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Using thermal mass for heating and cooling

Some building materials are good at absorbing and storing the sun's heat.

Thermal mass keeping your home comfortable

These materials are heavy and dense, and therefore high in what is technically called thermal mass. Common materials used for thermal mass include concrete or filled concrete block, stone or masonry usually used in floors or walls.

Used properly – the right amount in the right place, with proper external insulation – thermal mass can help maintain comfortable temperatures inside your home year-round. Thermal mass will absorb heat from the sun during the day and radiate it out as the temperature drops in the afternoon throughout the evening.

Graph showing the effect of thermal mass on room temperature throughout the day

Thermal mass reduces the room temperature during midday and early afternoon and increases the room temperature late in the afternoon and early evening hours.

Building thermal mass into your new home or renovation doesn’t have to increase costs. The money used for a carpet could for example instead be spent on polishing an exposed concrete floor.

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 ideal material is:

  • dense and heavy, so it can absorb and store significant amounts of heat (lighter materials, such as wood, absorb less heat)
  • a reasonably good heat conductor (heat has to be able to flow in and out)
  • has a dark surface, a textured surface or both (helping it absorb and re-radiate heat).

Different thermal mass materials absorb varying amounts of heat, and take longer (or shorter) to absorb and re-radiate it. For example, a brick wall has higher thermal mass than a timber framed cavity wall, so it will absorb more heat than a timber framed wall of the same thickness.

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 can absorb a lot of heat without warming up very much. Others will become quite warm after absorbing small amounts of heat. Thermal mass materials belong to the former. That means that if, for example, a concrete slab floor is exposed to direct sunlight it will be able to absorb and store a lot of heat and release it slowly.

A different material, for example a timber floor, cannot absorb and store as much heat, so what heat it does absorb is released quickly. As a result, much of the energy in the sunlight will quickly end up in the surrounding air, increasing your room temperature during the warmest parts of the day.

You can compare thermal mass to a sponge. Much of the water hitting it will be absorbed. A material with little thermal mass properties will behave more like a plain surface. Any water hitting it will bounce back and end up in the air.

In winter, properly designed thermal mass will absorb the heat from the sunlight on it 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 inside a dwelling should be shaded from direct sunlight for the entire day and be exposed to cooling breezes to provide some cooling on hot days and nights.

The interaction of insulation, glazing and thermal mass is complex and varies with climate and seasons. Because of this it is important to ask an expert in solar design, such as a designer, architect or building scientist who specialises in passive solar design, to advise you on the best option for your situation.

Where you should put thermal mass

Thermal mass should be placed where it will best be able to absorb heat in the colder months and be shaded in the warmer months. This means it is best to put it near windows or other glazed areas where it will be exposed to direct sunlight in winter. The north side of the house is generally best.

You can also put thermal mass near a woodburner, heater or another source of radiant heat.

Where you shouldn't 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. Concrete slab floors should preferably be insulated both underneath and around the edges, or at the very least for a metre in from the perimeter.

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, stained, tiled, painted or wall-papered). Various methods and proprietary systems are now available to design and retrofit insulated concrete block, precast concrete panel, or poured concrete walls. These include:

  • Insulating panels that can be fitted to the outside of new and existing concrete and concrete block walls
  • Precast concrete ‘sandwich’ panels, which have a layer of insulation between two layers of concrete.

Polystyrene formwork (or shuttering) systems are available for concrete walls. In some cases the polystyrene on the internal surface can be removed, however its insulation benefits usually outweigh the thermal mass benefits.

Other options

Other options include:

  • feature brick or stone walls
  • 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 the early evening as the temperature starts to fall. They can be used to maximise heat collection when views and glazing are oriented to the south or when site orientation is not ideal.

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.

Your passive solar design expert can optimise the amount and placement of both glazing and thermal mass by using hour-by-hour simulations of the design.

Thickness

Concrete slab floors should be 100–200mm thick for the best performance, while thermal mass walls should be 100–150mm thick. Very thick thermal mass walls and floors may take too long to heat, while those that are too thin won't store enough heat. The exact amount should be calculated as part of the design process.

To prevent the potential for overheating thermal mass in summer, it's important to design appropriate eave widths.

Designing Comfortable Homes, a booklet from the Concrete and Cement Association of New Zealand and EECA, has more information.

Other things to consider

Insulation

Good insulation (including glazing) is essential to maximise the benefits of thermal mass. It's vital that thermal mass is insulated from outside temperature fluctuations. Without insulation, thermal mass can be a liability – radiating cold and exacerbating damp conditions in winter.

Thermal mass that doesn't receive sunlight

Consider insulating the inside of high thermal mass walls which don’t receive direct sunlight or store heat from nearby radiant heat sources. This is particularly important if you only heat for part of the day – the thermal mass will absorb heat from the air until it is the same temperature.

Concrete drying

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 four months to dry out (longer in winter), and thicker slabs will take longer. During that time it is particularly important to ventilate the house regularly to avoid the build-up of internal moisture.

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. It may be worthwhile ripping up a carpet and putting ceramic tiles down to reduce some overheating in summer and capture some free solar gains in winter.

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. It will need to be exposed to direct sunlight or close to a radiant heat source, and be very well insulated if it’s an external wall.

You might also like…

Other resources

  • Design for the Sun

    You can order this guide from EECA’s Energywise website. It includes guidance for designing energy-efficient and passive solar houses in New Zealand.

  • Designing Comfortable Homes

    Download a comprehensive guide to designing more comfortable houses from the Cement and Concrete Association of New Zealand's website.

  • Eco Design Advisor

    Make the most of free advice on this website – and check whether there’s an Eco Design Advisor in your area. You can book them for free personal advice on your home design.

  • BRANZ Level: Passive design

    Get guidance on passive design, including heating and cooling, on the BRANZ Level website.

Note that this document is published by the Ministry of Business, Innovation and Employment Chief Executive as Guidance under Section 175 of the Building Act 2004. This is a guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case.