Stevenage Borough Council
Thermal mass and night cooling has been exploited in the refurbishment of these offices of Stevenage Borough Council to reduce daytime temperatures.

Exploiting thermal mass in existing buildings

Published:  03 September, 2004

Exposing the thermal mass of an existing building to night-time cooling has reduced internal temperatures by 5 K compared with ambient temperatures — with important implications for responding to climate change. Nick Barnard and Ant Wilson explain.

Recent hot summers and the escalating use of information technology in buildings has lead to increasing temperatures and overheating in many existing buildings. Climate-change predictions suggest increasing misery for building occupants in future summers. Rather than resort to fully air-conditioned solutions, many are seeking ways to meet this challenge in a cost-effective and environmental manner.

One option is to use thermal mass and night ventilation. Thermal mass available in building structures offers the potential to significantly improve comfort in many existing buildings. This approach has been adopted in the refurbishment of Stevenage Borough Council’s offices as a part of a European project known as REVIVAL (Retrofitting for Environmental Viability Improvement of Valued Architectural Landmarks). Stevenage is the UK REVIVAL site in a project which includes sites in France, Italy, Greece and Holland and aims to demonstrate that tertiary buildings from the post-war pre-energy-conscious era can be refurbished economically, with improvement in energy performance that lead to lower life-cycle carbon-dioxide emissions than the original building, or an equivalent new building, and would therefore make a significant contribution towards the EU policy of meeting the Kyoto Protocol.


Thermal mass can be combined with night ventilation of to provide passive cooling. Outside air is circulated through the building at night, where it comes into contact with and cools the building fabric. The cooling that is stored is then available to help maintain comfortable temperatures the following day. Mechanical cooling can be eliminated in applications with low to moderate cooling loads, with cost and environmental benefits.

This approach has been used successfully in a number of buildings. However, this often relies on the exposure of soffits to provide a direct thermal link between the slab and the space. Many existing buildings have false ceilings and floors that thermally isolate slabs from the conditioned space. Making effective use of the ‘hidden’ thermal mass in such buildings provides a notable design challenge.


The CoolDeck system has been developed to meet this challenge. Circulating air transfers heat between the space and the slab. The Cooldeck elements provide very good surface heat transfer between circulating air and the thermal mass. The elements are attached to the slab surface in the void. The air is circulated by a fan through the narrow paths formed by sealing along the edges. The turbulent air flow created through the paths enhances heat transfer between the slab surface and the circulating air.


The first CoolDeck systems to be installed in Stevenage Borough Council’s offices were based on using the building mass and night ventilation to provide passive cooling. The systems have been combined with other passive measures, including solar blinds, to reduce heat gains. Exposing the soffit by removing the false ceiling was not favoured due to aesthetic, co-ordination, acoustic considerations and concerns about heating in the winter. CoolDeck was therefore adopted to use the mass ‘hidden’ in the ceiling void.

During the summer, cool outside air is introduced into the offices by window fans at night. Ceiling fans circulate this air under the CoolDeck elements to enable the cooling to be stored in the thermal mass. During the following day, the ceiling fans operate to release the stored cooling.

The system was installed by Stevenage Borough Council. Estimated costs are about £40/m2, compared to £180/m2 for the air conditioning alternative. The design Coefficient of Performance (cooling/fan energy) is about 20.

Monitoring before and after indicate that a reduction in the region of 5 K in internal temperatures has been achieved relative to ambient temperatures. Areas with night ventilation are also reported to be fresher in the mornings.

In subsequent installations, phase-change material has been integrated with the CoolDeck elements to increase their thermal storage performance. This has enabled the number of elements and interconnecting ductwork to be reduced, easing co-ordination with existing services in the ceiling void. It can also be used to provide thermal storage where there is limited existing building thermal mass, such as for top floors with lightweight roof constructions. The cost of the system using the phase-change material is about the same as that for the system without. The cost of the phase-change material is offset by savings due to fewer elements and fittings.

The phase-change material is a salt that changes phase between about 20 and 24°C. This temperature needs to be high enough to be frozen by summer night ambient temperatures and low enough to provide cooling to the occupied space. The phase-change material is contained in flat pouches that lie in the CoolDeck elements so that the air passing through the gap exchanges heat with the slab above and the phase change material below.

The CoolDeck system has also been used with mechanical cooling to reduce the size of mechanical plant required in the new customer service centre. The system provides local stored cooling to peak lop the load imposed on the central plant. The CoolDeck system stores free cooling (plus mechanical cooling during peak periods) at night. This cooling is then delivered to the space the next day, reducing the size and cost of the mechanical cooling system. Phase-change material has also been incorporated to provide additional storage. This has a lower phase-change temperature (18 to 20°C) than the passive systems to provide an effective cooling temperature differential (the maximum space design temperature is 25°C).

These installations have demonstrated the potential of using thermal mass to improve comfort in buildings in a cost-effective manner. This thermal mass can be either thermal mass of the building structure or added in the form of phase-change material. Detailed monitoring of the installations is being undertaken as part of the European REVIVAL project.


The authors would like to thank the following for their contribution to the work: Corus, Senior Hargreaves, Stevenage Borough Council, Climator and the REVIVAL project team. More information on REVIVAL is available from the website below.

FaberMaunsell is at Marlborough House, Upper Marlborough Road, St Albans, Herts AL1 3UT.

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