Why use a concrete slab?

Lesson #1: Why a concrete slab?

One of the most important elements of an energy efficient home is the inclusion of  thermal mass. In simple terms, this is what will keep your house warm in winter and cool in summer – in conjunction with good solar passive design and a high level of  insulation. All of these elements combine to minimise the need for active sources of heating and cooling.

Thermal mass design is particularly effective in Canberra’s climate, with its extreme day/night temperature range. A concrete slab enables ‘earth coupling’ – in other words, the slab sits directly on the earth which is itself a thermal mass with a comparatively stable temperature. The below ground temperature is warmer than the ground surface and air temperature in winter, and cooler  than the ground surface and air temperature in summer. Earth coupling actively helps the house to achieve similar temperatures to the earth below the building. This method of ‘climate control’ is nothing new – civilisations have been using these methods for thousands of years, not only for comfortable living conditions but also for storage. This is why wine cellars are traditionally located under ground!

A concrete slab acts as a means of energy storage and clever design will make the most of this. In winter energy from low-angled sunlight coming into the north side of the house will be absorbed into the slab. As night falls the heat stored in the slab will gradually release to keep the house warm overnight. With Canberra’s cold climate it’s therefore crucial that the slab edge is insulated to prevent stored warmth from escaping. In summer, design elements such as the right width eaves to shade windows and external walls prevents the sun from shining onto the internal concrete floor. This helps prevent the house space from gaining heat. Cooler temperatures from the ground below the slab will help keep the house cool, creating a comfortable environment inside the building.

As the table below shows, concrete is the most effective construction material for creating a thermal mass.

MATERIAL	THERMAL MASS (volumetric heat capacity - KJ/m³.k)
Water	4186
Concrete	2060
Sandstone	1800
Compressed earth blocks	1740
Rammed earth	1673
Compressed fibre cement sheeting	1530
Brick	1360
Earth wall (adobe)	1300
Autoclaved aerated concrete (AAC)	550

Source: EDG

The benefits of using a concrete slab don’t end with energy efficiency – it’s also termite-resistant and concrete is a recyclable material.