Thermal Mass

Overview

Thermal mass refers to the mass of a structure that can act to retain and release heat. This is an important component of passive solar designs.  Thermal Mass works to regulate and dampen down temperature fluctuations in buildings.

So how does it work

Dense materials such as concrete, rammed earth and bricks all have good thermal mass properties. These materials could be built into walls, dividers or floors.

Your home, built from appropriate materials, can stay at a comfortable temperature year-round. Solid materials such as brick, concrete or stone, appropriately placed, provide sufficient thermal mass to absorb and store heat from the winter sun.

In winter the strategy is simple – let the sun in and let the floors and walls store the warmth, releasing it back into the home at night and on cloudy days. It helps if carpets and other insulating materials do not cover floors exposed to the sun.

In summer, the thermal mass of a building can “soak up” excess heat from within the building. This works if the thermal mass surface is shielded from direct sunlight by blinds, sails, a pergola or other means.

A building with little thermal mass (or incorrectly installed thermal mass) can experience large internal temperature fluctuations over a 24 hour period – heating up during the day and cooling down overnight. A building with well designed thermal mass would experience smaller temperature fluctuations over the same period in the same location. It would stay cooler during the day and warmer overnight.

Take Action

Building

When choosing building materials, try to incorporate as much dense material as practicable.  The thermal mass that works effectively will be insulated from the outside weather.  (In practice, therefore, brick veneer should have the bricks on the inside, insulated and protected by an outer skin).

Of all the material choices, the wall selection accounts for about 60–70% of the thermal mass of the building, with the floor area accounting for about 30–40%.  Generally, the more thermal mass the better.

Renovating

Try to rebuild walls (and floors) using thermal mass, particularly where the winter sun is going to shine on an inside wall.  A brick chimney is a good lump of thermal mass.

 

Resources

Sustainability Victoria’s Energy Smart Housing Manual will help you understand thermal mass and how it works in winter and summer.

This Technical Guide from Ecospecifier seeks to explain thermal mass, its benefits and limitations to enhance an understanding of mass in common building materials and it’s design implications and strategies for use in buildings.