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In the heat of the moment

Adding thermal mass in the form of phase change materials could help provide comfortable learning environments, says Tony Heslop

Posted by Dave Higgitt | April 17, 2015 | Interiors

Lightweight construction techniques such as timber frame, structural insulated panels (SIPS) systems or prefabricated modular construction solutions are often used to meet a challenging construction programme. Schools built using lightweight or modular construction techniques, however, have very little inherent thermal mass. This characteristic means that the buildings do not offer the same resistance to temperature rises that heavyweight construction does.

Now new construction materials incorporating affordable, safe phase change materials are giving designers the opportunity to build using lightweight construction techniques whilst enjoying the performance benefits of thermal mass without the weight associated with conventional materials.

The advantage of using lightweight and prefabricated construction techniques are that they save project time by allowing the bulk of a school to be built in an off-site factory and then delivered to site by lorry. Other reasons for its popularity are: 

• fewer materials are, generally, consumed in the manufacture of lightweight modular constructed buildings compared to a more traditionally constructed building

• the construction process is factory-based and therefore more controlled so the components will be of a higher quality with fewer defects

• because the construction process is more controlled, the volume of construction waste is also minimised while any waste that is produced is more likely to be recycled 

While lightweight construction does offer some clear school-building programme advantages a lack of thermal mass can make these schools prone to overheating in summer, particularly if they have been designed with large areas of glazing and/or a highly efficient airtight construction. The impact of summer overheating is set to increase as the climate continues to warm, particularly for high-occupancy spaces such as classrooms.

Sustainability consultant Bill Gething recognised the overheating challenge in his report ‘Design for Future Climate’ for the Technology Strategy Board. In it, he said: “Of all the projected climate change impacts, hotter summers will affect the design of buildings the most.”

Gething, however, also highlighted the scope for  innovative technologies to help meet the challenges of designing buildings for a changing summer climate. In particular, he acknowledged the potential for “affordable, safe, phase change materials to provide the performance of thermal mass without the weight of associated with conventional materials”. Now, five years on, phase change material (PCM) technology is being adopted to help schools comply with new design guidance, under the government’s priority school building programme (PSBP).

Launched in 2011, the PSBP is the successor to the building schools for the future programme. In February the government named 277 schools that would receive a share of a £2billion funding pot to refurbish or rebuild school buildings under the second phase of the programme. This followed phase one which, in May 2012, awarded funding to 260 schools. The PSBP heralds a new approach to school design with draught-free ventilation now mandatory, an expectation that passive cooling using thermal mass will be included in school design and the need to comply with a facilities output specification which includes onerous criteria to limit overheating in schools.

The PSBP places a greater emphasis on the comfort of school occupants. It introduces the concept of operative temperature as a way of defining occupants’ comfort. Operative temperature is defined as a function of air temperature, humidity, air speed and the temperatures of the surfaces to which the occupants are exposed. The PSBP also introduces the concept of a variable maximum internal air temperature, termed the running mean temperature, which is based on outside air temperatures recorded over the previous seven days.

Designers have to use the internal temperature and the running mean temperature to prove a design’s compliance with a new set of overheating criteria. These define: 

• the number of hours the maximum internal temperature can be exceeded in a year

• the number of hours a day above an allowable maximum temperature, multiplied by the number of degrees above the allowable maximum

• an absolute maximum value of 4 degrees centigrade for the indoor operative temperature above the running mean temperature 

A design must pass two out of the three criteria.

For schools built using lightweight construction, an effective way to maintain internal comfort in classrooms in summer is to add exposed thermal mass in the form of PCMs to the internal room surfaces.

Weight for weight, PCMs can absorb far greater amounts of thermal energy than an equivalent conventional material as a consequence of their ability to change phase at room temperature, usually from a solid to a liquid and back again. BASF’s Micronal PCM, for example, comprises thousands of microscopically small polymer encapsulated spheres, each with a wax core developed to make use of latent heat storage. Since melting and solidifying happens inside each microcapsule, the material remains solid even if heat is being absorbed or released. This feature has enabled BASF’s Micronal PCM to be incorporated into various construction products such as plasterboard (Knauf’s comfortboard, for example) and in ceiling tiles.

Armstrong’s Ceiling Systems’ CoolZone tiles incorporate BASF’s Micronal PCM. For this application the BASF developed the Micronal to change phase as the room air temperature approaches 23 degrees. As it changes phase the wax absorbs heat from the room. This process continues until all of the wax has changed phase. Night ventilation can then be used to remove this absorbed heat. As the room temperature drops below 23 degrees, the PCM will begin to return to its solid state, releasing its accumulated heat.

The effectiveness of adding PCMs to a school built using lightweight construction was tested in two classrooms at Belvoir High School, Bottesford, Nottingham. This school was constructed, pre-BSBP, in 2009 to meet the requirements of Building Bulletin 101. The classrooms featured Breathing Buildings’ R-Series e-stack through-the-roof ventilation units, sized to meet summertime overheating requirements and minimum daily average ventilation demands to limit CO2 levels. Both rooms had opening windows and both had a conventional suspended ceiling supporting 600x600mm ceiling tiles on a metal grid. In one of the classrooms, about half of the conventional ceiling tiles were replaced by Armstrong’s CoolZone PCM tiles. The PCM tiles covered area of approximately 20 sqm.

Armstrong worked with natural ventilation specialists Breathing Buildings to monitor the two classrooms. When the heat equivalent to that of the occupants and solar gains were added during the day, the room with BASF’s Micronal PCM embedded in the ceiling tiles was found generally to be 1-2 degrees cooler than the room without the PCM. While this might sound like a small temperature reduction, it meant that the room with the CoolZone tile would have passed the new PSBP output specification criteria, while the room without the addition of the PCM’s would fail. Shaun Fitzgerald, chief executive of Breathing Buildings, says “This is about being comfortable, and a one-degree saving could be a tipping point, especially in a south-facing classroom.”

Tony Heslop is regional market development manager for BASF

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