Thermal Mass and Insulation are not the same

Do you still design with concrete and glass only? then this article is for you.

What is referred to as thermal mass, is a combination of material properties in building physics, including insulation, which collectively determine the material's ability to store and release heat.

Because both are interconnected it can be confusing to separate their individual function but lets give it a try :-)

In building science thermal mass is actually a combination of:

• Heat Capacity: The amount of heat energy a material can store per unit volume or mass.

• Thermal Conductivity: The speed at which heat enters and exits the storage medium.

• Density: The mass per unit volume of a material

While Insulation is typically referring to the property of thermal conductivity alone - to slow down the speed at which heat exits the building -

thermal mass is typically referring to high mass (high density) materials, most popular as in concrete, and describes that the timely capture of sunlight can be utilised to release heat at a later time - hence reducing the need for active heating and lower power bills.

Sounds good in theory because the sun has a lot of power. Well, while it shines....

And you will need that extra power because...

High mass also needs high energy input!

High volumetric heat capacity paired with high thermal conductivity, like concrete, is not the most effective way to reduce energy bills due to how quickly it transfers heat. For instance, heating 1 cubic meter of concrete by 20 K requires 42,240,000 joules of energy, but because of its high thermal conductivity, it dissipates heat rapidly at a rate of 34 watts per square meter.

In contrast, an insulation product like mineral wool, while requiring only 1,120,000 joules to heat the same volume by 20 K, has a much lower thermal conductivity, resulting in a significantly slower heat loss rate of just 0.8 watts per square meter.

This means that mineral wool not only requires less energy to achieve the same temperature increase but also retains heat much longer. Consequently, buildings insulated with materials like mineral wool maintain stable indoor temperatures more efficiently.

But you want to do better than that and combine the best of both worlds, so you ask yourself -

High mass inside or outside the building?

(Strap-and-line - I keep coming for ya!)

Using high thermal mass with interior insulation doesn't make ANY! sense when trying to build energy-efficient structures in any climate because it undermines the fundamental benefits of thermal mass. Thermal mass' one and only job is to absorb and store heat from the environment, moderating indoor temperatures by releasing stored heat during cooler periods and absorbing excess heat during warmer periods. When insulation is placed on the interior side, it creates a barrier that prevents the thermal mass from interacting effectively with the indoor environment, hence failing to regulate indoor temperatures.

Consequently, the interior insulation renders the thermal mass ineffective, leading to increased reliance on mechanical heating and cooling systems, and ultimately resulting in higher energy consumption and costs. For optimal energy efficiency, thermal mass should always be exposed to the internal environment, with insulation placed on the exterior to protect against external temperature fluctuations.

Just think about the temperature range we keep inside our buildings (18-25Cº), vs. the temperature ranges outside our buildings (-5-40Cº) and decide for yourselves where we should place the barrier for rapid heat transfer...

And now you've done it all correct and insulated your thermal mass on the outside but -

High mass and insulation combined leads to massive overheating problems!

High mass paired with good insulation can lead to overheating problems, particularly in well-sealed, energy-efficient buildings. When a building absorbs a large amount of heat during the day through excessive glazing and is also well-insulated, the absorbed heat can become trapped inside, as the insulation effectively prevents it from escaping.

Without adequate ventilation or cooling strategies, this can compromise comfort and may necessitate the use of air conditioning or other cooling systems, which can offset the energy savings achieved through insulation.

Its a shame you didn't get your project modelled for energy performance during the design phase. Back then you could have found out that the solar heat gains were problematic and some minor changes in the design, or specification could have mitigated that. But, ok - maybe next time...We're here to help.