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.

Rule 2: Stop fighting the universe.

The universe wants you to design an energy-efficient house. No, seriously; it does. Why else would so many things, from the tilting of the earth to the laws of thermodynamics, give you the tools to heat or cool your house for free?

More energy lands on the roof of your house than you’ll ever need to heat it. Last summer’s heat lies stored only a few metres under your feet, and last winter’s ‘coolth‘ only a short distance below that. Hot air rises, where it can be recirculated as required or vented to the sky at night. The evaporation of water draws latent heat, cooling the air. Glass transmits light (short wave radiation) but traps heat (long wave radiation). Light-coloured surfaces reflect. Mass provides thermal capacitance, carrying the day’s heat into a colder evening. Heat travels from warm to cold, but at different speeds through different media.

These basic physical laws give us passive solar heating, earth-coupled heat storage, night-time heat purging, subsidence (cool) towers, sunrooms, reflective foil and bulk insulation and the many other tools we can use for energy efficient house design.

The same tilting of the Earth on its axis which gives us winter also sets the sun lower in the sky, allowing heat and light to pass under deep eaves and further into a north-facing room (south-facing in the northern hemisphere). The increased azimuth of the sun in summer allows it to be screened, provided you calculate your sun angles and eave dimensions carefully. (Pritzker-winner Glenn Murcutt calls this “Letting the planet do all the moving”.)

These are not new discoveries. Xenophon of Thebes wrote about passive solar design in his Memorabilia (3.8.8-10) in 371AD, but the 20^th Century experience of cheap ,easily accessible fossil fuel has allowed us to heat and cool our buildings through the application of brute force.

Now, brute force is a fantastic thing if you’re planning to wrestle a gorilla or launch a payload into orbit, but using it to warm or cool your house is just pure bloody-mindedness if there are cheaper and easier options available.

So before you turn the first sod or draw the first line for your new, energy efficient house, we recommend you ask yourself (or your architect/designer) the following questions:

• Are we best using the house’s environment, climate and conditions?

• Is there a path of lesser resistance to achieve the same levels of comfort and delight?

And my personal favourite:

• What is the universe trying to give me for free?

By all means, create an energy efficient house because you’re concerned about energy independence, peak oil or climate change. But personally, I like energy efficient design because any other choice is just beating your head against the laws of the universe.