Confused about thermal mass, where it belongs in your home renovation or new build, and how to use it to maintain even indoor temperatures year-round?

Learn about the specific conditions to consider so it works to improve your home’s comfort.

And see how underslab insulation, orientation and floor finish all shape whether the mass you’ve invested in actually performs.

Listen to the episode now.

Hello! This is Episode 420, and Way #20 of the 44 Ways to Create Your Sustainable Home series here on the podcast. We’re continuing through Section Five: The Building Envelope, A Sustainable Skin.

In this episode, we’re looking at one of the most commonly discussed, and commonly misunderstood, elements of the building envelope: thermal mass.

Way #20 is: Use Thermal Mass to Maintain Even Temperatures.

Thermal mass is a powerful tool when the conditions support it, and it’s designed with the right approach for the climate and site. However, when those conditions aren’t in place, it can make your home harder to heat, harder to cool, and less comfortable overall.

The website ‘Your Home’, created by the Australian Government, shares this about thermal mass…

“Thermal mass is the ability of a material to absorb, store and release heat. Thermal lag is the rate at which a material releases stored heat. For most common building materials, the higher the thermal mass, the longer the thermal lag.

“Materials with high thermal mass and long lag times are often simply referred to as ‘thermal mass’. These are typically heavyweight construction materials like concrete, brick and stone. Materials with low thermal mass are typically lightweight construction materials, like timber frames.” [see resources for link]

Thermal mass provides a buffering effect that smooths out the peaks and troughs of daily temperature change. 

On hot days, well-shaded thermal mass can stay cool and help moderate indoor air temperatures. 

In colder months, thermal mass exposed to warming winter sun heats up over the course of the day, and then releases that warmth into the indoor air as outdoor temperatures drop at night.

However, thermal mass doesn’t generate heat or cooling. It only responds to the environment. And so, it only performs as intended when the environment provides the right inputs, and it’s effectively positioned in your home to optimse those inputs.

Before including thermal mass, consider whether these occur at your site and home so the thermal mass can function as a benefit. 

First, a significant daily (diurnal) temperature swing, so the mass can absorb heat during the day and release it overnight (or vice versa in summer). 

Second, good passive solar access in winter, so the mass has something to absorb during the cooler months, from a well-oriented home with windows positioned to allow low winter sun to reach the floor, walls or other mass surfaces. 

Third, a well-insulated building envelope, because insulation is what keeps that stored heat inside the building.

When thermal mass works against you…

In climates where the daytime and night-time temperatures don’t have that swing or difference, the thermal mass will absorb heat during the day but can’t release it overnight because the night air is also warm. Instead of buffering temperature swings, it holds heat and continues to radiate it into the living space through the evening and overnight. 

In a cooler or even temperate climate, thermal mass can still work against you if the orientation doesn’t support it. I’ve reviewed homes where the floor is polished concrete, there are heavy brick walls, and the design has a real sense of mass and permanence. 

But the home wasn’t designed with passive solar design in mind, and the result is a home that’s slow to warm in winter and slow to cool in summer, because the mass is absorbing and holding the wrong temperatures at the wrong times. 

That mass creates thermal inertia. When it’s working for you, that inertia is a benefit. When it’s not, the home will be thermally uncomfortable.

For a long time, the no-brainer choice for thermal mass has been a concrete slab on ground. 

But if the slab is connected to the ground, its temperature is impacted by that ground temperature. 

In cold climates, or even in winter in milder climates, an uninsulated concrete slab will conduct any heat it absorbs directly into the ground, so the mass you invested in becomes a conduit for heat loss.

Cooler regions like Canberra and Melbourne have ground temperatures around 12 to 15°C, well below the optimal human comfort range of 20 to 24°C. And so, if the average ground temperature is outside that range, it’s preferable to insulate your slab. 

Ground temperature is also rising, so increasingly, warmer climates like Brisbane are also being designed with underslab insulation to future-proof for these increased ground temperatures.

What covers the thermal mass can matter too…

Tiles, polished concrete, slate, stone and similar hard, dense finishes allow the slab surface to absorb and release heat freely. When you cover it up with inert materials such as carpet and timber, you diminish the direct benefits of the thermal mass. 

And a quick aside on waffle pods…

These are the polystyrene blocks often used under slabs in volume-built homes. They’re often beneficial in an energy efficiency assessment, and they facilitate speedy and efficient construction. However, they’re terrible environmentally, and I’d suggest avoiding them altogether.

Test your thermal mass design before committing to your approach…

If you’re in the process of modelling your home’s energy performance, which we covered in Way #15 and Way #16, this is exactly the kind of decision that NatHERS modelling, or its equivalent in your country, can help you test before you build. 

Your assessor can compare the modelled outcome of different floor construction types, wall materials, and surface finishes for your specific climate and orientation. This makes the decision specific to your project and the climate it’s in, and you’ll be able to see the impact on your home’s heating and cooling loads. 

Remember your building is a system, so when working with your energy efficiency assessor like this during design, you can see which levers to pull, and what parts of your project to adjust, to improve its performance… all whilst your home is lines on a page and change is relatively cheap.

In this Episode, I cover:

• What thermal mass and thermal lag actually mean, and which materials are considered high thermal mass
• The conditions that should be in place for thermal mass to work as a benefit in your home
• Where thermal mass can quietly work against you, including in hot and humid climates and in homes without good passive solar access
• Why the ground your slab sits on matters, and when underslab and edge-slab insulation are worth including
• Why waffle pods are a poor sustainability choice, even when they perform in an energy assessment
• Where floor finish choice matters, and how carpet or timber diminishes the thermal mass benefit in a well-oriented living space
• Why thermal mass and insulation are complementary, not substitutes, and how modelling can help you test the right balance for your climate
• What to ask your architect, designer, builder and energy efficiency assessor so thermal mass is genuinely optimised for your site and climate

Plus a whole lot more.

LISTEN TO THE EPISODE NOW.

RESOURCES:

Episode 401 >>> 44 Ways #01: Design Your Future Home for the Orientation of Your Site

Episode 403 >>> 44 Ways #03: Naturally Heat + Cool Your Home

Episode 415 >>> 44 Ways #15: Use Energy Modelling During the Design Phase

Episode 416 >>> 44 Ways #16: Use Other Options to Model Your Design Before You Build

Australian Government’s sustainable design guide on thermal mass >>> YourHome: Thermal Mass

Renew’s article on how thermal mass affects home performance >>> Mass effect: The messy realities of mass

Powerhaus’s guide on when to install underslab insulation >>> When to Use Underslab Insulation

A free guide to simplifying sustainability across research, planning, design, materials, and building, from Amelia Lee’s 30+ years in residential design and construction >>> 44 Ways to Create a Sustainable Home

Access the support and guidance you need to be confident and empowered when renovating and building your family home inside my signature online program >>> HOME Method