What is Thermal Mass?
Thermal mass, in the most general sense, describes the ability of a material to store heat Exploiting thermal mass so that it helps to reduce heating requirements in the winter and cooling requirements in the summer is not difficult. However, it does need to be considered at the outset of the design process when the building's form, fabric and orientation requirements are being determined, Get it right and you can have significant energy savings and carbon savings over the life of a building.

For a construction material to provide a useful level of thermal mass it must have a high specific heat storage capacity, be of high density and have moderate thermal conductivity so that heat conduction is roughly in synchronisation with the daily heat flow in and out of the building.
Timber has a high heat capacity but a low thermal conductivity.
This limits the useful heat absorption rate and so provides a low thermal mass.
Steel also has a high heat storage capacity but it also has a very high rate of thermal conductivity which means that heat is absorbed and released too quickly for any meaningful thermal mass efficiency.
Concrete with its high heat capacity and density but moderate thermal conductivity offers a good balance. It steadily absorbs heat and stores it until the ambient temperature drops causing stored heat to migrate back to the surface from where it is released.
Heat moves in a wave like motion alternatively being absorbed and released in response to the variations in day and night-time conditions. The absorption and release of heat enables buildings with thermal mass to respond naturally to changing weather conditions, helping to stabilise the internal temperature and provide a largely self-regulating environment. This action helps to prevent summer overheating and reduces the need for air conditioning. It can also reduce the need for heating during the winter by capturing and later releasing solar and internal heat gains.
During warm weather, much of the heat gain in heavyweight buildings is absorbed by the thermal mass in the floors and walls thereby reducing the risk of overheating. This heat is then removed by allowing cool night-time air to ventilate the building.
This daily heating and cooling of the thermal mass works relatively well in New Zealand as the air temperature at night is typically 10 degrees less than peak daytime temperatures during the summer. The benefits of thermal mass, which is well understood in warmer parts of Europe, will become increasingly recognized in New Zealand as climate change results in hotter summer temperatures,
As well as cooler internal temperatures, these benefits also include reduced heating bills in the winter as instead of purging the day-time heat gains with night-time air, the stored heat is allowed to radiate back into the building.
For the winter, thermal mass works best when it is used as part of a passive solar design strategy (PSD).This approach seeks to maximise the benefit of solar gain in the winter, using thermal mass to absorb gains from North facing windows, as well as internal heat gains from electrical equipment, cooking and lighting. These gains are slowly released overnight as the temperature drops so helping to keep the building warm and reducing the need for supplementary heating.
Applying simple passive solar design techniques can result in fuel savings of up to 10 per cent. This saving can increase to 30 per cent if more sophisticated passive solar techniques such as sunspaces are adopted. To meet these challenges STEADFAST provides a whole-building approach where the materials, structure and systems work in unison to maximize the building's overall performance.
Efficient use of thermal mass used in conjunction with high insulation, orientation, solar gain, ventilation and shading can do much to reduce the whole-life carbon footprint of buildings and reduce energy consumption by up to 60%.

What is the best place for thermal mass?
The best place for thermal mass (concrete) is on the inside of the insulation. If the thermal mass is not insulated from the outside cold it is not effective and can become more a problem than a benefit, this is why it is vital that there is no thermal bridging (breaks) in the wall. Steadfast walls and floors are continuously insulated with no thermal bridging.
Thermal mass is best located where it receives direct sunlight (Radiant Heat), this is generally the floor and strategically placed concrete walls. Thermal mass floors and walls are also placed best in rooms where there is excessive convectional (air) heat gain

What is a thermal break?
A thermal break is a material, which is part of the wall or floor where via heat can escape and cold can get in. Theses materials such as timber, steel and concrete are not insulators.
Concrete with its thermal mass properties needs insulation

Steel is a conductor, the opposite of an insulator. It is the reason why 80% floor heat loss is through the edge foundation perimeter. The steel in a concrete floor is connected to the
foundation. This steel conducts the heat in the floor and rapidly transfers it to the cold foundation outside. This also explains why some underfloor heating systems are expensive to run, as the heating pipes are connected to this same conducting floor steel.
 Timber is not an insulator. The average timber frame wall is 25% Timber and 75% insulation. If the insulation is R2.6, the True R-value of the entire wall is only R-2.  All STEADFAST walls are True R-values

Is polystyrene the only available insulation?
No, the STEADFAST patent and systems apply to any insulation; even a STEADFAST straw bale home can be constructed.