Passive Design & Thermal Mass

What is energy efficiency and how do we achieve it?

An energy efficient home is a building which provides a high level of thermal comfort without an over reliance on artificial heating and cooling, as well as adequate lighting and ventilation.

Passive design features make the difference

Passive design is energy efficient design which makes the most of local conditions to make your home more comfortable while reducing your bills. Passive design costs no more when included at the planning stage. Good passive design uses natural heat from the sun and natural night-time cooling to keep your home at a comfortable temperature year round. It can significantly reduce the need for expensive mechanical heating and cooling.

The interaction of passive design features is illustrated in Figures 1 & 2 and the key considerations are:

• Orientation and solar access
• North-facing shaded glass
• Sealing and ventilation
• Insulation
• Thermal mass

It is important to tailor the passive design features to each climate. For example, in southern parts of Australia, prominent north-facing shaded windows with eaves that overhang permit the entry of winter sun and restrict summer sun.

The importance of thermal mass in building design?

Thermal mass is the ability of a material to retain heat energy when subjected to a temperature differential. Clay brickwork and concrete floors have relatively high thermal mass.

In summer, a high thermal mass wall can reduce the transfer of heat by absorbing the heat energy flowing in from the outside. This process is slow and results in a delay called thermal lag. The capacity to absorb large quantities of heat energy for a small rise in temperature combined with the thermal lag, effectively increases the R-value performance over the complete day-night cycle.

Maximum external air temperature is usually reached between noon and 2pm. A lag of six hours, which is typical for brick construction, means the maximum heat flow would not reach the interior until six hours later. By then external air temperature will usually have dropped and thermal flow will reverse, allowing the building to cool for the following day.

Thermal mass should be incorporated not only in external walls but in other areas as well.

• A concrete slab is an essential part of the equation as it increases mass significantly.
• Solid partition walls, as opposed to stud walls, will also add significantly to the mass.