Concrete Slabs and Thermal Mass: How They Affect Your NatHERS Rating

Thermal mass is one of the most misunderstood ideas in home design. Used well, it steadies indoor temperatures and can lift your NatHERS rating. Used badly, it can make a home harder to heat or cool. Here is what thermal mass is, and how concrete slabs and other heavy materials affect your rating.

What is thermal mass?

Thermal mass is the ability of heavy materials such as concrete, brick and tile to absorb, store and slowly release heat. A concrete slab warmed by winter sun during the day releases that warmth into the evening, smoothing out temperature swings.

When thermal mass helps

• Paired with winter sun – a slab that receives direct north sun stores free daytime heat for the evening.
• In climates with big day-night swings – mass buffers the highs and lows.
• With good insulation and shading – mass works best when the home is well sealed and summer sun is kept off it.

When thermal mass works against you

Mass in the wrong place is a liability. A slab or masonry wall that bakes in summer afternoon sun stores unwanted heat and radiates it overnight, making bedrooms uncomfortable. Mass that never sees the sun simply stays cold and can make a home slow to warm up.

Insulation and slabs

How a slab is detailed matters. Slab edge insulation and the floor covering both influence how the mass behaves and how it is scored in NatHERS. Carpet over a slab, for example, insulates the mass away from the room and changes its effect.

It depends on your climate zone

NSW spans many climate zones, from coastal to alpine. The value of thermal mass differs across them, which is why the same slab design does not perform identically in Sydney, the Hunter or the Southern Tablelands. Your assessor models mass for your specific site.

Frequently asked questions

Is a concrete slab always good for my rating? Not automatically. It helps when it catches winter sun and is protected from summer heat, and it can hurt otherwise.

Does tiling or carpet change things? Yes. Hard finishes let the slab work as mass, while carpet insulates it away from the room.

Want your floor and mass working for your rating, not against it? Get a fast, fixed-fee quote from our assessors.