Diagram Architects

Rule 5: Mass is Critical

If you’re interested in energy efficient house design, you’ve probably come across the very impressive sounding terms “thermal capacitance” or “thermal mass”.

The principle of thermal capacitance is pretty easy to understand: Heavy (or, more accurately, massive) stuff takes a long time to heat up. Once it’s hot it takes a long time to cool down again.

You’ve probably noticed this if you’ve ever stood against a west-facing brick wall on a cool evening after a hot day and felt the day’s heat still radiating off, or if you’ve felt last night’s coolness of concrete under bare feet on a summer morning.

Thermal mass, coupled with smart glazing and well-designed insulation, is one of the most important efficient-design tools available to an architect or building designer. It is most useful where temperatures swing between hot and cold, or in cold areas where heat sources, like the sun, are occasionally available. (It is less useful in hot, humid climates, where average temperatures are high, and cooling breezes must be maximised.)

Massive materials can store heat for a long time, and if designed properly, release it when it is most useful. Depending on the soil type, depth and structure, some well designed underground houses can access heat or ‘coolth’ from last winter or summer, 6 months or more ago. A concrete slab or thick masonry wall (300-400mm brick, stone or concrete) can release heat or ‘coolth’ from almost 6 hours ago.

Thermal mass can also be useful to draw heat from a space when too much heat is available, and re-radiate it later when temperatures drop. Some house designs can allow you to draw heat from one area (from the outside, for instance) and relocate it to another (to inside) – roof ponds or geothermal heating/cooling work on this principle.

There are situations where thermal capacitance is not useful. In spaces which must heat up or cool down quickly (for instance, when a heater is turned on in an irregularly used room), thermal mass will slow the heat response; a well-insulated lightweight room may be more useful.  And thermal mass can be awful when inappropriately used, as anyone who has baked in an uninsulated hot-box brick apartment on a stifling summer night (well after the outdoors has cooled) will know.

To use thermal capacitance advantageously there are a few questions you should ask:

• Is thermal capacitance going to be useful to me? This leads to some other questions: What are my maximum and minimum temperatures? What is the average between these extremes? How long is the cycle between hot and cold?

• How can I admit the useful and exclude the troublesome? (This might include shielding from extreme western sun, designing windows and eaves to admit more sun in winter than in summer, or insulating the thermal mass from unwanted temperature extremes).

• How much thermal mass do I need? There are rules of thumb for ratios of glazing-to-mass. For underground houses some site testing may be required.

• How do I plan to use the space? Will the usage be regular or irregular? When is heat or cold more useful?

The thermal mass in the earth-coupled polished concrete slab and the internal paddock-stone walls in our Wistow Smart Farmhouse (pictured above) allow the living spaces inside to remain comfortable year-round. The farmhouse needs no mechanical cooling and a minimum of heating in winter, despite baking summers and cold, grey winters.  Note the overhead ‘clerestory’ windows - these can be opened during cool summer evenings to purge the heat absorbed by the stone walls and concrete slab during the day.