Thermal Mass
see also decrement delay and passive solar design
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 same for coolth during hot weather. This quality is utilised in passive solar design, including the Passivhaus design standard. However it is a complicated issue.
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.
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 ballance out and you will pretty much have an even temperature. It would simply be a matter of calculating the mass off 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 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 effects 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 may have a 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 profesional 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. 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 lead to complex software which models the variables to get the best overall result.
New kid on the block
Recently BASF have developed a new type of wallboard which incorporate microcapsules of eutectic wax substances which increase the effective thermal mass very considerably. These are phase change materials (PCMs) which effectively melt when a set temperature (23ºC or 26°C) is reached and due to latent heat absorbed they effectively soak up a considerable amount of heat. They then give it out again when the room temperature goes down. This is equivalent to having a high thermal mass. The data sheet for Micronal PCM SmartBoard 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. However literature on the material is still difficult to get hold of in English and the product is very expensive compared with plasterboard. See the BASF eco house at Nottingham
Been there before
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 and hot for a relatively short time and then close off the air intake to prevent loss of heat. The flue is chanelled 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 carful management and constant restoking 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 is often done by using a fan to draw cool night time air in through ducts in concrete floors. 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. Imagine a mobile phone charger which takes so many hours to charge and then gives you 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 (without much sunshine) 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!
Long term 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.
