Making the most of thermal mass Buildings must be designed carefully to get all the advantages of a high thermal mass, a new study from the BRE shows
Many building designs now use night cooling in conjunction with thermal mass for passive cooling during the summer. However, the effect of the thermal mass affects building performance throughout the year. Particularly important is a significant penalty of increased space-heating demand that can result from increasing the thermal mass of a building. There are also issues of integrating building services, for example, to achieve suitable lighting levels and acoustic performance. The importance of year-round performance was a key finding of a research project on the effective incorporation of thermal mass. The project was carried out by bre, Oscar Faber and Derrick Braham Associates.
The research identified a number of issues that should be considered by architects when they design a building with thermal response in mind.
The research analysed a five-storey, naturally ventilated, open-plan office of approximately 3000m2 to compare summer cooling with winter heating demand. Three levels of thermal mass were characterised as follows:
Light: false ceiling, false floor, lightweight walls and partitions
Medium: exposed lightweight concrete soffit, false floor, medium- weight walls and partitions
Heavy: exposed heavyweight concrete soffit, false floor, heavyweight walls and partitions.
The use of night cooling in conjunction with thermal mass to eliminate or reduce the need for refrigeration has been well documented. In summer, air is circulated through the building at night, thus cooling the building fabric. This stored cooling is then available the next day to offset heat gains. The graph below shows the benefits available from increasing levels of thermal mass in terms of a reduction in overheating based on a simulation exercise carried out within the study.
Winter heating demand
However, the results (bottom graph) indicate that there is an associated net annual space-heating demand penalty of 10-20 per cent that accompanies these benefits. The graph shows significant increases during the winter months. This is caused by the slab being cooled by conduction and infiltration heat losses at night (unwanted night cooling) which must then be overcome by the heating system. An alternative way to view this is in terms of the internal temperatures. The graph above gives typical heating curves for lightweight and heavyweight buildings. The heavyweight building has a higher internal temperature at night and so heat losses (proportional to the internal/external temperature differential) will be greater.
A number of parameters will influence the magnitude of the heating demand penalty. Poor insulation and airtightness will increase losses and exacerbate the problem. This may be a particular concern for retrofitting older buildings that may have low levels of insulation and be relatively leaky. Heat gains will influence the length of the heating season and, in turn, the amount of 'unwanted' night cooling. Thus, the daily operating period determines the night period during which the losses occur.
In contrast, there is a reduction in heating demand in summer due to higher levels of thermal mass. This comes about because excess heat from internal gains can be stored more effectively by heavier constructions - a form of heat recovery. With lighter constructions, excess heat tends to be lost to the outside.
Day and night ventilation can be achieved either by natural or mechanical means, or with a mixture of these. With natural ventilation, night ventilation needs particular consideration and issues related to automated vent opening and security. As noted above, the control of unwanted ventilation in winter is of increased importance in high-mass buildings. Openable windows should be well sealed when closed to minimise infiltration losses. Trickle ventilators may be used in winter to provide a minimum fresh-air rate.
Mechanical ventilation will incur energy costs for fans but may offer advantages in other areas. On the heating demand side, ventilation heat recovery can be implemented and building airtightness more readily achievable. With mechanical ventilation there is also the opportunity to use solutions which achieve a high level of performance by improving the air/mass heat transfer.
Another key issue is the design of the thermal mass, in particular determining the optimum thickness and achieving good thermal access. The optimum thickness of concrete storage elements is typically quoted as 50-100mm, but this can vary depending on:
the air/surface heat transfer
the thermal properties (capacity and conductivity) of the material itself.
The rate of air/surface heat transfer will be a function of area and rate of heat transfer from the surface. There are several variations for basic system types (see box, overleaf). The heat capacity of a material will determine the thickness required to store the available thermal energy efficiently. Relatively high thermal conductivity is also necessary so that thermal energy can flow through and use the whole stored energy in a 24-hour charge and discharge period. Admittance values provide a simple measure of dynamic storage performance for difference construction elements. The values equate to the rate at which an element can absorb heat from the air assuming a 24-hour sinusoidal variation in air temperature.
Basic system types
For an exposed surface, heat transfer is by a combination of radiation and natural convection. Typical values for exposed plane surfaces are 5W/m2K and 2-3W/m2K respectively. High emissivity is needed to achieve good radiant heat transfer. Improvements in surface heat transfer can be achieved by increasing surface area through the formation of coffers or profiling the surface.
Partial thermal exposure of a slab surface can be achieved by using open- cell or perforated ceiling tiles. This permits air to circulate between the ceiling void and space below making direct use of the convective surface heat transfer. Research by others has suggested that percentage open areas as low as 15 per cent of the building could be sufficient to allow significant air circulation. In addition, open-cell ceilings with a high reflectance may permit a significant level of radiant heat exchange between the slab above and the space below.
The use of a solid false ceiling will obviously limit heat transfer by effectively insulating the slab from the space below. However, a significant level of heat transfer may still be attainable if the ceiling itself is made of a conductive rather than insulating material. Surface finishes such as carpets can also insulate the slab from the space below or above, although thin layers of relatively conductive materials such as plaster should not have a significant effect.
Mechanical ventilation can be used to pass air through floor voids, cores or air paths to exchange heat with the store before entering the space. The main surface heat transfer mechanism is convective heat transfer between the air and the store. If convective heat transfer is poor, as is normally the case for air flow in floor voids, performance can be limited. Convective heat transfer coefficients may be increased by using mechanical means to create forced convective heat transfer, rather than relying on natural buoyancy forces.
High rates of forced convective heat transfer (10-15W/m2K and upwards) can readily be affected by creating highly turbulent air flow at the surface. This can be achieved by blowing air through hollow cores in slabs or creating air paths through which air can be blown across the surface of a slab. Air flow through cores/paths may be modulated to control when the thermal mass is linked to the ventilation system and hence vary the effective thermal mass of the building to suit conditions (heavyweight in the summer, lightweight in winter).
Exposing a concrete soffit to take advantage of its thermal mass in place of a false ceiling can have significant design implications in terms of acoustics and integration of services, in particular lighting. The absence of a suspended ceiling can give rise to increased reverberation time and increased reflected sound across an open-plan space. Measures available to overcome these problems include acoustically absorbent partitions, absorbent banners hung from the ceiling, acoustic plaster, integration of acoustic elements at high level with lighting and profiled slabs to reduce propagation. Sculpted coffers can be designed to focus sound onto acoustic absorbers located in suspended light fittings or back on its source, or below carpet level. However, note that such measures can have a detrimental impact on access to the thermal mass which must be taken into account when assessing its performance.
Integration of lighting
Where suspended ceilings are provided, modular light fittings can easily be integrated. More careful consideration is required where the soffit is exposed, although it may be possible to integrate the lighting within the coffers. Other options identified by the Steel Construction Institute report, Environmental Floor Systems, include pendant sys- tems, floor- or furniture-mounted uplighters, cornice and slab recessed. For uplighting, the soffit form is highlighted as an important consideration, together with a high surface reflectance of at least 70-80 per cent and a gloss factor of no more than 10 per cent (otherwise lamp images will be visible). Perforations in the light fitting can be used with downlighters to avoid a cavernous coffers effect.
A bre report, due out later this year and incorporating this research, will highlight key architectural and engineering issues to help designers maximise the potential and avoid the pitfalls of using thermal mass.
Nick Barnard is associate director of environmental engineer Oscar Faber. This project was funded by the detr's Construction Research Programme