Understanding the Significance of Thermal Mass for Energy Efficiency in Rendered Walls

As any builder or home renovator knows, thermal mass is a key consideration in the construction of energy-efficient homes. But what exactly is thermal mass and how can it be used to improve the efficiency of rendered walls?

Thermal mass is the ability of a material to absorb and store heat. It is usually measured by its specific heat capacity, which is the amount of heat required to raise the temperature of one kilogram of the material by one degree Celsius. The higher the specific heat capacity, the more heat a material can absorb before its temperature starts to rise.

Concrete has a high specific heat capacity, which means it can absorb a lot of heat without getting much warmer itself. This property makes it an ideal material for energy-efficient construction.

heat loss comparison using infra red camera

What is Rendered Wall Construction?

Rendered wall construction consists of applying a coat of cement-based material to the exterior surface of a wall. This layer can be incredibly thin, sometimes even less than 1mm thick, and it serves several important functions. It provides an extra layer of protection from the outside elements, reduces air infiltration, and most importantly for energy efficiency, it increases the thermal mass of the wall by trapping any heat that passes through it.

Rendering with concrete or other high-density materials can significantly improve the energy efficiency of walls by increasing their thermal mass and acting as a form of insulation against both hot and cold temperatures. In fact, studies have shown that the use of rendered walls can reduce thermal losses by as much as 40%.


Benefits of Rendering with High Thermal Mass Materials

Rendering with high-thermal mass materials offers several advantages for energy efficiency. Firstly, it increases the thermal mass of the wall, allowing it to absorb more heat before its temperature starts to rise. This means that in hot climates, if the sun is shining on one side of a wall, the other side will stay cooler than it would without rendering.

Secondly, high-thermal mass materials are also excellent insulators against both hot and cold temperatures. This helps to keep your home warm during colder times of year and cool during summer months. The increased insulation also reduces the amount of energy required to heat and cool your home, further improving its energy efficiency.

Lastly, as well as increasing thermal mass and insulation, rendering can also act as a vapour barrier. This is particularly important in humid climates where it can help to prevent condensation and mould growth on walls.


How to Render Walls for Energy Efficiency

There are several things to consider when rendering walls for energy efficiency. Firstly, you need to choose a high-thermal mass material such as concrete or brick. These materials have a high specific heat capacity and will therefore absorb more heat before their temperature starts to rise.

You also need to make sure that the wall is properly sealed so that there is no air leakage. Any gaps or cracks in the rendering should be filled with a suitable sealant to prevent air infiltration. Finally, you need to ensure that the rendered wall has adequate ventilation so that it does not become too hot and cause problems such as condensation and mould growth.



The importance of thermal mass for energy efficiency in rendered walls cannot be overstated. Not only does it help to keep your home warmer in winter and cooler in summer, but it also reduces the amount of energy required to heat and cool your home. In addition, rendering walls with high-thermal mass materials can act as a vapour barrier, preventing condensation and mould growth on walls.When correctly installed, rendered walls can reduce thermal losses by up to 40%, significantly improving the overall energy efficiency of buildings.