Thermal Mass: How to Add It to Your Home
Last Updated: Apr 9, 2025Most savvy homeowners are interested in keeping warm and comfortable during the winter while decreasing their heating bills and reducing their carbon footprint. They typically first consider upgrading the insulation in the walls, floor, and roof. Fortunately, there are several healthy and environmentally friendly insulation options available today. Few people, however, understand that the materials they choose for their walls and floors have a significant impact on their homes' thermal performance.
Before the age of insulation, air conditioners, and central heating units, incorporating thermal mass into a house was the primary strategy for regulating the house's indoor temperature. Our grandparents' homes were mostly heated by a fireplace or a cast iron wood stove. The brick, stone, or other wall materials were built purposefully to be much thicker than the 2x4 framed walls that define most modern-day construction.
This article explains what thermal mass is and how it affects the temperature inside our homes. We then look at the pros and cons of opting for this natural heating and cooling strategy.
Table of Contents
- What is Thermal Mass?
- What Does Thermal Mass Do?
- How Do You Calculate Thermal Mass?
- Typical Usage of Thermal Mass in Homes
- Potential Benefits of Thermal Mass
- How Much Thermal Mass Do I Need?
- Where is Thermal Mass Not Useful?
- How to Add Thermal Mass to a House
- Bottom Line on Thermal Mass
What is Thermal Mass?
Thermal mass is simply any solid or liquid material that can capture, hold, and store heat. Essentially, almost every object inside our homes can be considered to have thermal mass. Everything, from your living room couch, the rug in your dining room, and even the can of soda sitting on your counter, can store heat inside your home.
However, different materials have different heat storing capabilities. Denser materials used to build homes tend to capture and store heat better than lighter construction materials. When we talk about thermal mass in home construction, we are usually referring to dense, solid materials such as concrete, stone, brick, adobe, and gypsum. Some examples of homes with a high thermal mass are:
- Those that have solid stone or brick walls (thin brick facades over plywood and 2x4 frame have much less thermal mass)
- Stone or concrete floors
- Adobe or earthen materials incorporated into the home construction
Ideally, a home that integrates thermal mass into the construction will "suck up" the sun's heat that penetrates the home during the winter. While the thermal mass will take a long time to heat up because of the material's density, the same is true for the cooling process. The density of rock floors and adobe or brick walls will hold the heat captured from the sun during the day and slowly release that heat into the home throughout the cooler nighttime.
Building materials that have a high thermal mass can potentially offer a source of natural heat that is slowly released into the home, even when the sun is not shining. It is worth mentioning that wood, because of its thinness and relative lack of density, does not provide much of a thermal mass function.
How Do You Calculate Thermal Mass?
Concrete, adobe walls, and rammed earth all have a high thermal mass rating above 1,300. We measure the thermal mass of a structure as the heat capacity per unit of volume, or, in other words, the volumetric heat capacity (VHC). VHC is measured in kilojoules per cubic meter Kelvin (KJ/m³k).
For those who want to dive deep into calculating thermal mass, we found this resource, from GreenSpec in the UK, quite useful.
How Much Thermal Mass Do I Need?
There are varying opinions on this topic, but Smarter Homes New Zealand offers some useful guidance. When calculating the amount of thermal mass you need, you must balance this with the area of glazing. In general, you should have around six times the area of the sunlit windows, and the concrete floor should be between 100 and 200mm (4-8") thick.
Where is Thermal Mass Not Useful?
Not everyone is fortunate enough to live in a climate with mild winters and steady fluctuations between daytime and nighttime temperatures. In areas with long, cold winters, thermal mass will certainly not capture enough warmth to keep a home warm at nighttime. Wintertime in many parts of the country is often characterized by those dreary, gray days where weeks can go by before the sun ever appears. The sun's lack of direct energy and heat will not give the thermal mass enough heat to capture in those climates.
A home with thermal mass without enough insulation will lose heat more quickly than a well-insulated home. While the density of thermal mass can store captured heat, it also allows for heat to pass more rapidly through the walls. In cold climates, non-insulated homes will undoubtedly have a much higher heating bill.
How to Add Thermal Mass to a House
It should be clear by now that thermal mass walls work best in areas with particular types of climate. Regions with mild winters and large differences between day time and night time temperatures are naturally suited for thermal mass integration. In general, to benefit from thermal mass, you need to live in an area with a minimum temperature differential of 10 degrees between the day and nights.
Suppose these climatic conditions don't define your region. In that case, thermal mass can still be incorporated into a home that uses high-performance insulation. Thermal mass floors, mantles, and walls can capture the sun's heat that flows into the house during the winter. The slow release of heat into a well-insulated home will mean that your heating requirements will be even lower, benefitting from another source of natural heat. However, you will need to implement strategies that block or shade out the light and heat from the summer sun. Since, in the summer, that same thermal mass could release excess heat into your home, thus increasing your cooling requirements.
Whether you are building new or considering a renovation, in-floor heating is a great way to use thermal mass. Materials such as concrete, slate, and tile will work better with in-floor heat because they will store the heat and slowly release it. And in the warm summer months, that same material remains cool to the touch.
Another strategy that you can implement to build thermal mass in your home is to create a stone or brick accent wall, with a direct line of sight to south-facing windows. Using this strategy will, like a concrete floor, collect heat during the day to release at night.
Bottom Line on Thermal Mass
Thermal mass construction might not work for every home and situation. Still, suppose you are renovating or building new. In that case, it's important to pay attention to the choice of materials and their relative thermal mass, particularly for your walls and floors.
Tobias Roberts
Tobias runs an agroecology farm and a natural building collective in the mountains of El Salvador. He specializes in earthen construction methods and uses permaculture design methods to integrate structures into the sustainability of the landscape.
