NatHERS Design Factors
Thermal mass can help a home absorb, store and release heat. When it is used well, it can support comfort and improve thermal performance. When it is used poorly, it may add little benefit or even create performance issues.
Thermal mass refers to dense materials that can absorb and store heat, then release it later. In NatHERS, thermal mass can affect heating and cooling demand because it changes how a home responds to temperature swings, solar gain and internal conditions. Its value depends on climate, orientation, insulation, glazing, shading and whether the mass is exposed in the right places.
Thermal mass is the ability of a material to absorb, store and release heat. Dense materials such as concrete, brick, blockwork, stone and tiles generally have higher thermal mass than lightweight materials.
In a home, thermal mass can help moderate indoor temperature changes. During the day, it may absorb heat from sunlight, warm air or internal gains. Later, as conditions cool, it can release some of that stored heat back into the space.
This behaviour can support comfort when the home is designed well. But thermal mass is not automatically beneficial. It needs the right climate response, solar access, shading and insulation strategy to work effectively.
A NatHERS assessment estimates how much heating and cooling a home may need to remain comfortable in its local climate. Thermal mass can influence this result because it affects how quickly a home heats up, cools down and responds to daily temperature changes.
For example, exposed concrete floors that receive winter sun can help store warmth during the day and release it later. In another design, unshaded thermal mass exposed to too much summer sun may contribute to overheating. The outcome depends on how the mass is used.
This is why NatHERS modelling is useful. It does not treat thermal mass as automatically good or bad. It assesses how the material interacts with the whole design, including climate, orientation, glazing, shading and insulation.
Thermal mass works best when it is part of a passive design strategy.
It needs useful solar access when heat is wanted, shading when heat is not wanted and insulation that helps the stored heat work for the home rather than escape from it.
Thermal mass is usually associated with dense building materials. These materials can be part of the structure, the floor finish or internal exposed surfaces.
• Concrete slabs and polished concrete floors
• Brick and blockwork walls
• Stone and masonry surfaces
• Tiled floors over suitable substrates
• Dense internal walls or exposed structural elements
For thermal mass to be useful, it usually needs to be thermally connected to the interior. Dense material hidden behind insulation, lightweight linings or disconnected from internal conditions may not provide the same benefit as exposed mass in the right location.
Thermal mass behaves differently in different climates. In climates with warm days and cool nights, thermal mass can help smooth temperature swings when it is paired with appropriate shading and ventilation. In cooler climates, it can help store useful winter solar gain when the home is well oriented.
In hot humid climates, thermal mass needs more careful consideration. If night time temperatures remain high and ventilation is limited, stored heat may not be released effectively. In those conditions, shading, ventilation and cooling load reduction may become more important than simply adding mass.
This is why the relevant NatHERS climate zone matters. The same material strategy can support performance in one location and be less effective in another.
Thermal mass is closely connected to window design. If winter sun can reach an exposed concrete floor or masonry wall, that mass may store useful heat. If summer sun reaches the same surface without control, it may store unwanted heat instead.
This makes shading critical. Eaves, awnings, balconies, external screens and landscape elements can help control when sun reaches thermal mass. The aim is to allow useful heat when needed and block unwanted heat when conditions are already warm.
For this reason, thermal mass should be coordinated with window design, glazing performance and shading from the beginning of the project.
• The home has good orientation and useful winter solar access
• External shading controls unwanted summer sun
• The mass is exposed to internal conditions
• The climate has daily temperature swings that the home can use
• Insulation and ventilation are coordinated with the passive design strategy
Thermal mass may offer limited benefit if it is not exposed to the indoor environment, does not receive useful solar gain or is placed in a climate where stored heat cannot be released effectively.
It may also create issues if the home has large unshaded glazing, poor ventilation or too much solar exposure during hot periods. In those cases, the mass can absorb heat that the home does not need, increasing cooling pressure rather than reducing it.
This is why thermal mass should be tested through the NatHERS model rather than assumed to improve the rating automatically.
For new residential projects, thermal mass can influence whether the home reaches the required NatHERS star rating. It may support projects targeting or required to achieve a 7 Star Rating, but only when it works with the rest of the design.
In NSW, construction details that affect the NatHERS result may also need to align with BASIX commitments. If material selections, slab details, floor systems or wall systems change after assessment, the NatHERS and BASIX pathway may need to be reviewed.
Thermal mass also sits within the broader building fabric picture. It works alongside insulation, glazing, shading and construction details, while Whole of Home considers the wider energy systems and equipment used in the dwelling.
Thermal mass should be designed for the specific climate and site. In some homes, exposed concrete floors, masonry walls or other dense surfaces can support stable indoor temperatures. In others, the priority may be reducing unwanted heat gain before adding more mass.
The surrounding design decisions matter. Orientation, glazing, shading, insulation, roof colour, ventilation and floor coverings can all influence whether thermal mass supports or weakens the NatHERS outcome.
The best time to consider thermal mass is early, when floor systems, room orientation, glazing and shading can still be coordinated as part of the overall thermal strategy.
Certified Energy provides NatHERS assessments for new homes, townhouses and multi residential projects across Australia. Our team can model the proposed design and help identify how thermal mass, construction materials and floor systems are influencing the rating.
Where needed, we can help project teams understand how the rating is affected by thermal mass, glazing, shading, insulation, orientation, climate zone and construction details. We can also help connect the assessment with related requirements such as NatHERS, BASIX, 7 Star Rating and Whole of Home.
For a broader explanation of the rating framework, visit our NatHERS Knowledge Hub.
Yes. Thermal mass can affect a NatHERS rating because it influences how heat is absorbed, stored and released within the home.
Is thermal mass always good for NatHERS?No. Thermal mass can help when it is used appropriately for the climate, orientation and solar access. Poorly placed or poorly shaded thermal mass may not improve the rating.
What materials provide thermal mass?Concrete, brick, blockwork, stone, tiles and other dense materials can provide thermal mass when they are exposed to internal conditions and used as part of the home’s passive design strategy.
Can concrete floors improve NatHERS?Concrete floors can support NatHERS performance when they are exposed, correctly shaded and used in a climate where storing and releasing heat is useful. The result depends on the whole design.
Should thermal mass be considered early?Yes. Thermal mass is most useful when it is coordinated early with orientation, glazing, shading, insulation, ventilation and floor or wall construction details.