Build a Concrete House and Reap the Energy-Saving Rewards of Thermal Mass Home Design


If you are planning to build a house, consider concrete instead of traditional wood framing. Beyond the strength and durability of concrete, you can harness the energy-saving potential that concrete’s thermal mass offers, and using Insulated Concrete Forms (ICFs) makes building a thermal mass concrete home easier than ever before.


To design a successful thermal mass home, it’s necessary to understand the synergy between the architectural design, construction materials, and surrounding natural environment. Here’s what you need to know:

What is Thermal Mass?

Any solid or liquid material able to trap heat, store it, then release the heat as it cools is a thermal mass. Just about every object imaginable is by definition a thermal mass, but the object’s size, material, and density determine how much heat it can store and how gradually it radiates that heat upon cooling.

In the construction industry, thermal mass typically refers to solid, dense materials like concrete, brick, stone, and rammed earth because they are best able to store heat. Less dense construction, like a wood-framed wall, has a much lower capacity for heat storage.

How Does Thermal Mass Heating and Cooling Work?

Put simply, thermal mass homes work by soaking up the Sun.

When combined with the principles of passive solar design, a thermal mass home is naturally heated and cooled. Passive solar design employs several strategies to control how heat from the Sun interacts with the architecture.

To provide passive heating, interior spaces and their thermal masses - such as concrete walls and floors - are oriented to capture as much sunlight as possible. South-facing windows are carefully sized and placed so that the low winter sunlight shines directly on the thermal mass, which collects and stores that heat during the day. The heat is then released into the home at night when the outdoor air temperature is lower.

Passive cooling is achieved through the use of shading elements to block sunlight from streaming through the windows. Because the Sun is much higher in the summer, roof overhangs, awnings, or other horizontal shading elements that extend out above south-facing windows will block summer sunlight while letting winter sunlight in. Please note, however, that these shading elements are not effective on east and west-facing windows.

Thermal mass in the home provides additional cooling by absorbing heat. At night, thermal masses can be cooled with natural ventilation, preparing them to absorb even more heat the following day.

Get Sun-Savvy

The key to passive solar design is controlling and directing sunlight to thermal masses. To maximize your home’s comfort and energy-savings, become familiar with the Sun’s path throughout the day in different seasons. All year, the Sun moves in an arc from east to west across the southern sky. In the summer, the Sun’s arc is high in the sky and the days are longer. As the days grow shorter, the Sun’s arc becomes lower and lower.

The Sun impacts interior spaces in different ways depending on which direction the windows face. Think about the rooms you have planned for your house: where would you enjoy the morning sunlight streaming in? Is there any space – a TV room maybe – where the glare of afternoon sunlight would be undesirable?

Rooms that face south are exposed to sunlight throughout the day. The sunlight from south-facing windows is easy to control with roof overhangs or awnings, and thermal masses are able to capture the most heat from south-facing windows.


East windows bring morning sunshine, great for a kitchen or breakfast nook – not so great in the bedroom of someone who enjoys sleeping in. Since east windows are the first to access the sunlight, it might make sense to have a thermal mass wall or flooring so that morning heat is captured right away.

Rooms with only north windows do not get any direct sunlight. Thermal masses intended for passive heating should not be located in a space with only north windows.

West windows bring blazing afternoon sunlight and are difficult to shade. In a hot climate, this makes for an unwelcome evening heat gain right when cooling is supposed to occur. In cold climates, west windows provide the last opportunity before sunset to capture more heat.

Understanding the path of the Sun and your climate will inform your architectural design and help you to make good choices along the way.

Begin the Design with Thermal Mass in Mind

To be successful, a thermal mass home requires thoughtful design. Every site is unique, so careful site analysis at the beginning of the design process is essential. Some early questions for the design team might be:

  • What is the path of the Sun across the site?

  • How much south-facing glazing can be incorporated into the design?

  • Is sunlight obstructed by other buildings, terrain, or trees?

  • Is heating or cooling the top priority?

Architects and homeowners can then choose appropriate locations for thermal masses, place windows to optimize solar heat gain in thermal mass, and choose structural materials. Concrete is one of the most effective and versatile thermal mass construction materials available, and what used to be a slow and expensive installation process has been made easy and efficient with Insulated Concrete Forms.

What Are Insulated Concrete Forms (ICFs)?

ICFs are building blocks that consist of two high-performance rigid insulation panels held together – yet spaced apart - by a series of vertical ties. How far apart the panels are spaced depends on how thick the concrete core needs to be to meet the project's engineering needs. The interlocking ICF blocks are stacked on-site to form walls. Once stacked, rebar is placed in the cavity between the rigid insulation panels, and the cavity is filled with concrete. The continuous double insulated mass concrete core still captures heat from the sun and performs as a ‘heat battery’ controlling temperature transfer through the wall in all climate zones- hot or cold. The calculated EFFECTIVE wall assembly R-value of R23 is enhanced by the mass concrete building envelope, exceeding energy codes for all climate zones.

Fox Blocks ICFs: Build a House with Life-Size Legos

Insulated concrete forms by Fox Blocks combine structure, air barrier, insulation, and vapor retarder into one product and one installation process, which simplifies the general contractor’s life and expedites the project schedule by eliminating coordination between trades.


Building a concrete house with Fox Blocks ICFs is like construction with life-sized Legos. They come in different shapes and sizes to accommodate inside and outside corners, brick siding, a curved wall…you name it. The reversible interlock design tightly fits the blocks together for a solid wall assembly, above or below grade. Fastening strips are molded into the EPS panels for attachment of interior finishes and exterior siding.


Fox Blocks has a full line of pre-assembled forms, knock-down forms, in concrete core sizes - 4",6",8",10" and 12", plus accessories. Cost savings, energy-efficiency, and ease of installation are just a few of the many benefits of ICF construction.

You Can Build a Pool with It! And Other Benefits of ICFs

You already know why concrete’s thermal mass is integral to a home’s energy-saving passive solar heating and cooling system. However, the airtight comfort and durability of a home built with ICFs has yet to be discussed.

Fox Blocks ICFs insulate the concrete wall on both the interior and exterior sides with rigid insulation that has a minimum R-value of 23.5. The continuous insulation provides an airtight barrier that regulates indoor temperature and prevents heat loss. The result is superior thermal comfort and continual operational cost savings.

The strength of an ICF home is unmatched. As weather events become more extreme, ICF construction provides safety from hurricanes, tornadoes, and flooding. In addition, ICF construction minimizes sound intrusion for a calm, quiet home.

And yes, you really can build a pool with ICFs.

Curious to Learn More? Contact Fox Blocks Today

The team at Fox Blocks is happy to answer any questions and would welcome the opportunity to work with you on your project. Contact our experts today.