See also “One Year of House Performance: I”.

Like the graph in the post linked above, this is a log of indoor and outdoor 7-day mean temperatures at my low-energy solar-passive house at Manilla, NSW.
In place of the curves for normal air temperature and comfort zone limits, this graph includes two (raw value) logs of subsoil temperature at 750 mm below the surface. The green trace is the subsoil temperature outdoors in the garden. The orange trace is that below the middle of the main floor slab. The mass of material below the slab is surrounded by insulation at the edge so as to form a “heat bank”.

Outdoor subsoil temperature

For over half the year, the outdoor subsoil temperature (green) closely followed the outdoor 7-day average temperature, with fluctuations reduced to less than one degree, and lagged about four days. It peaked at 27.7° just after the outdoor averaged air temperature peaked at 30.6°.
From mid-March to mid-August, the subsoil temperature was several degrees warmer than averaged air temperature, as seems to be normal in this climate.

Consequently, outdoor subsoil temperature reached its minimum value of 13.3° about a month after the averaged outdoor air temperature minimum of 6.7°. This year, the subsoil temperature range at 750 mm had a range of 14.4° ( from 27.7° to 13.3°).

Heat bank temperature

The temperature of the heat bank (orange) followed a cycle like that of the outdoor subsoil, but with the much reduced range of 6.0°, from 23.0° to 17.0°. That is mainly within the comfort zone (not that anyone lives under the slab). One notable difference between heat bank and subsoil temperatures is that, due to solar heating of the interior brick walls and the tiled concrete floor slab, the heat bank warmed slightly during April, while the outdoor subsoil cooled by three degrees.

Influence on house temperature

Through winter, it seems clear that the heat bank temperature has a very strong effect on temperature in the (unheated) house, or that heat is shared between the two. Both hold temperatures very much higher than either outdoor subsoil or outdoor air.
In summer the heat bank, due to the shading and insulation provided by the house, remains much cooler than the house or the air or the outdoor subsoil. This must feed “coolth” into the house, yet the house is not very cool.
I suggest that, despite extremely effective shading, the heat load on this house in summer is so high it defeats the heat bank.

This material was posted in a weatherzone forum on 6th June 2011. It is posted here and made sticky on 29th July 2014, but back-dated to 24th July 2011.