Thermal mass: exposing the myth

Thermal mass is the ability of the fabric of a building to absorb excess heat and as such it can reduce cooling loads and, in some cases, remove the requirement to provide air conditioning entirely and its associated energy consumption.

Over a typical 24 hour 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 (typical of steel composite construction) will provide the maximum amount of fabric energy storage possible. If more mass is provided, it will not be utilised and is a waste.

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Thermal Mass FAQ's

Q.		What is thermal mass?
Q.		How does thermal mass work ?
Q.		What is thermal capacity?
Q.		Is it correct to say that the more mass is in a building, the more thermal capacity is available?
Q.		How much mass is mobilised in a building to optimise the available thermal capacity?
Q.		What is the typical thickness of a floor slab in construction?
Q.		What studies have been carried out to assess the difference in fabric energy storage potential between different types of flooring systems?
Q.		What of the appearance of exposed soffits? Are they acceptable?
Q.		Does a metal deck reflect the heat and reduce the ability of the slab to absorb excess energy?

Thermal Mass FAQ's

What is thermal mass?

Thermal mass, more correctly called Fabric Energy Storage is the ability of the fabric of a building to absorb excess heat. It is important because, effectively utilised, it can reduce cooling loads and, in some cases, remove the requirement to provide air conditioning entirely. 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 ?

	Solar 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.
	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.
	Where utilised successfully, fabric energy storage has the capacity to significantly reduce maximum daytime temperatures. This figure shows internal dry resultant temperatures for a typical south facing office.

What is thermal capacity?

Fabric energy storage can be used to remove excess heat, improve thermal comfort and reduce energy use in buildings. The amount of heat which the structure, or fabric, of the building can absorb is the thermal capacity.It 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?

No, 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 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 is that 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?

The 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. 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.

Lightweight building materials, such as dry lined partitions, have a low admittance, or a low ability to store and release useful amounts of energy. By contrast, exposed structural elements such as floor slabs have a high admittance.

On a daily 24 hour 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 construction?

	This 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. 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 fabric energy storage potential between different types of flooring systems?

In 2007 AECOM carried out a study on a four storey, naturally ventilated office block using five different flooring systems. The results are summarised here.

The 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 systems.

What of the appearance of exposed soffits? Are they acceptable?

Steel decking for composite floors can be painted to provide different colours. Decking is also available with a plasticol coating although this will not usually be compatible with composite construction.

For concrete soffits the issue is more difficult but can still be solved. An aesthetically pleasing finish requires that more care is require in forming and shaping the surface. Significant care in preparation is required if an aesthetic concrete surface is to be provided. Formwork must be new and complete sections between designated construction joints must be capable of being concreted in a single operation. Also, formwork must be adequately supported to prevent differential movement. High standards of dimensional tolerance in casting the slabs is required and aesthetic surfaces must be protected after preparation to retain the specified surface finish.

Does a metal deck reflect the heat and reduce the ability of the slab to absorb excess energy?

An oxidised, galvanised surface has an emissivity value of around 0.3 (reflector or shiny). For a plasticol coated deck it is 0.82 and concrete 0.9. The underside surface resistance when the slab is cooling the space for an emissivity of 0.3 is 0.107 m²K/W and for an emissivity of 0.9 it is 0.091 m²K/W. So overall the concrete is about 15% better in terms of the ability to allow heat flow across the surface.

This essentially means that assuming a flat steel deck the concrete will absorb heat at a quicker rate than the steel. The steel decks are not flat however and a typical CF60 deck has 1.42m² of available surface area per m² of ceiling area so that more than compensates for different resistance