Using thermal mass for heating and cooling

Where shouldn’t you put thermal mass?

Thermal mass can be a liability if used incorrectly, so it should be kept away from:

• cold, draughty areas such as entryways or unheated hallways
• rooms that face south or don’t get much winter sun
• areas with poor insulation.

Well-insulated lightweight construction walls are suitable for cool, draughty areas and south-facing rooms.

Thermal mass in floors

In most climate conditions a concrete slab, insulated underneath and around the edge, where it is in direct contact with the ground, is the most effective way to increase thermal mass. Other floor options include brick, compressed earth or a suspended concrete slab, with external insulation.

The surface can be polished or tiled where exposed to the sun. Surfaces exposed to direct sunlight shouldn't be covered with rugs or carpet, as this reduces the amount of heat the thermal mass can absorb.

Thermal mass in walls

Brick, concrete, concrete block (including insulated and aerated types) and rammed earth all provide good thermal mass. All can be used for internal walls if they catch the sun or are close to a radiant heat source. They must be insulated on the external surface to prevent heat loss.

Other options

Other options include:

• Trombe walls - a trombe wall is a north-facing heavy wall made of concrete or some other thermal mass material, located behind a layer of glass. The wall’s exterior is dark-coloured to attract the sun’s heat. The heat takes several hours to travel through the wall before it is released into the home’s living areas - properly designed, it should start to release heat in late afternoon or early evening as the temperature starts to fall. Trombe walls can range in size, from being window replacements to covering a whole north-facing wall. They can be used to maximise heat collection when views and glazing are oriented to the south or when site orientation is not ideal.
• Conservatories ‑ thermal mass can be used in a north-facing conservatory, either on the floor or in the wall separating the conservatory from the main part of the home.
• Gabion baskets - these are low wire or metal baskets containing rocks, placed behind glazing. Gabions can be used as internal window seats or feature walls.
• Feature brick or stone walls.
• Large earth-filled or water-filled pots, behind glazing.

Water features and heated pools can provide internal thermal mass if located near radiant heat sources. Pools require good ventilation to avoid condensation problems.

How much thermal mass do you need?

Area

The area of exposed thermal mass should be balanced against the area of glazing. You don't want to have so much glass that the room overheats in summer and loses heat too fast in winter.

As a rule of thumb, the exposed area of thermal mass should be about six times the area of glass that receives direct sunlight. For example, a north-facing room with a 1m² window should have about 6m² of exposed thermal mass, located where it will be exposed to direct winter sun.

The exact glass-to-mass area ratio will of course vary with climate and design.

Thickness

Very thick thermal mass walls may take too long to heat, while those that are too thin won't store enough heat. In most cases, the correct thickness for a thermal mass material is about 100mm to 250mm. However, the exact amount must be calculated as part of the design process.

What else do you need to consider?

Insulation

It's vital that thermal mass is insulated from outside temperature fluctuations. Without insulation, thermal mass can be a liability - creating cold, damp conditions in winter.

To maximise the benefits of thermal mass, the floor, walls and ceiling of any space it is keeping warm should be well insulated.

Thermal mass that doesn’t receive sunlight

If you have a high thermal mass wall in a room that does not receive direct sun and is not needed for heat storage, consider insulating it on the inside as well. Otherwise, the thermal mass might absorb warmth from inside your home - providing a cooling effect when you don't want it.

Ventilation

When you're deciding where to put thermal mass, you'll also need to consider summer cooling. Especially in warmer climates, exposure to cool breezes might be needed to ensure thermal mass doesn't absorb and release too much heat.

Floor coverings

Thermal mass floors should be left exposed to direct sunlight. Covering them reduces their effectiveness. Carpet or other coverings may be used on floors that aren't used for heat storage - for example in south-facing rooms.

Adding thermal mass to an existing home

Some homes have thermal mass that isn't being used. Any concrete slab in a north-facing room can absorb and store heat, so long as it's uncovered and insulated. So ripping up a carpet can help reduce your heating bills.

Thermal mass can be added to existing homes during renovations, for example by replacing a timber floor with a suspended concrete floor, or by adding a brick or stone feature wall to the interior of a house (the wall must be very well insulated from outside temperature extremes).