What is Thermal Mass in Passive Solar Building?

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Thermal mass is a property of construction materials to absorb, store, and release heat energy. It is an important element of passive solar house design and can be utilized for passive solar heating, passive solar cooling, or both.

Some of the examples of thermal mass construction materials are concrete, brick, stone, water, etc. The most commonly used thermal mass structural elements are concrete slabs and walls. They should be placed at a certain position to receive adequate sunlight. These locations may vary based on the direction of the building to the sun.

The utilization of thermal mass in passive solar house design should be considered when there is a difference between day and night temperatures; otherwise, it will not serve its purpose, which is improving comfort and maintaining moderate temperature in a building.

For passive heating, thermal mass can absorb and store enough heat energy during days and release it at night to increase the indoor temperature. Whereas for passive cooling, thermal mass can be prevented from exposure to direct sunlight, storing the temperatures at evenings and nights and releasing it during days to keep the temperature low inside the house.

What is Thermal Mass in Passive Buildings?

Thermal is a critical element in any passive solar design as it controls the heat and minimizes temperature variations in the house.

A building material with a high thermal mass can capture and store a great amount of heat energy. As a result, the quick rise of temperature inside living space is prevented, see Figure-1.

On the contrary, a material with low thermal mass cannot capture and store a large amount of heat energy, and subsequently, the temperature in the house will rise quickly, see Figure-2. The higher the thermal mass, the longer the lag time.

The conductivity level and density of materials help keep the internal temperature of a building stable. Building materials with thermal mass properties are employed in constructing floor or inside walls of a passive solar structure.

These structural elements are located near the solar glazing to permit the sun's energy to strike directly on them.

What is Thermal Lag?

The rate at which thermal mass emits absorbed heat energy is called thermal lag. Thermal lag is controlled by material thickness, surface area, texture, color, surface coating, heat capacity, conductivity, temperature differentials between each face, exposure to air movement, and air speed.

A twelve-hour thermal lag cycle is appropriate and desirable in moderate weather conditions. In colder weather conditions, a thermal lag of up to seven days is beneficial if there is adequate solar exposed glazing to charge the thermal mass on sunny days.

Building Materials with Thermal Mass Properties 

Materials with thermal mass characteristics generally have a high density, such as concrete, brick, ceramic tile, and stone. 

Water possesses high density and can store and emit heat energy. However, it cannot be integrated into the building similar to other construction materials, but it is doable using water-filling containers for passive solar cooling. Timber frame is an example of low thermal mass material, i.e., it has short thermal lag.

Commonly, materials with thermal mass properties have high density, so they are capable of conducting heat. Therefore, thermal mass in a passive solar building is not to be exposed to the sun completely, but the exposure of a large portion is enough. Then the absorbed heat is spread slowly throughout the mass and then released during the night to heat the living space.

The ability of thermal mass to absorb a large amount of heat can be used to provide a cooling effect inside a passive house. Initially, the mass absorbs the heat and keeps the inside space cool. Then, the heat energy of the mass is released and driven out of the building using proper ventilation. 

In addition, the thermal mass can capture and store cooler evening and night temperatures during nights that aid in cooling the house during days.

The use of thermal mass in a passive solar building is suitable for regions with a reasonable difference in day and night temperatures. Construction materials with high thermal mass can considerably increase comfort and decline home energy consumption.

Table 1: Thermal Mass Construction Material

Thermal mass material	Specific heat capacity	Thermal conductivity	Density	Effectiveness
Water	4200	0.60	1000	high
Stone	1000	1.8	2300	high
Brick	800	0.73	1700	high
Concrete	1000	1.13	2000	high
Unfired clay bricks	1000	0.21	700	high
Dense concrete block	1000	1.63	2300	high
Gypsum plaster	1000	0.5	1300	high
Aircrete block	1000	0.15	600	medium
Steel	480	45	7800	low
Timber	1200	0.14	650	low
Mineral fibre insulation	1000	0.035	25	low

Considerations for Effective Thermal Mass Installations

The thermal mass effectiveness would be questionable unless integrated with suitable passive design techniques. The amount of heat energy captured by thermal mass depends on the glazing areas, type, and shading. 

In cold climates where the need for heat is low, a small area of high-solar transmission glass is required. Conversely, the reverse is true for hot weather conditions. Moreover, airtightness and insulation will affect how long the absorbed heat is held within a home.   

How Much Thermal Mass is Needed to be Provided?

Area

The exposed thermal mass area should roughly be around six times the area of glass that is being struck by direct sunlight. So, it is important to balance the exposed thermal mass area against the glazing area. 

If the glazing area is large, it will overheat the summer living space and lose heat too quickly in winter. The accurate thermal mass to glass area changes with weather conditions and design.

Thickness

The thickness of thermal mass should be determined to prevent potential overheating during summer. 

The thickness range of the thermal mass wall is between 100-150 mm, whereas the thickness of the concrete slab floor should range from 100-200 mm. 

If the thickness of thermal mass is great, it takes too much time to heat, but a thin thermal mass will not be able to store adequate heat.

What are the Principles used to Locate Thermal Mass?

The first step is to locate the thermal mass to determine whether it is used for passive solar heating or passive solar cooling. After that, use the following principles to determine the location of thermal mass in the building:

1. If thermal is used for passive heating, place it in areas that receive direct sunlight or radiant heat from the heater.
2. Protect thermal mass from the summer sun with insulation and shading for passive cooling. Make sure cool night breezes and air currents pass over the thermal mass to draw out absorbed heat energy.
3. If a thermal mass is employed for passive heating and passive cooling, position it on the ground for summer and winter efficiency. Locate thermal mass in rooms with good solar access and it should be exposed to a night breeze in summer.
4. Providing proper shading to protect thermal mass from the summer sun is a good strategy to improve passive cooling.
5. Place extra thermal mass close to the center of the building, especially if an air conditioner is positioned there. Feature brick walls, slabs, water features, large earth, water-filled pots, and water tanks can be used.  

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