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Passive houses have long been considered the ultimate in low energy
buildings. So when it comes to BERs, why don’t they always get a
straight A? Lenny Antonelli investigates.
With awareness of the carbon footprint of buildings higher than ever, the passive house has been heralded as the future of high performance, low energy buildings. The European Parliament has proposed that all buildings in the EU be passive from 2011, while it’s anticipated that the planned 60% energy reductions in 2010’s revision to Part L of the Building Regulations will come close to passive standards.
However, a recent analysis carried out on Lars Pettersson’s detached passive house in Moycullen, County Galway using the Dwelling Energy Assessment Procedure (DEAP) - the standard test for assigning building energy ratings - awarded the building a B1-rating (82kWh/m2/y). Pettersson is the founder of Scandinavian Homes Ltd. , which started building prefabricated timber frame houses in Ireland in 1991, and has offered passive houses since 2004.
The rating is disappointing considering the building’s impressive performance. Passive houses have no need for traditional heating systems, instead relying typically on optimising orientation, generous insulation, air-tightness, and heat generated by occupants and appliances to provide heat, though small back-up heating systems are usually installed. Further to earlier work conducted by Peter Keavney of Galway Energy Agency for Scandinavian homes using Heat Energy Rating, Ronan Rogers of building energy consultancy Energy Matters provided an interpretation of passive house design using DEAP, which led to Energy Matters performing a series of air-tightness and thermography tests on the house, and producing a detailed study of the ways in which it, like all passive houses, is at a potential disadvantage under DEAP and the new building regulations (TGD L 2007).
Tests carried out by staff at Energy Matters helped to identify and correct points of air leakage. Sealing measures were applied to the attic hatch and lock, doors were adjusted, and the gable walls of the roof were opened from the inside and a vapour barrier was resealed, reducing air leakage from the building by 21 per cent. Energy Matters’ study stresses the “need to perform this type of testing on these high performing buildings to fine tune the performance once constructed.”
Blower door tests then verified the impressive air-tightness of the house, with an air permeability of 1.17m3/h/m2 at 50 pascals (or 0.88 air changes per hour) – significantly better than the UK ‘best practice’ standard of 3m3/h/m2, though just short of meeting the Passivhaus Institut certification standard of 0.6 air changes per hour.
However, the delivered energy for space heating in Pettersson’s house is 10.2kWh/m2/year, almost a third better than the 15kWh/m2/y certification standard, while Pettersson says the house’s constant heating load is just 3.2 W/m2. The low heating demand is particularly impressive considering the house is only occupied on a part-time-basis but temperatures are measured to ensure the house constantly meets the passive house standard in terms of comfort. In other words, the house maintains the right interior temperature but does so without the benefit of heat generated by full time occupants and their use of appliances, and therefore must rely on passive solar gains and a small heating system to achieve the right temperatures. The Energy Matters study adds: “Given the high performing nature of this passive house in Moycullen, combined with relatively mild Irish climatic conditions, theoretically the annual heating requirement for this house must be one of the lowest in Europe.”
So how could such a low energy building fail to achieve an A-rating? “DEAP may not be the most appropriate tool to accurately assess a high performing passive house,” the study says. Why not?
One reason is that, under DEAP, every building is presumed to have a
primary heating system, though the unit installed in passive houses is
typically a little-used back-up device. Energy Matters’ study says
that, “(Pettersson’s) house is fitted with a heating element on the
heat recovery ventilation system, as a back-up measure, which is
standard in most passive houses in Europe. Although the heating element
is never used, the assessor is required to include it…as the primary
heating source.” Additionally, as the admittedly little used heating
system is electrically powered, it is penalised by DEAP, which marks a
home’s electricity usage on the basis of primary energy consumption–
thereby including substantial power generation and transmission losses.
According to DEAP, every kilowatt of electricity used for heating or
lighting requires 2.7 kilowatts of energy to be inputted at the
generation site.
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