Thermal Mass in Buildings

What is Thermal Mass?

Thermal mass, more appropriately called Fabric Energy Storage, is the ability of the fabric of a building to absorb excess heat. Effectively utilised, it can reduce cooling loads and, in some cases, remove the requirement to provide air conditioning entirely in some buildings. Since air conditioning is one of the major users of energy, this can have a significant effect on reducing CO2 emissions in building use.

How does Thermal Mass work?

How does thermal mass workSolar gains, equipment use and human activities within the building generate heat. The warmed air rises, flows across the exposed surfaces and is absorbed into the material. For this to happen efficiently, the soffits of the ceiling must be exposed. This can significantly reduce air and radiant surface temperatures. These are often combined as the dry resultant temperatures, an indication of the temperature perceived by the occupants.

How does thermal mass workHeat stored in the structure by day is expelled at night by the flow of cool air across the exposed surfaces

At night, cool air is allowed into the building. This flows across surfaces which have been used during the day to absorb heat, purging that energy and thus allowing the process to begin again the following day.

How does thermal mass workIt can not only reduce peak temperatures but also delay the time at which that temperature occurs.

Is it correct to say that the more mass is in a building, the more thermal capacity is available?

How does thermal mass workNo, this is incorrect. The concept that more mass is good arose from a fundamental misunderstanding of how fabric energy storage works. The origin of the theory lay in observations that large, monolithic buildings such as the Radcliff Camera (pictured) never overheated, even in the warmest weather. The assumption then made was that this was because the large, thick walls absorbed the excess heat in the building. The actual reason was more prosaic. These buildings did not overheat because they sheltered relatively low levels of activity, and therefore generated little internal energy, and also because small windows limited solar gain.

More recent studies have demonstrated that relatively small amounts of mass are mobilised in maximising the available thermal capacity in a modern building.

How much mass is mobilised in a building to optimise the available thermal capacity?

How does thermal mass workThe quantity of mass in a building which can be linked to the internal atmosphere to provide fabric energy storage is governed by the admittance of the material of which the construction, usually the ceiling, is manufactured. Generally this will be concrete. The admittance is a measure of the ability of a material to exchange heat with its surroundings. The admittance is limited by the rate of heat transfer between the material and its environment. On a daily 24 hour (diurnal) cycle, the maximum value of admittance for a slab exposed from underneath only may be achieved with only 75-100mm of concrete. This means that, where heating and cooling takes place over a daily cycle, a floor thickness of 100mm will provide the maximum amount of fabric energy storage possible. If more mass is provided, it will not be utilised.

What is the typical thickness of a floor slab in steel construction?

How does thermal mass workThis varies according to the form of construction used. The most common flooring system used in the UK in composite metal deck supported by a steel beam (shown). This will typically be of the order of 130-150mm thick with 70-90mm of this above the rib of the deck. Steel beams with precast planks are also common and will typically have a floor thickness of 200-250mm. Reinforced concrete floors will typically be 250mm+ thick.

What studies have been carried out to assess the difference in thermal mass potential between different types of flooring systems?

How does thermal mass workIn 2007 Aecom carried out a study on a four storey, naturally ventilated office block using five different flooring systems. The results are summarised here.

How does thermal mass workThe results are presented as percentages of occupied hours in which temperatures lay within certain limits. As can be seen, there is little difference between the different systems.

Are there examples of steel framed buildings that have used thermal mass?

How does thermal mass workThere are numerous examples. One of the most recent was highlighted in the June 2010 edition of New Steel Construction. This is St. Johns Square in Seaham, Co. Durham; a combined library/office construction for Durham Council.

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