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Post-occupancy evaluation of five schools by Architype

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Architype’s Chryssa Thoua and Jonathan Hines on the environmental monitoring of five of their practice’s schools and how their Passivhaus buildings compare with previous designs

In the UK it has become accepted that there is a significant gap between the predicted and actual performance of buildings. Architype has always been keen to understand how our completed buildings are performing so we can improve the design and performance of future buildings and eliminate any potential performance gap. In 2010 we began a two-year programme in partnership with Oxford Brookes University to monitor the energy consumption, environmental performance and user satisfaction at 10 of our buildings. This study highlighted a number of areas – particularly internal environmental performance – that we believed would be addressed by improved design and by adopting the Passivhaus standard.

Architype completed its first Passivhaus schools in 2011 and our first-generation Passivhaus schools were monitored during the first year of occupation. While they generally performed well, we learned a number of important lessons, which we then applied to improve the design of our second-generation Passivhaus schools, the first of which was completed in 2013. In 2014 we secured EU funding for a Knowledge Exchange and Enterprise Partnership (KEEN) with Coventry and Wolverhampton universities, enabling us to undertake further monitoring.

CO2 levels were dramatically improved in the Passivhaus buildings with MVHR, especially in the  winter 

Over 12 months we monitored five of our schools: two naturally ventilated non-Passivhaus schools completed in 2009 and 2010, both designed to a high environmental standard and typical of most new UK schools in being naturally ventilated in both winter and summer; and three Passivhaus schools – Oak Meadow and Bushbury Hill Primary schools, completed in 2011, and Wilkinson Primary school, completed in 2014. To further the comparison, a sixth school was also monitored: an unmodernised 1970s school building, typical of many in the UK.

Environmental factors monitored

Environmental conditions were monitored during all seasons over the full 12-month period within four control classrooms in each of the six schools.

A range of environmental conditions was continuously logged: temperatures, using two methods – air temperature thermometers and black globe thermometers; CO2 concentration levels in parts per million (ppm); and relative humidity. The operation of blinds and windows was monitored in selected classrooms using time-lapse cameras; and extensive user surveys were undertaken of both staff and children.

While all factors are important, CO2 concentration is of particular interest to schools, with a number of studies and guidelines recognising that it provides a good approximation of indoor air quality (IAQ) in learning spaces. High CO2 levels in primary school classrooms can have a negative impact on academic performance and long exposure can affect health and development. A maximum average CO2 concentration of 1,000ppm is currently required in classrooms to prevent CO2  having a negative impact on pupils’ performance.  

For many years the UK standards for schools (set out in Building Bulletin 101: ventilation for school buildings) have required that: CO2 concentration should never exceed 5,000ppm; the average CO2 concentration during occupied hours should not exceed 1,500ppm; and that occupants should be able to reduce CO2 levels to 1,000ppm at any occupied instance. In 2013 the standard was tightened so that the average CO2 should be 1,000ppm, with 1,500ppm not being exceeded for more than 20 minutes a day.

Key environmental findings

Figure 1: Seasonal monitored CO2 levels comparing non-Passivhaus and Passivhaus schools

Figure 1: Seasonal monitored CO2 levels comparing non-Passivhaus and Passivhaus schools

The findings of the study are unequivocal. Both temperature and CO2 levels were better during all seasons in the Passivhaus buildings using mechanical ventilation with heat recovery (MVHR) than in the non-Passivhaus buildings that relied on natural ventilation.

In particular, CO2 levels were dramatically improved in the Passivhaus buildings fitted with mechanical ventilation with MVHR, especially in the more critical winter season (see Figures 1, 2 and 3).

Figure 2: Seasonal CO2 levels during occupied hours in a pre-Passivhaus naturally-ventilated school

Figure 2: Seasonal CO2 levels during occupied hours in a pre-Passivhaus naturally-ventilated school

Figure 3: Seasonal CO2 levels during occupied hours in a Passivhaus MVHR school

Figure 3: Seasonal CO2 levels during occupied hours in a Passivhaus MVHR school

We were able to further optimise performance in our second-generation Passivhaus schools, with improved summertime temperatures (see Figure 4). This was achieved by more accurate calculation of internal heat gains, which enabled a modest reduction in south-facing glazing. This was achieved by raising glazing sills from ground to 800mm, which reduced unwanted solar gain and made classrooms more practical to use, while not affecting daylighting.

Other lessons learned from monitoring the first-generation schools included simplifying control systems and heating water locally by electricity – rather than centrally by gas.

Using these measures of temperature and CO2 levels, we are able to confidently conclude that Passivhaus schools provide significantly better levels of internal environmental comfort than non-Passivhaus schools, and that monitoring can be used to further improve performance.

Figure 4: Air temperature levels in non-Passivhaus and Passivhaus schools

Figure 4: Air temperature levels in non-Passivhaus and Passivhaus schools

Monitored energy consumption

In parallel with monitoring environmental conditions, energy consumption was also monitored. Figure 5 illustrates delivered energy consumption in the three Passivhaus schools and the two non-Passivhaus BREEAM schools compared with UK good practice benchmarks set by the Chartered Institute of Building Services Engineers. The two BREEAM Schools use gas, biomass and electricity; the three Passivhaus schools use gas and electricity only. The first-generation Passivhaus schools, Oakmeadow and Bushbury Hill, heat water centrally by gas; the second generation Passivhaus school, Wilkinson, heats water electrically near to point of use. This proved more efficient and cuts unwanted internal heat gains in summer.

Total delivered energy consumption

Total delivered energy consumption

Conclusion

Our study has proved conclusively that Passivhaus schools with MVHR achieve a better level of internal comfort than non-Passivhaus schools. Key measures of temperature and CO2 levels are both significantly improved in all seasons, while annual energy consumption is also significantly reduced.

It is only by learning such lessons through rigorous monitoring that the design of buildings can be improved and the performance gap eliminated. After many years of such monitoring, we are now working towards our 2020 target of being able to guarantee the performance of our buildings.

Engineer’s view

Architype is to be commended for undertaking this study. We all need to better engage with, and understand, the performance of our buildings so we can learn how best to improve them.

The essential ingredient for achieving Passivhaus certification is a great attention to detail – detail that needs to be carefully considered by the designers and highly scrutinised on site. Combined with a rigorous testing regime, this results in a high quality of workmanship. It’s not surprising that this approach results in a building that performs closer to what was intended.

Architype says its study has ‘proved conclusively that Passivhaus schools with MVHR achieve a better level of internal comfort than non-Passivhaus schools’. I would question the certitude of that statement. No doubt the Passivhaus schools examined for this study performed better than the non-Passivhaus schools, but there are many ways to approach natural ventilation and some are more effective than others. In many cases, the way the school is used and managed can influence the effectiveness of natural ventilation.

An observation from the study is that the air quality between the two building design methods is similar in the summer, but the Passivhaus schools had superior air quality in winter. This suggests that the background ventilation is not so effective in the naturally ventilated schools – it is possible, for example, that teachers are reluctant to open windows in winter because of perceived losses of energy efficiency, or to avoid cold draughts.

Detractors of naturally ventilated buildings often assume that summertime poses the greatest challenge, but as Architype’s study indicates, it is winter where the greater challenges exist. It is possible, however, that some of these issues can be designed out.

It is certainly interesting to see that the electricity consumption in the Passivhaus schools is comparable with the naturally ventilated schools, indicating that MVHR’s energy consumption is not a significant burden.

As a practice, Architype is at the forefront of sustainable architecture. Its ambition to close the performance gap and its committed pursuit of that aim serves as an exemplar to the industry.

Tamsin Tweddell, senior partner, Max Fordham

 

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