Comfy, healthy homes hard to resist
Building a super energy-efficient Passive House requires careful planning, design and applied science. Will Harvie reports.
It’s a simple house. It’s a rectangle, with a pitched roof. It’s got three bedrooms, two levels and is relatively modest at 157 square metres. It sits on a quiet street in suburban Christchurch, surrounded by a mix of new earthquake rebuilds and older houses that have been repaired and a few still roughed up.
‘‘It’s just a house,’’ says Glenn Murdoch, design director at Theca Group. ‘‘There’s nothing special that you can see. The only thing [visible] is that the windows and doors are different.’’
But those differences are important.
All of the windows and doors are triple glazed. They’re 90 millimetres thick, compared to the 60mm or so of double-glazed conventional doors and windows. Triple glazing costs more, of course, but comes with significant advantages with regard to heat control.
In winter, power bills amount to less than $20 a month. In summer, the power bills are nil, he says. In part, the home doesn’t need power from the grid because of the solar panels on the northfacing side of the roof. In summer, they generate enough power to run the home and send some electricity back to the grid.
Getting this level of energy efficiency costs 5 per cent to 10 per cent more than a conventional build, Murdoch says. The owners of the Christchurch house – who wish to remain anonymous – spent about $675,000 to build their house.
‘‘It doesn’t cost more, you’re going to invest more,’’ Murdoch says with some salesmanship.
‘‘It’s an important distinction … everyone talks about the capital cost and nobody talks about the ongoing running costs.’’ It’s been estimated that lower power bills offset the increased mortgage costs month to month, leaving the owners ahead, he says.
This is a Passive House, a European-devised building standard that leads to ‘‘truly energy efficient, comfortable and affordable’’ housing, according to Passipedia website.
‘‘It’s a building performance standard, not a design guideline,’’ says Murdoch, who is an architectural designer.
‘‘Passive House specifies an outcome ... and the underlying principle is that it provides a comfortable and healthy building. Energy efficiency is a secondary benefit,’’ he says.
How the standard is achieved is up to the architects and engineers, although there are commonly seen elements across many Passive Houses in New Zealand, including triple glazing and an exacting thermal envelope.
Passives Houses are not zero energy or off-the-grid houses. There isn’t a composting toilet, although owners could have one if they wanted. There isn’t a heat sink or a battery, although owners could have those too. Woodburning fires, however, are hard to include.
So far, 22 New Zealand Passive Houses are listed on the PassivHaus website, 13 on the North Island and nine on the south. Others are under construction or not yet certified. And some owners chose not to certify. The websites lists almost
4300 passive buildings around the world, including schools, fire stations, nursing homes and factories.
As a rule, they have to meet the Passive House standard comfort level of about 20 degrees Celsius
365 days a year. In summer, the building cannot be warmer than
25C for any more than 10 per cent of the occupied time.
The standard for winter is expressed as a mathematical formula that is hard to display in print. But Murdoch estimates it’s about one-tenth the heat requirement for a typical Building Code-compliant house.
In addition to the triple glazing and photovoltaics, this is achieved through the thermal envelope. It comprises three layers – an airtight layer, an insulation layer, and a wind- and weathertight layer. In colder New Zealand climates, the insulation layer is usually thicker than in warmer parts of the country.
These layers must be installed with exacting standards to minimise the number of ‘‘air changes per hour’’ to less 0.6 per hour at 50 Pascal or, in other words, on a pretty windy day.
In a conventional houses, draughts and heat loss are most common around windows and doors and from corners, Murdoch says.
Those exacting installation standards mostly solve these leaks and can be tested with a blower door test, a technical measure of air tightness. But house design helps too, which is why the Christchurch house has a simple form that reduces the number of corners.
Ali Wilkinson and family have a more complicated Passive House. Their 100-year-old character villa in St Albans, Christchurch, was uneconomic to repair after the earthquakes and they wanted a contemporary home that looked like a villa. Only after the 250sqm design was complete did they decide to make it a Passive House.
The result cost 5 per cent to 10 per cent more than the conventional house and they had to top-up the insurance money to achieve the standard, she says.
But after living in a damaged old villa, the family notices how quiet and still the home is. As for the temperature, ‘‘I’m barefoot all the time’’, she says. Her hay fever symptoms are reduced.
When temperatures in the city soared above 30 degrees Celsius last summer, the Wilkinson home was 6C to 8C cooler than the neighbour’s, she says. They are in credit with their electricity supplier after the summer.
Even in extreme heat, Passive Houses designers must account for water vapour from outside sources and interior sources, such as kitchens and bathrooms. The Passive House standard doesn’t specify how these should be dealt with, but a common solution is a ventilator system that extracts vapour from the kitchen and bathrooms and expels it.
In conventional houses, vapour is removed with kitchen and bathroom extractor fans and through windows.
‘‘The evidence is clear that doesn’t work,’’ says Murdoch. ‘‘The evidence says we must mechanically ventilate.’’
Cleverly, the ventilators typically include heat exchange technology that captures heat from the kitchen and bathrooms and recirculates it around the house.
This helps with the ‘‘warm and comfortable’’ goal, while only requiring two small fans, Murdoch says. These ventilators have fallen in cost in recent years and now cost about $10,000 installed. Filters in the unit remove pollutants.
The biggest energy drain in the first Christchurch house is heating water. In other circumstances, solar could have been used but Murdoch ran out of space on the roof and installed a heat pump.
The first Christchurch house also achieved a higher standard, called Passive House Plus, which stipulates the amount of energy that must be created on site.
Murdoch, who ran into financial difficulty with a previous company building Passive Houses and is now employed by Theca, notes the design include shutters on north-facing windows. These put shade on windows on clear days and help control heat gain.
In conventional houses, curtains are often closed to achieve the same effect. But once the heat is through the glass, it’s too late, says Murdoch. The home won’t be warm and comfortable.
"Passive House specifies an outcome . . . and the underlying principle is that it provides a comfortable and healthy building." Glenn Murdoch, design director at Theca Group