can have a major influence on the internal climate by adding thermal inertia (capacitive insulation) which is assisted by thermal coupling with the mass of earth underneath the floor.

The coupling effect of the earth and building structure increases with depth.  Walls and roofs can also be earth coupled if the structure is excavated below ground. (As in “mineshaft” houses in White Cliffs and Coober Pedy).  Once a structure lies about 3.0 metres below ground, it has such great thermal inertia that it is no longer subject to day/night temperature swing, but only to slight effects from seasonal variation.

Ground coupled floors will benefit from convective insulation at the edges and perimeter.  This helps isolate the floor from the cold surface zones of the surrounding earth.  The ground layers around a floor in the first 300mm from the surface are subject to severe diurnal and seasonal temperature fluctuation.

FLOOR INSULATION: ground-coupled floors are usually insulated with closed-cell, extruded polystyrene.  High-density expanded polystyrene can also be used in some situations.  Perimeter insulation should be taken under the floor for 800-1200mm, or alternatively, out from the wall-line for 1200-1500mm.  By doing this, the connection between the cold external earth and warmer sub-floor earth is broken.  Continuous under-floor insulation can be beneficial for sites with shallow water tables.

Suspended floors will benefit from insulation to the complete exposed underside in order to prevent convective transfer to the outside air.

MATERIALS: most floors are made from either concrete or timber framing with board or sheet lining.  Steel framing is also common, however, thermal bridging will be greater with steel than with timber.  Thermal inertia is much greater in concrete floors (both ground-coupled and suspended) than with framed floors.  Less common materials include cement-reinforced, compacted earth and masonry paving on earth base.

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