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Thermal Mass

Energy storage

High thermal mass is generally seen as a useful quality in a building because it can be utilised to trap and save any spare heat which might turn up.

It can also do the reverse equivalent for coolth during hot weather. This quality is utilised in passive solar design, including the Passivhaus design standard. However it is a complicated issue which is now increasingly being covered in the SAP calculations.

It works a bit like the rechargeable battery in a mobile phone which captures energy when on charge and provides energy later when it is needed. With a building the energy is from sunlight (or maybe heat from cooking or human bodies etc.) It is stored in the fabric of the building when there is a surplus and lets itself out later when needed. In particular, it is the first 50mm to 100mm of the material closest to the inside surface which is of most use for this.

ARUP publishes free calculator software called the Passive Design Assistant tool for thermal calculations of houses on a room by room basis. It is intended to have a “simple and intuitive user interface” and to be somewhat educative about thermal modelling. Commercial software is available to calculate insulation values, thermal mass and risk of condensation from Build Desk

A kind of ideal situation is where you would like your house to remain at 21°C. all the time and the outside temperature goes up to 25 during the day and down to 17 at night (a swing of 4°  each way). With the right amount of thermal mass to absorb the extremes (at the correct speed) everything would balance out and you will pretty much have an even temperature. It would simply be a matter of calculating the mass of building materials needed to absorb the daytime surplus energy so it could let it back out at night.


So far so good but there are several complications –

  • The better a house is insulated, the less thermal mass matters. To take it to an extreme, imagine an almost perfectly insulated house with almost perfect MVHR and excellent shading of windows. No matter how hot or cold it is outside it will have almost zero effect on the inside temperature and so thermal mass would be irrelevant. Conversely, a rather poorly insulated house may benefit considerably from high thermal mass.
  • The weather is seldom as described above. Sunlight comes in irregular doses from varying directions at different times of the year so this may alter the amount of thermal mass worth building in.
  • The speed at which thermal mass has its effect is critical and difficult to calculate. Any surplus heat starts to find its way into all the surfaces of a dwelling including walls floors, ceilings, furniture fabrics etc. All these materials absorb heat at different rates and the outer surfaces get affected fastest and most. Another factor is the layout of rooms and circulation spaces. If surplus heat from one corner of the house can spread itself around by air movement it is better than one room simply overheating.
  • The way heat from sunlight enters the house affects thermal mass in different ways. Solar gain through windows generally heats the air inside quite quickly which then moves around by convection. Heat coming in through the wall is forced to go through the thermal mass of the wall before it heats the inside. This can be utilized to create decrement delay.
  • The usage pattern of the house will have a major bearing. Imagine a house where everyone is in all the time – big family, children crawling about on the floor – that kind of thing. And then imagine the opposite. Young single professional who is only home half the time (but more importantly has an erratic lifestyle). The first example works well with high thermal mass but not the second. High thermal mass is the last thing required. The ideal with the second is minimal thermal mass so that it can heat up very quickly when the person comes home and then cool quickly when they leave so as not to waste heat. The same is true for holiday homes.

All this has led to complex software which models the variables to get the best overall result.

Chimneys as heat stores

Historically there have been ways developed to utilise high thermal mass with fast burning wood. In some areas of central Europe there are examples of houses which are basically built around a massive masonry chimney with a large wood stove built in at the bottom. The idea is to charge the stove with tightly packed small section firewood which is burned fast, hot and cleanly for a relatively short time and then close off the air intake to prevent loss of heat. The flue is channeled to distribute the heat into as much of the masonry as possible where it is stored for a day. The whole chimney then slowly radiates heat into the house around it. It means that the careful management and constant restocking of the stove is not necessary. Given the clean nature of a fast burn and the generally agreed surplus of low quality timber in the UK there may be merit in developing this here.

Thermal mass and cooling

Thermal mass can be used for night time cooling. This can be done by using a fan to draw cool night time air in through ducts in concrete floors.  It may be possible to draw air through without using a fan. This is usually employed in situations where street noise is a problem if windows are simply left open for cooling on a night.

It is worth noting that thermal mass only works within certain bounds. With the mobile phone analogy for storing energy, a charger takes so many hours to charge the battery and then you get so many hours of use. Once it is full you can’t get any more into it. Once it is empty you can’t get any more out of it. So in long periods of high or low external temperature thermal mass is of little help because it has ‘run out’ of heat or coolth. Not much use in a heatwave or a long freeze. Only good insulation will help then!

A recent (2006) report by Arup looked at four types of construction ranging from lightweight timber frame through to heavyweight masonry and concluded that the heavyweight type was more energy efficient in the long run, even after the embodied energy of concrete was taken into consideration. This would be true because more air conditioning in summer would be needed to cool the lightweight type over the next century.

The Concrete Centre covers this report on page 14 of their Concrete Quarterly publication to counter criticism of concrete being so energy intensive to produce. However it is interesting to look at the amount of insulation in the various types of wall. They are all set at 0.3W/m².ºK (roughly 50mm of insulation – no more than has been in use for the last half century) whereas a more modern standard such as the Passivhaus standard will have approximately 300mm of insulation. Of course the more insulation, the less heat gets into the house in hot weather. So this is a completely outdated study.

Furthermore it could be seen as somewhat disingenuous because of the scaling of the various wall thicknesses. The lightweight construction is drawn to look the same thickness as the heavyweight one by having twice the thickness of airspace cavity and twice the thickness of insulation. In reality the heavyweight construction would be about 100mm thicker. See the section on building footprint. This is not to argue against thermal mass: just to point out that vested interests can emphasize what suits them.

Phase change materials

There are now a handful of companies producing phase change materials incorporated in wallboards. These effectively work like high thermal mass but utilize the latent heat of phase change. If you think back to your school physics lessons you may remember the latent heat of evaporation and fusion. For instance if you heat up ice at 0°C. it doesn’t get hotter for some time. It absorbs the heat and uses it to turn itself into water.

In a similar way, these wall boards have microcapsules of wax built in, which absorb excess heat in the room to turn the wax from solid into liquid, so the room stops getting hotter. Conversely when the temperature drops the wax goes solid and gives out heat at the same time

Two of the companies producing these boards are:

  • Datum Phase Change with their RACUS®, and F.E.S-Board® which use bio waxes.
  • BASF do Micronal PCM SmartBoard. The data sheet gives the latent heat capacity of their 15mm wall board as 330 kJ/m² and this is equivalent to 90mm of concrete (according to a quote in Building magazine).

The implications for increasing the thermal mass of lightweight buildings is considerable.

See the BASF eco house at Nottingham

Inter-seasonal heat storage

All the above is about thermal storage acting in the fabric of the building over a short time span, usually 24 hours. There are a couple of different approaches. Surplus heat can be saved into a specially constructed store such as a water tank or rock store which can act over a longer time period. In fact water stores have been used successfully to store summer heat right through into the winter. See more here.  Water is extremely good at storing heat. It has a specific heat capacity of 4.18 joules per kilogram compared with iron at 0.45 , concrete at 0.75 and brick at 0.9.

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