Using thermal mass for heating and cooling
Some building materials are good at absorbing and storing the sun's heat.
These materials are high in what's known as thermal mass. They're usually dense materials - such as concrete and brick - and they're usually used in floors or walls.
Used properly - the right amount in the right place, with proper insulation - thermal mass can help maintain comfortable temperatures inside your home year round. When the weather is cool, thermal mass will absorb heat from inside the room during the day and radiate it out as the temperature drops in the afternoon or evening.
Thermal massare also useful for cooling because they keep absorbing heat as long as the air temperature is warmer than the thermal mass.
If you incorporate thermal mass into your building or renovation plans, there's no reason for it to increase your costs.
How does thermal mass work?
Thermal mass works by using a simple principle of physics: heat moves from warmer surfaces to cooler surfaces.
When the sun is shining into a room and the air is warm, heat will be absorbed by the walls, floor and other surfaces in the room.
How much heat they can hold depends on what they're made of and how thick they are. Some materials take longer to absorb heat, but can hold it for longer. For example, concrete floors will absorb more heat and hold it longer than timber floors.
Properly designed thermal mass will absorb heat during the day. Then, as the air temperature drops, the heat will move from the warmer thermal mass to the cooler air and other surfaces in the room.
In summer, thermal mass absorbs heat from the inside air, providing a cooling effect during a hot day.
In other words, thermal mass evens out variations in temperature. It helps keep you comfortable, day and night, all year round.
Thermal mass materials
Probably the simplest form of thermal mass is a concrete slab floor. You can also use concrete blocks, tiles, brick, rammed earth and stone. Three factors determine how good a material is at absorbing and storing heat:
- The material has to be dense and heavy, so it can absorb and store significant amounts of heat (lighter materials, such as wood, absorb less heat).
- The material has to be a reasonably good heat conductor i.e. heat has to be able to flow in and out.
- The material should have a dark and/or textured surface. This helps it to absorb and re-radiate heat.
Different thermal mass materials absorb different amounts of heat, and take different lengths of time to absorb and re-radiate it. For example, a brick wall will absorb more heat than a rammed earth wall of the same thickness.
Where should you put thermal mass?
The simple answer is: put it where it will be able to absorb heat. This means putting it near windows or other glazed areas where it will be exposed to direct sunlight. The north side of the house is generally best.
You can also put thermal mass near a woodburner, heater or other source of radiant heat. Thermal mass should be in rooms that are well insulated.
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. This can form the whole floor, but just as effective is a strip of polished or tiled concrete along the sunny side of the room or a slab floor which is exposed around the edges with mats or carpet in the centre.
Other floor options include brick, compressed earth or a suspended concrete slab in rooms with north-facing windows, 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 can be used for internal or external walls if they catch the sun or are close to a radiant heat source. Walls can either be solid or an internal veneer.
External thermal mass walls must be insulated on the external surface to prevent heat loss and exposed on the inside of the house (that is, without internal lining, but it can be plastered, painted or wall-papered).
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?
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.
Very thick thermal mass walls and floors may take too long to heat, while those that are too thin won't store enough heat. Concrete slab floors should be 100-200mm thick for the best performance, while thermal mass walls should be 100-150mm thick. However, the exact amount must be calculated as part of the design process.
What else do you need to consider?
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.
Concrete slab floors should be insulated underneath the slab to an R value of at least 1.9 and around the slab edge so that heat is not lost into the ground or to the outside air. If you have an existing concrete slab floor, retrofit insulation around the edges. External thermal mass walls should be insulated on the exterior face.
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.
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. You may also need to consider summer shading to prevent overheating, particularly from the late afternoon sun.
Thermal mass floors should be left exposed to direct sunlight. Covering them reduces their effectiveness. Carpet, lino, cork or timber will not et the heat through.
However, floor coverings may be used on floors that aren't used for heat storage - fo example, in south-facing rooms or in areas of the room which don't receive the sun.
If you are using concrete as thermal mass in a floor or wall, you need to be aware that it will not perform at its best until it has dried out. Drying time will vary depending on humidity and thickness. A 100mm thick slab can take 4 months to dry out (longer in winter), and thicker slabs will take longer. See Concrete construction for more information.
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 by:
- laying a concrete floor in a new extension
- adding a brick or stone feature wall to the interior of the house. It will need to be exposed direct sunlight and be very well insulated
- adding a conservatory with high thermal mass in the floor or wall.
On Smarter Homes
- Design overview
- Passive heating
- Passive cooling
- Construction systems
- Concrete construction
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From other sites
The Energywise website has a web page on thermal mass and passive design. And you can download the Energy Efficiency and Conservation Authority publication Designing for the Sun, which provides comprehensive and conceptual information and guidance for designing energy-efficient and passive solar houses in New Zealand.
Designing Comfortable Homes is a guide to energy-efficient design using thermal mass, glass and insulation, produced by EECA and the Cement and Concrete Association of New Zealand (CCANZ). The booklet can be downloaded for free from the Energywise website.
The Australian Your Home website has technical information on designing for thermal mass.
The Level website has more technical detail on thermal mass.