Thermal mass: what you need to know

Thermal mass: what you need to know
Thermal mass: what you need to know

With ever tightening budgets, builders and developers are increasingly being challenged to deliver cost efficiency, both during the building phase and in the long-term use of the property.  Improving energy efficiency is one important aspect, particularly with the rising cost of energy and more and more occupiers are seeking buildings that can save money in the future.

Improving environmental efficiency has largely focused on the carbon content of building components (embodied carbon dioxide), with industry targets recognising environmental performance during the construction phase. However, the amount of carbon dioxide generated by occupation over the life cycle of the building is significantly greater.  For example, the amount of energy used for heating and cooling a building accounts for almost a third of all carbon emissions from UK buildings.

Finding ways to reduce the amount of energy waste is an important consideration for any developer, particularly in light of the revisions to Part L of the Building Regulations which require greater levels of insulation and reduced air leakage.

In response, extensive research has been carried out by the industry into the environmental impact of a building’s lifecycle.  One of the important conclusions is that by incorporating materials that maximise thermal mass properties, the energy efficiency of a building can be improved all year round.  Careful design positively utilising the thermal mass properties of materials can significantly reduce the energy required for heat and cooling.

What is thermal mass and how does it work?

Thermal mass is the ability of a material to store heat, meaning that the thermal mass of a building is governed primarily by the materials it is built from. The thermal mass of a material depends on several factors – its specific heat capacity, density and thermal conductivity.  Concrete, for example, is a material that has a high thermal mass because it is able to absorb a great deal of the heat generated during the day-to-day operations of a building.  Once the temperature starts to drop the concrete is exposed to cooler conditions and the stored heat is subsequently  released.

Using materials with a high thermal mass means that the building is able to regulate its own temperature and respond to the changing weather conditions both throughout the day and during the year.  Overheating in the summer can be prevented, reducing the need for air conditioning.  While during the winter months, concrete will absorb the heat from appliances and people in the building and radiate this back – lowering the amount of fuel required to heat the building.

Concrete is not the only material with a high thermal mass. Masonry and stonework are also highly effective, due to their ability to absorb and re-radiate at a moderate rate. However, not all materials are suitable.

Incorporating thermal mass into the building

The way a building is constructed also has an impact on its thermal mass as the thickness of its walls and the insulation can also affect its performance. For example it is necessary for the concrete wall to be left exposed to ensure effectiveness.   As a result any insulation must only be fitted to the outer surface as by insulating the inner layer of a concrete wall blocks the thermal flux attempting to enter or leave the room.

During the design phase it is important to ensure that the materials are exposed to the internal air – allowing heat to be absorbed and radiated within the building.  Fit outs that incorporate false ceilings or use tiling or artex can undermine the building’s thermal mass as they create a barrier and prevent the circulating air coming into contact with the concrete.

Presently there is no prescribed level  of thermal mass recommended for developments as any amount can benefit the building.  However, the more internal space that uses this approach, the greater the improvements in energy efficiency.  Larger internal areas are also the perfect environment for using thermal mass as they allow for greater amounts of exposed concrete.

The key to success is for thermal mass to be built into the shell of a building from the outset to optimise the thermal mass capacity of the building as a whole.

What are the benefits of thermal mass?

Incorporating thermal mass into the design of the building at the outset can have important benefits.  With more occupiers seeking efficient buildings to reduce running costs, and the government increasingly legislating to improve environmental performance, enhancing energy efficiency and reducing carbon is important for any developer.

Incorporating thermal mass can also help to future proof against climate change.  Improving the flow of air and cooling a building in increasingly hot temperatures will be a concern for developers and occupiers in the coming years and helping to counteract this now will have significant long-term benefits.

Portland House case study

Lafarge Cement’s head office Portland House, near Birmingham, demonstrates the environmental benefits of cement and optimising thermal mass.

Portland House makes the most of concrete’s ability to provide natural temperature control.  Air circulation is provided through a ‘floor supply upflow’ system. Each storey has a concrete base and a ‘raised floor system’ built on a series of stilts. The stilts are topped with raised floor tiles, covered by carpet.  The ceilings are barrel vaulted exposed concrete.

Airflow is created by introducing treated air via the floor which is then circulated by ‘swirl diffusers’. This creates a large mass of swirling air, which is circulated around the room. As the air is warmed by the people and computers in the office, the air mass moves up to ceiling level, where heat is extracted by the exposed concrete. The air itself comprises a mixture of re-circulated air and fresh air, which allows for natural cooling without the need for mechanical cooling.

The benefit of using concrete in the construction of Portland House is not limited solely to its energy saving potential. The smooth surface of the barrel-vault ceilings also enhances the acoustics of the workspace and the exposed concrete maximises daylight penetration, improving the lighting for the building’s users.

Ben Crowther
Ben Crowther