Some time back I was at a ‘passive energy’ seminar where the speaker said that any interior thermal mass must be located so that the sun could shine on it; if not, then the mass would be of no benefit.  I wasn’t sure if I had heard correctly so asked for confirmation, which I was given.  If this is correct, then hypothetically an ice block at the back of the solar-warmed room – which is always out of the sun’s path – would never melt.  Yes, the temperature of a sunlit face of an interior thermal mass will rise faster than that of a similar face in the shadow but the shaded surface will still warm to the ambient temperature of the room.

Any material will react to a temperature difference between itself and the adjacent environment by absorbing or emitting 'heat' energy.  With regard to building materials the ability to do this is commonly labelled as the 'thermal mass' of the materials.  When this feature becomes significant for a given volume of the material, it is popularly referred to as a 'thermal mass product' e.g. concrete, bricks, etc.  This is in contrast to, say, fibreglass insulation pads, which do not add to the thermal mass of a building.

In a low thermal mass room the input of solar or network energy (for a heater), results in a rapid temperature rise; in a high thermal mass space the temperature rises more slowly because the 'heat' energy is 'disappearing' into the construction materials.  When the heating source is removed, the temperature of the low thermal mass space falls much faster than in the high mass space because there is much less stored energy to be released. (Think of the rapid temperature fluctuations in a tent.) In Australia, thermal mass is used to even out the excessive climatic temperature variations.

Thermal mass is greedy in that it will aggressively soak up heat energy if the adjacent space has a higher temperature, but becomes generous when it gives it up if the adjoining temperature is lower than itself.  The rate of movement is governed by the degree of temperature difference.  Therefore in a building the same thermal mass product will significantly influence the interior environment differently if it is part of a northerly sunlit room compared to one with a dark southerly aspect.  Equally, materials of differing thermal masses in similar rooms will have significantly differing effects between night and day, summer and winter, weather fluctuations, etc. 

Recently I analysed a timber framed house in northern New Zealand where the introduction of appropriately placed thermal mass reduced the energy deficit (the energy needed from heating and cooling appliances to maintain a sensible comfort range) by 25% – that is, a 25% reduction in this portion of the monthly power bills over the lifetime of the house.  A room that does not receive a solar input will, to maintain a given temperature, require more additional energy to heat it if there is a greater thermal mass in the room.  The south wall bathroom is a classic case if it has a tiled concrete floor instead of a timber floor.

While it is possible, through experience and practice, to intuitively design for the positive and negative effects of thermal mass, the analysis I am able to do through Ecorate Ltd’s thermal simulation services enables more precise, objective results of these effects to be quickly obtained so as to better inform the other design and material selection options.  Minimal thermal mass is not useful (think of the tent), much as 100% thermal mass is not either (as in the Waitomo Caves), which makes changing the adjacent temperature very difficult.  However, there is an ideal ‘sweet spot’ between.  This differs for each building and each site, meaning that there is no simple rule-of-thumb that can be applied; each project requires a unique solution.