Concrete slab floors come in many forms and can be used to provide great thermal comfort and lifestyle advantages. Slabs can be on-ground, suspended, or a mix of both. They can be insulated, both underneath and on the edges. Conventional concrete has high embodied energy. It has been the most common material used in slabs but several new materials are available with dramatically reduced ecological impact.
Some types of concrete slabs may be more suitable to a particular site and climate zone than others.
Slab-on-ground is the most common and has two variants: conventional slabs (raft slab) with deep excavated beams and waffle pod slabs, which sit near ground level and have a grid of expanded polystyrene foam pods as void formers creating a maze of beams in between. Conventional slabs can be insulated beneath the broad floor panels; waffle pods are by definition insulated beneath. Both may benefit from slab edge insulation.
Suspended slabs are formed and poured in situ, with either removable or 'lost' non-loadbearing formwork, or permanent formwork which forms part of the reinforcement.
Precast slabs are manufactured off site and craned into place, either in finished form or with an additional thin pour of concrete over the top. They can be made from conventional or post-tensioned reinforced concrete, or from autoclaved aerated concrete (AAC (Autoclaved aerated concrete)).
'Thermal mass' describes the potential of a material to store and re-release thermal energy. It is sometimes referred to as 'building conditioning', which is much more effective than air conditioning. Materials with high thermal mass, such as concrete slabs or heavyweight walls, can help regulate indoor comfort by acting like a temperature flywheel: by radiating or absorbing heat, they create a heating or cooling effect on the human body.
Thermal mass is useful in most climates, and works particularly well in cool climates and climates with a high day–night temperature range. To be effective, thermal mass must be used in conjunction with good passive design and should also consider the inclusion of high mass walls, as they can provide the benefits of 'building conditioning' instead of, or as well as, concrete slab floors.
In winter, slabs should be designed so they can absorb heat from the sun (or other low energy sources). This heat is stored by the thermal mass and re-radiated for many hours afterwards.
In summer, slabs must be protected from direct sunlight and exposed to cooling night breezes and night sky radiation so that heat collected during the day can dissipate.
A slab-on-ground can be ground coupled (uninsulated) or insulated. An uninsulated slab in a good passively designed house has a surface temperature approximately the same as the stable ground temperature at about 3m depth. Depending upon your location, this may or may not be desirable. Ground coupling in mild climate zones such as Perth, Brisbane or coastal NSW allows the floor slab of a well insulated house to achieve the stable temperature of the earth: cooler in summer, warmer in winter. In winter, added solar gain boosts the surface temperature of the slab to a very comfortable level.
In climates with colder winters, such as Melbourne or the southern highlands of NSW, the deep ground temperature is too low to allow passive solar heating to be effective enough. In these locations, slabs should be insulated underneath, which reduces the amount of heat required to achieve comfortable temperatures. In northern Australia, ground coupling still works well, unless the building is to be air conditioned, in which case insulating the slab — especially the edges — is essential.