PASSIVE
New low carbon buildings are often designed to have minimal heat loss, but real performance is often disappointing. Sally Godber looks at some key Passivhaus principles that can help bridge this gap
s building services engineers, we should be able to predict the variables in heating loads accurately. Almost all of the
calculations involved in this process are in our hands. And yet, it seems, we often fail to achieve this basic function. A well- publicised Joseph Roundtree Foundation report on the heat losses of dwellings shows just how wide the gap is between the original design assumptions and the actual performance of buildings (see Figure 1). This discrepancy was exactly what interested Wolfgang Feist, the co-creator of Passivhaus, 20 years ago. He wrote: ‘I was working as a physicist. I read that the construction industry had experimented
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with adding insulation to new buildings and that energy consumption had failed to reduce. This offended me – it was counter to the basic laws of physics. I knew that they must be doing something wrong. So I made it my mission to find out what, and to establish what was needed to do it right.’ The major underlying cause of the
‘reality gap’ is a lack of appreciation of the less obvious heat loss mechanisms. This article looks at some key Passivhaus design principles and gives examples of where the building may fail to live up to expectations.
Modelling Heat demand is calculated as the balance of heat losses and gains. The two charts in Figure 2 show typical annual energy values per square metre of floor area for housing in the 1980s, and for a modern low energy building. The balance of losses and gains gives the annual heating demand – shown in yellow. The two charts demonstrate how the dominating factors have altered over the past 30 years. Previously, the fabric and infiltration losses dominated. Now solar gains, internal gains, window losses and thermal bridges have become much more critical. This is why the Passivhaus design principles have been developed with a much more stringent interrogation of these factors. This means that, for modern buildings, Passivhaus should provide a more accurate estimate of the heating requirement, a fact borne out in the European CEPHUS study (www.
passivehouse.com/07_eng/news/ CEPHEUS_final_long.pdf).
Powys County Council’s new IT and adult learning centre, which opened in 2009, was the UK’s first Passivhaus Institut- certified office building, and underwent extensive performance monitoring to compare design intentions against outcomes. It won the New Build category in the CIBSE Building Performance Awards 2011 (see the Journal, April 2011, page 34)
Correct U-value assessment Manufacturers should not be relied upon to calculate opaque U-values. Instead, this
34 CIBSE Journal November 2011
www.cibsejournal.com
JPW Associates
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