Thermal Mass – Design Tips

Note: This article appeared originally in Green Magazine as in the image above. The text below may differ slightly from the original.

Thermal mass is a critical component of passive solar design, and probably the most misunderstood. The role of thermal mass is to effectively stabilise interior temperatures at all times of the year, and this at zero running cost which makes it most attractive. But how does it work?

What is thermal mass?

We know that like the doona on our bed, insulation is light and fluffy and contains lots of air pockets to slow down heat loss. Thermal mass however is hard and heavy and the perfect complement to insulation. Imagine a boulder that has been sitting in the winter sun all day. Were you to bring it indoors at the end of the day, you would have a gentle heat source that would release its energy during the evening and through the night. If the house envelope is well insulated, the energy released would escape the building slowly, thereby keeping the house warmer for longer. If you left this boulder behind a sunny window during the day, it would in the same way absorb solar heat and release it at night. The same principle applies in summer. A boulder left in a cool room all night will gently release its cool energy if placed inside a house during a hot summer day, counteracting the house heat uptake. The boulder would need to be left in the shade and the windows kept closed to maximize the daytime benefit. At night however, when the temperature drops, the windows need to be left open to cool the house down including the thermal mass.

Best materials

The materials that act as thermal mass in a home are concrete, bricks, sand and stone. Interestingly, water has a very high thermal mass, as it can absorb more energy than a masonry product. It is not effective in the fabric of a building however because it releases its energy too quickly, which does not work with the 24 hour cycle. A masonry floor or wall will release its energy over 6-10 hours provided it is 100mm thick or more – this means a tile floor on a timber structure will not suffice. Suitable finishes to a concrete slab include polishing, tiling or stone flooring. Insulating materials such as timber flooring, cork or carpet prevent the thermal mass from fully exchanging energy with the indoor environment and are best avoided. Area rugs are acceptable.

Best location

The ideal location for thermal mass is the floor as it is the surface that is most hit by the sun in winter. The heat rises uniformly from the floor which is another advantage. A second best option is to have thermal mass in the walls, either internal walls or external walls, using double brick with cavity insulation or reverse brick veneer, where the insulation is on the outside. Thermal mass located outside of the insulation barrier – read conventional brick veneer walls – is nowhere as effective thermally, because the indoor environment is thermally separated from the outdoor one with wall insulation. If we use the analogy of a esky (a well-insulated house) and ice blocks (thermal mass), it is like placing the ice outside the esky and expect it will stabilise the air temperature inside the esky. The golden rule is that thermal mass needs to be insulated to work – i.e. kept inside the building envelope.

Best climates for thermal mass

Thermal mass will absorb the free heat of the sun in winter and the cool of the night in summer, but it will also do a great job at absorbing paid energy if the winter day is overcast or the summer night is warm. Whatever the energy source, it will absorb it and release it slowly for the next 6-10 hours. A home with functional amount of thermal mass will take a bit longer to heat and is therefore not recommended in large amounts for a weekender where instant heat might be required. Another factor to consider is that climates with greater daytime and night time temperatures will make the most of the thermal mass as it will be best able to operate on free rather than paid energy. Its ability to regulate indoor temperatures with paid energy is just the same however, it will just cost you a bit more to run the energy uptake / release cycle.

The right amount of thermal mass

A home works best thermally when the right balance of thermal mass, passive solar gains, insulation and ventilation is achieved. Solid brick and mud brick homes for example feature large amounts of thermal mass. If this is not balanced by a significant amount of passive solar gains in the form of unshaded north facing windows, the house will be too cold year round in cooler and temperate climates except on very hot days. Timber homes in contrast do not offer enough thermal mass and will quickly follow outside temperatures, whether on the rise or fall. Insulation will help somewhat, but no more than it helps the air temperature inside an empty esky. I find that in Victoria, a well-orientated home with good insulation in the roof and in timber framed walls, teamed with a concrete floor and functional shading – for the hot season only – will perform well on a reasonable budget and can be expected to gain an 8 star rating. Where climates are more extreme, additional thermal mass in walls will be useful as a second measure.

Insulating concrete slabs

Cooler European climates mostly used suspended slabs with the surface facing the ground lined with insulation. Milder Australian climates would benefit from such construction as well although a slab on ground with edge insulation is also suitable for most climates bar the tropical one, as the ground temperatures are more temperate. Insulating the underside of a slab on ground will be most beneficial where in-floor hydronic heating is installed.

Typical objections to concrete and to concrete floors

Many people worry that a non-resilient floor will be too cold and hard on their knees. My understanding is that there is no scientific evidence that low-impact activities such a walking on a hard floor has an impact on joints – it is an unheard of objection in Europe for instance where all homes and apartments have been built on concrete floors for more than a century. The floor can be somewhat cooler but it often takes living in a passive solar design home to become convinced that it is not uncomfortable in any way. Concrete itself however is not a green material, as the production of its cement content is responsible for large generation of greenhouse gases worldwide. I find that using concrete in a floor where it will minimise energy use is a satisfactory response, whereas using concrete panels or blocks in walls or other places where alternatives such as lightweight structures or recycled bricks are suitable, is harder to justify. I have successfully used recycled bricks set on aerated concrete floor panels and found that the thermal mass thus provided was adequate. Alternatively, specifying fly ash to replace some of the cement or using the new cements of the geopolymeric variety is a good idea. The latter release only 40% of the greenhouse gases of ordinary cement and are gaining recognition. Check whether you have a plant near you and use that small footprint concrete in your next project!    

Retrofitting thermal mass

Thermal mass is often a structural building component and is best integrated at the planning stage. Timber floors can be replaced with inset slabs and reverse brick veneer can be installed in walls, but these are not cheap solutions and they will require some engineering. Another avenue would be to use the new Phase Change Materials. They are lightweight products with exceptional thermal mass properties which can be installed in ceilings to absorb summer heat in particular, a very useful product in hot climates. The case is still unclear as to its use in temperate and cooler climates as they may provide excessive amount of thermal mass in winter situations.