In Australia, cooling has always been the challenge, and climate change predictions are that cool temperate heating climates like Melbourne will become cooling climates within the lifespan of a new building.
How do we achieve energy efficiency, healthy and safe indoor environments and enjoy comfortable buildings with that special relationship with the outdoors and have the greatest impact on reducing GHG emissions?
An alternative to sealed and insulated conditioned buildings is mass-linked ventilation in naturally conditioned buildings. Buildings should be designed so they are appropriate to climate so they are responsive to natural external conditions like solar radiation, cool night air and breezes and offer adaptive comfort.
Occupants wanting adaptive comfort don’t require precise temperatures. They accept satisfactory indoor temperatures more related to recent and prevailing external conditions with a supply of fresh air. It is much healthier to have expectations of internal comfort more in tune with sensible seasonal and daily dress decisions. As occupants move from inside to outside, they don’t experience the discomfort of thermal shock and the inconvenience of changing several layers of clothing.
Comfort, energy efficiency and good health are worthwhile outcomes, and each one should not be considered in isolation but as a set of goals. Of course, energy efficiency means full life cycle analysis not the smaller fraction that is heating and cooling energy.
Best results are achieved when design is optimised for climate and site because the design focus is on maximising gains and balancing and storing natural conditions within the massive fabric of the building to minimise the need for heating or cooling as first principle. Engineers sometimes refer to thermal mass as FES or fabric energy storage.
As is often the case with renewable energy sources, the sun, breezes and night-time cool are not always there when needed, so storage is the key. Sometimes in renewable systems, there is too much energy, so a heat sink is required. Mass is that heat sink. Best practice is achieved with earth providing the lowest embodied energy thermal mass available (Cement 2,640, Fired Brick solid 1,140, Lime 900, Fired Brick extruded, Natural sand/gravel 45, Earth five to 10 kWh/m3).
Furthermore, earth balances humidity, and mass provides radiant surfaces that offer the greatest level of comfort.
With mass-linked ventilation, fresh air and air movement – the other fundamental factors of comfort – are considered along with humidity control, radiant surfaces and temperature. Drafts and loss in int/ext movement through doors is not such an issue with mass buildings. Open all windows and doors in winter and close them some time later and the mass will recondition the fresh air and the building will lose some energy, though it will recover comfort without heating. The fire goes out at 2 am and in the morning it doesn’t need relighting for breakfast because mass has retained the heat and does the work conditioning air until the sun returns. There can be enough mass in a building to balance conditions for several days.
The thermal mass in the building may need to be orientated, zoned, shaded or exposed according to season, wall thickness may need to be increased, best colours can be chosen to either reflect or absorb heat, earth wall density can be optimised or indeed insulated (though it all depends on climate), and natural conditions able to be harnessed. The building ventilation needs to be managed by the occupants. It sounds complicated, though once principles are established according to climate, it is relatively simple.
Mass-linked ventilation is the key to the energy efficiency and improved indoor air quality in mass buildings. The thermal mass is a built-in fabric energy storage heat exchanger and air conditioner. The practice is ancient and automatic – the building responds to climate.
In warmer weather, ventilation is used to draw cool night air into the building for storage. Ventilation control is simple though the logic and practice is different in mass buildings and lightweight buildings. In a lightweight building, the heat of the day is usually flushed out quickly and windows are closed against the cold night air.
A heavyweight mass building is ventilated all night long, though inside it is not too cold as the exchange takes place between radiant warm walls and cool night air. The mass slowly relinquishes the heat absorbed during the day whilst it conditions the cold fresh night air and stores coolness for the next day. In winter, sun drenched mass walls absorb the sun’s energy and store it so that at least minimum ventilation can take place all day and night without losing everything gained from natural conditions.
Humidity is controlled through the hygrothermal properties of earth, so whilst there is heat exchange through ventilation, there is also a balancing of humidity. Thus, natural conditions are stored and offer natural conditioning. In addition to air changes, excess heat or moisture from cooking, washing or heating sources are balanced and stored by thermal mass. Lightweight sealed buildings easily become stuffy from excess heating and they store nothing but conditioned air. Excess heating and cooling is never lost in mass buildings; it is balanced and stored more like a bank account than a hole in your pocket.
The real benefit of thermal mass and appropriate climate responsive design is that air conditioning can be avoided in all climates with good design, whilst natural ventilation can be utilised to achieve minimum air changes without compromising adaptive comfort or energy efficiency. Heating, not cooling, is the challenge with mass buildings, and heating is far easier to achieve with abundant sun, many low-carbon portable energies like gas as well as carbon neutral sources like wood and wood pellets and renewable sources such as solar hydronic heating.
The qualities I enjoy in mass-linked ventilated buildings include the quiet, the stillness and stability of comfort that is more predictable and attuned to external conditions. There is no wonder, as they are designed to respond to climate and they naturally achieve adaptive comfort with simple control of ventilation.