Industrialised, Integrated, Intelligent sustainable Construction - I3con

HANDBOOK 2 SUSTAINABLE CONSTRUCTION
“Indeed, termites must live in a constant temperature of exactly 87 degrees (F) to
survive.” 7
“Termites farm fungus deep inside their mounds. To do so, the internal temperature must
remain at a steady 87 degrees F.” 8
“The fungus must be kept at exactly 87 degrees …” 9
There is just one problem: at least in the nest of Macrotermes michaelseni, there is no evidence that
nest temperature is regulated. Indeed, there is good evidence that it is not. In the subterranean nest of
Macrotermes michaelseni, for example, while temperatures are strongly damped through the day, they
also closely track deep soil temperatures through the year (Figure 3). Consequently, the annual march
of temperature in the nest ranges from about 14 o C in winter to more than 31 o C in the summer, a span
of nearly 17 o C. Nor is there any evidence that mound ventilation affects nest temperature. In the nest
of Odontotermes transvaalensis, which builds open-chimney mounds, eliminating ventilation
altogether (by capping the open chimney) produces no discernible effect on nest temperature [13].
These observations have a straightforward explanation: when a nest is embedded in the capacious
thermal sink of the deep soil, the nest energy balance (and hence its temperature) is strongly driven by
this large thermal capacity. This produces the nest’s strongly damped diurnal temperatures, and the
mound infrastructure and nest ventilation has virtually nothing to do with it.
Figure 3. The annual march of temperature in the nest of a Macrotermes michaelseni colony in
northern Namibia. For comparison, ground temperature 15 m away and at 1 m depth (about
the depth of the nest) is also plotted.
Another surprising feature involves the relationship between nest temperature and mound
temperature. A core assumption of Martin Lüscher’s thermosiphon model is that buoyancy is
imparted to nest air by waste heat from nest metabolism. There is, in fact, a considerable production
of waste heat, estimated to range from about 80 watts, to as high as 250 watts, and this can elevate
nest temperature by a few degrees above soil temperature [14]. However, for the thermosiphon to
work, nest temperature must be warmer than mound temperature. Extensive measurements over the
year of mound vs nest temperature for Macrotermes michaelseni reveal that nest temperature is most
frequently cooler than mound temperature (Figure 4). In short, the nest air is most commonly stably
stratified with respect to mound air, which means no thermosiphon flow is possible. Where the nest is
warmer than the mound, the buoyant forces that result will be weak and unlikely to drive much flow
[15].
7 http://www.aia.org/aiarchitect/thisweek03/tw0131/0131tw5bestpract_termite.htm
8 http://database.biomimicry.org/item.php?table=product&id=1007
9 http://www.zpluspartners.com/zblog/archive/2004_01_24_zblogarchive.html
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