Thermal mass is a great way to increase the energy efficiency of your dream sustainable house, helping to keep your home warm in winter and cool in summer.
In the right climate, the more thermal mass your house has, the better it will insulate you from temperature fluctuations.
In this article we’ll explore why thermal mass can be a great design feature for an eco-friendly home and what materials are best for increasing it.
We’ll cover:
- What is thermal mass and how does it work?
- How climate affects whether or not to use thermal mass
- The benefits of incorporating thermal mass
- 7 stunning ways to incorporate thermal mass to create a sustainable, efficient, and comfortable living space.
What is thermal mass?
It’s a material that has the ability to absorb and store heat and release it later. Concrete, stone, bricks and rammed earth all have this ability.
The idea is to use one or more types of mass in your house design so that in winter the sun’s warmth can be absorbed and radiated throughout your home during the day as well as after the sun goes down and the temperature cools.
Incorporating thermal mass into your home design, like all the other elements of solar passive design, needs to be considered holistically alongside factors such as house orientation and layout, insulation, window size and placement and shading to ensure maximum energy efficiency and comfort for your home.
Due to the technicalities and interplay between solar passive elements, Architects and building designers often use computer modelling to simulate and evaluate expected performance of all the elements to ensure a good result is achieved.
Climate and thermal mass
In a climate where it’s hot in summer and cold in winter it’s important that the house is designed in such a way that the thermal mass you use to warm the house in winter doesn’t also warm the house in summer! It’s important that thermal mass is shaded in the summer months to prevent this from happening.
Our house is in the warm temperate zone of Australia where the temperature fluctuates between night and day, therefore we used a polished concrete floor to help heat the house in winter. Shading in the form of eaves and fixed awnings shade the concrete in summer to keep it cool and the louvre windows and doors are opened to cool the house.
In climates that are almost always hot OR almost always cold (i.e. don’t fluctuate much), thermal mass can actually be detrimental as it only has the capacity in those environments to either warm up or cool down (to match the outside temperature), not fluctuate at night time to bring the house into a comfortable temperature range.
This is why you find in warm or tropical climates buildings are constructed with open designs and lightweight materials that do not retain heat while in cold climates buildings are usually highly insulated with very little exposed thermal mass.
The benefits of incorporating thermal mass
The benefits of using thermal mass in the right climate (where temperatures fluctuate between day and night) include:
- Reduced need for artificial heating
- Reduced reliance on fossil fuels
- Lower energy bills
- Increased comfort and enjoyment of your home
What materials can be used for thermal mass?
It’s often used in the structure of the house, usually in the floors or walls, often becoming stunning feature of the home.
The best ways to incorporate thermal mass are:
- Polished concrete floors
- Concrete walls
- Rammed earth walls
- Brick (masonry) walls
- Stone walls
- Stone floors
- Tile floors over a concrete slab
Polished concrete floors
Polished concrete floors have substantial density and therefore offer exceptional thermal mass and natural temperature regulation. When strategically placed in rooms that receive ample winter sunlight, polished concrete floors capture solar energy and essentially function as a solar battery that stores free heat during the day and releases it throughout the evening.
Beyond their thermal benefits, polished concrete’s reflective surface amplifies these effects by bouncing light deeper into spaces, while its durability ensures these thermal mass advantages continue performing for decades with minimal maintenance.
Concrete floors tick the durability box however the lifecycle of concrete needs to be considered given the high carbon emissions involved in making cement, the core ingredient of traditional concrete. Low carbon concrete alternatives are now readily available and are a lot better option for an eco-conscious renovator or home builder. In Australia, two low carbon concrete brands that are available are Boral Envisia and Holcim EcoPact.
Concrete walls
Internal concrete walls offer exceptional thermal mass in sustainable home design. Their high density and specific heat capacity allow them to absorb and store significant amounts of heat energy, releasing it slowly as surrounding temperatures drop. This natural heat regulation helps maintain comfortable indoor temperatures while reducing the need for expensive artificial heating and cooling.
Rammed earth walls
In passive solar homes, rammed earth walls positioned to receive winter sunlight can capture solar heat during the day and radiate it back into living spaces throughout the evening hours. This natural heating cycle works particularly well in climates with substantial day-night temperature differences.
Brick walls (internal)
Internal brick walls positioned to receive winter sun offer excellent thermal mass and work particularly well in climates with significant day-night temperature swings, where they help flatten temperature fluctuations. For summer cooling benefits, protect brick walls from direct sunlight using appropriate shading devices during hotter months, allowing them to absorb excess indoor heat instead.
Stone floors
If you install stone flooring in areas receiving direct winter sunlight, particularly North-facing rooms (in the Southern Hemisphere), they offer excellent thermal mass in sustainable home design, using high density and heat capacity to absorb solar radiation or ambient heat during the day and slowly release it when temperatures drop, creating natural temperature regulation.
Stone walls
Stone walls offer excellent thermal mass benefits in sustainable home design. Their high density allows them to absorb heat slowly during warm periods and release it gradually when temperatures drop, naturally regulating indoor climate.
In passive solar designs, strategically placed interior stone walls in north-facing rooms (southern hemisphere) can absorb direct sunlight during winter days, radiating stored heat throughout the evening. This reduces heating needs while preventing temperature fluctuations.
Tiles over concrete
Tile-covered concrete slabs create an excellent thermal mass system for sustainable homes. The combination leverages concrete’s substantial heat capacity while the tiles provide a durable, attractive finished surface. Together, they effectively absorb, store, and gradually release heat, helping regulate indoor temperatures naturally.
For optimal performance, install dark coloured ceramic or porcelain tiles in areas receiving direct winter sunlight, particularly south-facing rooms in the Northern Hemisphere. The tile-concrete system works best when directly connected to the ground without intervening insulation, though proper perimeter insulation is essential to prevent heat loss at the slab edges.
Incorporating thermal mass into your home can dramatically enhance your home’s comfort, energy efficiency and sustainability. By thoughtfully selecting and positioning these materials, you can take full advantage of passive solar heating and cooling, reduce your reliance on artificial heating and cooling and lower your environmental footprint. Whether you’re building new or retrofitting an existing space, using thermal mass strategically is a smart, enduring investment in a more resilient and livable home.
