Passive temperature control in buildings

Passive temperature control uses the environment and the properties of the building to control temperature within the building. Active temperature control uses energy consuming methods such as heating by burning fossil fuels and cooling with air conditioning to control the tempertures in the building. Passive temperature control, while it has its limitations, and requires thought and possibly extra cost at the construction stage, uses little energy.

Thermal mass Substances vary in the amount of heat needed to change their temperatures. In general, heavy (massive) things like stone, bricks and steel have high thermal masses, it takes a lot of heat to raise the temperature of a stone wall for example. On the other hand, once warm, a stone wall can provide heat to slow the cooling of a room for a long time. But weight-for-weight, very few substances have a thermal mass as great as that of water.

There are several fundamental features of passive temperature control:

Insulation;

Insulation limits the amount of heat that unintentionally enters or leaves the building by conduction.

Thermal mass;

Thermal mass provides some stability to the temperature in the building.

Controllable ventilation;

If ventilation can be controlled then it may be possible to use the cool of the night to reduce the temperature in the building, or the warmth of the day to increase the temperature, whenever outside temperatures are suitable. Undesirable ventilation from gaps needs to be minimised.

Control of the entry of sunlight.

Stopping sunlight from getting into the building in summer, but allowed to enter in winter (supposing that the sun is shining), can give added control of temperatures without using energy.

Conduction;

When you touch a hot pot on a stove top heat is transferred to your hand by conduction.

Convection;

A fluid (liquid or gas) that is warmed will (in almost all cases) expand and become less dense than the surrounding fluid; it will then rise while the surrounding fluid falls to take its place. Applying heat to the bottom of a pot warms all the water in the pot much more effectively than by applying heat to the top of the pot would.

Radiation;

The heat that you feal on your skin when you expose it to direct sunlight is moving by radiation. Radiation is most important when the radiating body is very hot (the surface of the Sun is about 5500°C); although it can still be significant if the radiating body is large (for example, the ceiling of a room), but only a few degrees above the temperature in the room. It is the only way that heat can move through the vacuum of space.

Sun shining through windows; or onto blinds, warming the blinds, then heating the room by convection;

This can be minimised by:

• Ideally, stopping the sun shining on the window (by use of a shutter or similar on the outside of the building);
• Stopping the heat that gets through the window from getting into the room by the use of blinds or curtains.

Pasing through walls, roofs or floors by conduction; the unwanted heat that is conducted through the ceiling will then get to the rest of the room mainly by radiation.

This can be minimised by the use of insulation.

Coming in with hot air that blows through gaps under doors etc.

This can be minimised by blocking the gaps.