Understanding Thermal Mass in Building Design

While modern homes tend to look elegant and aesthetically pleasing, the materials used in their construction aren’t just for show. Each material has been carefully selected to play a critical role in the building’s durability and energy efficiency.

And, when it comes to building high performance homes, some materials are better than others in ensuring interior comfort. The thermal mass of these building materials determines just how well they play their part.

In this blog we’ll take a look at what thermal mass is, and why it matters in your building’s design.

What is thermal mass?

In simple terms, thermal mass is the ability that a material has to absorb, store, and release heat. (This releasing of heat is known as thermal lag, but these two concepts are generally combined under the single banner of thermal mass.)

Materials like brick, concrete, tile, stone, and rammed earth all have a high thermal mass, which means they absorb heat and then retain it for a long time. And, in practice, you probably already understand the concept: we’ve all placed our hand on a brick wall in an evening and still felt warmth radiating from it.

Materials like wood or cloth, on the other hand, have a low thermal mass and lose their heat quickly.

When used properly, thermal mass can be used to improve the comfort and energy efficiency of your home.

How does thermal mass work?

In theory, thermal mass is reasonably straightforward: most materials absorb heat, but some absorb more and retain it better than others. The density of the material has a lot to do with it, and materials with a high density tend to have a better ability to store heat.

When used in the construction of a home, materials with a high thermal mass help to stabilise indoor temperatures. Think about the old homes you’ve been in. The older buildings, with the wooden walls and floorboards, as well as often being draughty, have felt cold. This is because while the wood absorbs heat, it releases it quite fast.

Conversely, homes made from brick or concrete are more robust, and the walls and floors retain warmth that they’ve absorbed during the day, slowly releasing it throughout the evening and night.

It’s not just for warming, though. Thermal mass can also help to improve the cooling qualities of a building.

Thermal mass works to absorb heat. So if a material used in building a home, like a concrete slab for the floor, doesn’t receive any direct heat (for example, if it’s shaded from direct sunlight) then it will absorb warmth from inside the home—and in this instance, help to remove excess heat from your rooms.

However, it’s vital that these materials are used in the most effective way to achieve this.

The benefits of using thermal mass in building design

There are some big benefits to using thermal mass in the design of your home.

Improved energy efficiency. Utilising thermal mass intelligently means you can use the materials in your home design to help warm and cool your house, reducing your reliance on expensive heating and cooling.
Lower energy bills. By reducing your reliance on the heater or air conditioning, you can lower your electricity costs, both immediately and for the long term—something that’s front of mind for most people building new homes.
Enhanced indoor comfort. By its nature, high thermal mass means that you can create a more stable indoor temperature. Utilising thermal mass correctly helps to eliminate temperature fluctuations throughout the day that are common with more lightweight constructions like wood and plasterboard.
A more eco-friendly design. By taking advantage of thermal mass and reducing the need to run your air conditioner or heater as much, you can help to reduce the carbon footprint of your home over the long term.

How to incorporate thermal mass in home design

Incorporating thermal mass into your new home build borrows from passive house principles.

Passive solar design

To make the most of your thermal mass, it’s important that you utilise it in the right way. Passive solar design plays a big part in this.

To help keep a home cool, make sure thermal mass is protected from the sun during the hot summer months, either through shading or building design. Also, incorporate strategically placed windows and vents into the design to make sure the home can take advantage of cooling breezes or air currents that pass over your materials and help draw out any retained heat.

Wall and flooring materials

Compared with lightweight construction materials like wood or plasterboard, which lose heat very quickly, high thermal mass materials like brick and stone can retain warmth for longer.

This makes them excellent flooring choices. During the day, these materials will retain heat and make the room feel warmer, while also keeping the floor underfoot feeling more comfortable. Nobody likes to walk on a cold floor.

They’re ideal for walls, too. If the building’s thermal mass is properly shaded during summer, it can also work to draw down the room’s ambient heat, helping to keep the room at a more stable temperature.

When thermal mass might not be ideal

Materials with a high thermal mass don’t suit all climates.

Even though Perth can get hot, we do tend to see a high temperature variation between day and night. In situations like this (and in colder climates), thermal mass can be a big benefit, helping to keep the temperature inside more stable.

However, in hot and humid climates that have a limited temperature variation, thermal mass isn’t encouraged. This would retain unwanted heat, making houses feel hot and stifling instead.

Also, if your home doesn’t have adequate ventilation to make use of breezes, then thermal mass can warm up rooms, leaving the heat with nowhere to go. So be sure to build your home to take advantage of breezes that push out unneeded warm air with the incoming cooler winds.

Thermal mass is an integral part of a high-performance home

From aesthetically pleasing materials, to creating more comfortable and consistent indoor temperatures, using thermal mass in the right way can help you achieve your goals for a new home build.

At Trueline Homes, we’re proud to build homes that are designed with performance and comfort in mind. So if you’re considering a smarter design for your new home, contact TrueLine Homes to book a consultation.