Thermal Mass: How to Add It to Your Home

Most 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.

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

What Does Thermal Mass Do?

In theory, thermal mass can heat a home by capturing the warmth from the sun that floods through the windows in a home. This is why thermal mass construction is usually paired with passive solar design techniques that maximize the amount of winter sunlight that enters a home. While home insulation maximizes the amount of porous space that traps heat, the thermal mass density means that heat takes a long time to leave a space.

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.

Typical Usage of Thermal Mass in Homes

Let's imagine an old adobe home built somewhere in the southwestern part of the United States. In desert climates, temperatures generally have wide fluctuations between day and night. While it might routinely be upwards of 90 degrees during the day, the desert nights can bring a coolness that can drop that temperature 30 to 40 degrees. That same pattern holds throughout the year, both in summer and winter climates.

A home with lots of thermal mass, then, will take a long time to heat up during the warm days. By 2 or 3 pm, the home's interior walls might still be cool, allowing for a comfortable indoor temperature without relying on air conditioning. By the evening, however, the heat from the day will have penetrated the wall. As the walls cool during the cooler evening and nights, the day's captured heat is released. Older generations of homeowners often relied on this natural heating and cooling cycle to regulate their homes' temperature.

Potential Benefits of Thermal Mass

In an ideal situation, thermal mass can be utilized to lower your heating and cooling needs drastically. The heat recovery ability of homes that incorporate lots of thermal mass might limit the heating or cooling input from conventional HVAC units. Walls that include lots of thermal mass will create what is known as a "thermal lag." Thermal lag affects the flow of heat from the outside to the inside of the wall.

Since thick, high-mass walls will not allow the heat to penetrate to the interior wall for much of the day, air conditioning may not be needed until evening. Air conditioners tend to work more efficiently at night when the outside air is cooler. So, this function of thermal mass walls can also lead to lower cooling bills.

Another benefit of integrating thermal mass into your homes is that these materials generally mean that your home will be more durable, permanent, and resilient. To name just one example, solid rock walls are less susceptible to termites and water damage than treated plywood and 2x4s.

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.