Buildings designed to the Passivhaus standard are wonderfully sustainable on day one, but ignoring embodied energy distorts overall performance
The importance of the basic principles of Passivhaus design are now well known. A key feature is the interlocking ideas of ‘super’ insulation, thermal bridge free construction, ventilation and heat recovery, window design and of course, crucially air tightness. For this last item to be truly effective a variety of sophisticated tapes are often used to deal with air infiltration.
My questions are these: This is all great on day one, but what happens over 40-50 years? How fast do these tapes degrade, with the resulting ‘falling off’ of airtightness, and hence energy efficiency? How do successive owners say, 40+ years after PC know where to find the tapes? Does their replacement involve significant damage to internal fabric and finishes to maintain the Passivhaus standard? Does accessing and replacing these tapes therefore become a major embodied energy cost? My concern is that an outstanding product evolves over time into something that has an energy expensive future retrofit regime. In short what is the ‘whole life’ energy cost of a Passivhaus over 60 or more years?
My concern is that an outstanding product evolves over time into something that has an energy expensive future retrofit regime.
In a previous study for Building we established that the retrofit of a Victorian Terrace House to Enerfit Standard had the same whole life carbon footprint over 60 years as a new terrace Passivhaus. Obviously the Passivhaus outperformed the retrofit on an annual energy consumption basis, but it had much higher embodied energy costs in relation to build and materials replacement. The message is: ignoring embodied energy distorts the overall performance picture.
A parallel thought is to do with an owner’s requirement to adapt or extend his Passivhaus. If someone wishes to build an extension or put in a new window, do they have to have specialist (ie expensive) consultants and contractors, or can the local builder do it? Do Passivhaus projects therefore require detailed operating and maintenance manuals?
Passivhaus design concentrates on reducing operational emissions, but at potentially unacceptable embodied costs. Taking a ‘whole life’ view, ie considering operational and embodied emissions together would produce an overall lower carbon outcome. This may mean relaxing Passivhaus standards slightly, but with the benefit of a more flexible and maintainable building. This in turn would produce the lowest lifetime carbon footprint for Passivhaus, not just the best operational footprint at the outset.
Simon Sturgis is managing director of Sturgis Carbon Profiling