Thermal Mass
Thermal mass refers to the ability which a material has to absorb, store and release heat energy over a period of time. For instance, a wooden floor or shuttered wall would not have as much thermal mass as a concrete or tile floor. Thermal mass materials are important in Passive buildings as it allows for the heat energy (gathered through solar gain) to be released at night time.
The sunlight passing through the glazing to hit the tile floor.
Tile floor = high thermal mass material
So Surely the concrete and tile floor in my house are just as good?
Yes, its true, these materials in any home are effective in storing and releasing heat. However what is crucial is that this heat does not escape. The cartoon sketches below shows how heated air moves around a room which has thermal mass. Unless this room is thermal bridge free and Airtight, then this heat will eventually excape.
A well insulated room with thermal mass. The air moves in convection currents, which means that the hot air rises and tries to escape.
Why is thermal mass important?
Thermal mass is particularly important in Passive Houses, as it works in conjunction with Solar Gain.When the sunlight passes through the windows and hits the material on the interior walls/floors, it is imperative that these materials are proficient in storing this heat for a significant period of time. This means that the heat accumulated at the height of the sunlight (depending on the time of year) is still being released when the sun has set at night-time (see image below).
Thermal Mass gathering and releasing heat energy from low angle sunlight.
The temperature remains the same as the energy is released at night-time, ensuring that the occupant is always comfortable. This is known as PASSIVE COOLING.
How can overheating be prevented?
Although there are measures to prevent overheating in Solar Gain, it is imperative that similar measures are taken in the placement of interior thermal mass. Thermal mass is generally sheltered from direct summer sunlight through shuttering or the overhand at eaves level, but exposed to low angle winter sunlight. During the day heat gains from the sun and the occupants of the building causes the air temperatures within the space to increase, and this heat is absorbed by the exposed concrete slab.
The thermal mass (yellow layer) is only exposed to direct sunlight from low angle winter sunlight.
Shuttering blocking higher more concentrated sunlight from the interior thermal mass.
The limited amount of sunlight entering through shuttering and striking the floor from direct summer sunlight..
Calculating the ratio of Glazing Area to Thermal Mass exposure
Since the most important source of thermal energy is from the Sun for a passive house, the ratio of glazing to thermal mass is an important factor to consider. As a general rule, additional solar-exposed thermal mass needs to be applied in a ratio from 6:8:1 for any area of south facing glazing above 7% of the total floor area. For example a south-facing room with a 1m2 window should have about 6m2 of exposed thermal mass, located where it will be exposed to direct winter sun. The exact glass-to-mass area ratio will of course vary with climate and design.
Thicknesses
Very thick thermal mass walls and floors may take too long to heat, while those that are too thin won't store enough heat. Concrete slab floors should be 100-200mm thick for the best performance, while thermal mass walls should be 100-150mm thick. However, the exact amount must be calculated as part of the design process.
Thermal Mass and Solar Gain in non-domestic construction
Thermal mass has been combined with optimum solar gain design in industrial and commercial construction also. The video below gives detail of the use of techniques used by Termobuild, which also captures heat from other sources by using thermal mass. An Irish example of such an Industrial Building would be the Tesco in Tramore, Waterford. This building was the world’s first Passivehaus certified supermarket.