Get Busy Around The Haus
1 February 2017
For an idea that’s time had supposedly come, it’s taken PassivHaus a long time to arrive.
Back in October 2011, then Energy Secretary Chris Huhne – remember him? – addressed the first ever UK PassivHaus conference, giving the initiative his unequivocal support. “I would like to see every new home in the UK reach the PassivHaus standard,” he told the audience back then.
“Saving energy is the cheapest way of closing the gap between supply and demand. And energy efficiency is at the heart of the Government’s programme...This principle is at the core of the PassivHaus ethos. In its modern application of ancient principles, the PassivHaus standard represents a watershed moment in our relationship with the built environment.”
Apart from some of that being a candidate for Private Eye’s Pseud’s Corner (“modern application of ancient principles”), the ‘watershed moment’ seemed something of an overstatement – a bit like Mr Huhne’s long defunct political career.
(His Wikipedia page tells us that he is now a management consultant specialising in sustainability, whose specific role is “"growing the business in the European Union". Timing or what, eh?).
And yet while PassivHaus undoubtedly has had its adherents, it’s always felt like a slow mover. However, in North America at least, possibly that ‘watershed moment’ has arrived – for last year, according to a recent report, the number of PassivHaus builds in the US quadrupled. Admittedly, the base was infinitesimally small – yet the rate of growth cannot be dismissed as a small number being replaced with a not-quite-so-small number.
For those unfamiliar: the PassivHaus concept is of “a building in which thermal comfort is guaranteed solely by re-heating (or re-cooling) the fresh air that is required for satisfactory air quality” and was first developed in 1988 by Professors Bo Adamson and Wolfgang Feist, founder of the PassivHaus Institute. Since the Institute was founded in 1996, around 25,000 PassivHaus buildings have been built worldwide, most of them in Germany and Austria, and been found to consume up to 90% less energy compared to existing building stock.
For a dwelling to qualify as a 'PassivHaus' it must meet strict criteria – the energy used for space heating and cooling must be less than 15 kWh/m2/yr and the total primary energy use is limited to 120 kWh/m2/yr (including all end-uses).
UPVC Windows and UPVC doors are a crucial component, typically accounting for about 43% of the total heat gains but as much as 50% of the fabric losses. This means that U-values for windows and doors need to be 0.8 W/m2K or less (for both the frame and glazing), so the window frame must incorporate insulation and triple glazing.
This U-value is not only based on the energy balance for the building but also on comfort. In a conventional house there is usually a radiator heater below the window but not in a PassivHaus, where the heating is ambient, so the temperature on the window surface must always be above 17°C. Otherwise cold air will fall, creating a cold air spot on the floor that is not only uncomfortable but also creates a real risk of mould.
A point emphasised by many specifiers and architects using PassivHaus standards is the importance of using certified components, particularly with doors and windows. The reason for this is that the whole window U-value has to be calculated in accordance with EN 10077 and includes the frame, the spacer bar and the glazing.
Also, when designing a PassivHaus using the PassivHaus Planning Package developed by the Institute, the thermal performance of each window component must also be considered but this data is not readily available from most manufacturers and suppliers. Using certified windows and doors ensures that the manufacturers have this information available and that it has been verified.
There is no doubt that it is possible to design certified PassivHaus buildings without using certified windows, say architects, but in practice the extra work it causes wipes out any savings that might be made.
The physical composition and installation of a window are not the only considerations in a PassivHaus – size, orientation and solar transmittance are important too because the windows effectively constitute the building’s primary heating system. Too little south-facing glazing and the shortfall in heat demand cannot be met by the ventilation system. Too much and the building is at risk of overheating. For this reason, many architects building PassivHaus buildings are unhappy to specify rooflights because they are always positioned outside the thermal insulation and tend to maximise heat loss on cold days and solar gains in summer.
But there are strong incentives to pay this attention to detail as Chris Huhne promised his audience all those years ago.
It’s been a long time coming, yet will be fascinating to see if that US growth trend in PassivHaus builds – they had 13 buildings in 2010; and built 2000 last year – continues exponentially; not least as the UK energy efficient window sector is not only well-placed to reap the gains from such a trend here; but as a key advisor as to how its aspirational standards can be cost-effectively met.