Paola Sassi: Home is where the hearth isn't

You don't expect to find a contemporary, not to say minimalist, eco-house tucked into a quiet cul-de-sac of Victorian terraces in the Roath district of Cardiff. But that's only half the story because Paola Sassi has built her house, comprising two one-bedroom flats, to the German Passivhaus standard, which requires no central heating. In fact, no heating at all. On the mid-January day when I visited, the outside air temperature was just 7C, so it would not have been unreasonable to expect a chilly welcome. But the internal temperature was a comfortable 19C. And there was no heating – no central heating, no fireplace, no wood burning stove, no electric fan heaters, nothing.

The Passivhaus standard was established by the Passivhaus Institute in Darmstadt, Germany in 1996. Since then about 6,000 houses have been built and certified to the standard across Europe and the US. What Passivhaus means, in broad terms, is that the building is insulated to a level that allows the sun and other passive heat gains to produce enough energy to heat the home. Passive gain is the heat from daily activity, given off by people cooking, using the shower, making toast and boiling the kettle. Pretty much everything we do produces heat that can be captured and circulated from warmer rooms (bathroom and kitchen) to cooler rooms (lounge and bedroom) by a heat recovery and ventilation system.

Sassi has gone to some trouble to ensure that the sun warms the house with big triple glazed windows to the south elevation and solar panels on the roof. These produce most of the hot water she needs for washing, showers, laundry etc. and about half the electricity she needs. There are dual-flush toilets, flow-regulated showers and taps and a rainwater harvesting system which meets about half the water needs of the house. "I needed somewhere to live and could have bought a house cheaper," Sassi says. "But I wanted to prove that it is possible to build a house in a city centre that is sustainable, comfortable and cheap to run."

Sassi, a professional architect, was born in Turin and educated in Munich. She worked for 15 years in London at two architectural practices before taking up a post at Cardiff University's School of Architecture. It was working in London that informed her decision to focus on sustainable buildings. While working on large commercial projects, she became disenchanted with the levels of waste in the construction and use of the buildings. "It was our arrogant attitude that we can do whatever we want that struck me," says Sassi.

She decided that her house in Cardiff would be built entirely from sustainable materials and that every part of it, from the floor covering to the zinc roof, would be reusable or recyclable. She used hemp insulation throughout. Hemp has very similar thermal properties to its non-sustainable mineral wool alternatives, but has three significant advantages: it uses less energy in manufacture; it "locks in" the CO2 that the plant absorbed as it was growing; and it is entirely recyclable. It has been installed in such a way that when the house is finally demolished, it can just be taken out and used again in another house.

The internal walls are lined principally in plywood sheets from sustainable timber, coated with a natural wax. This has the twin affect of allowing the ply to be completely recyclable and giving a warmer feel to the house. Brick, plaster or stone walls feel cold to the touch, whatever the air temperature, which makes a room feel cooler. The highly insulated plywood walls are what make a 19C internal temperature comfortable when 21C or even 24C would be more usual.

A house that needs no heating needs a lot of insulation. Sassi has installed 380mm in the walls, 200mm under the floor and 430mm in the roof. This compares to the normal UK standard of 90mm in the walls, 75mm under the floor and 270mm in the roof. In also needs a very high level of air-tightness to prevent heat losses from air movement, and that is where Sassi's contractors encountered most of their problems.

Building air-tight houses needs precision and the typical British builder is not used to building houses in this way and to this standard. Sassi's contractors were forced to learn on the job and this led to a longer build time than expected. It also led to mistakes, and the extra cost involved in rectifying them.

"It was very difficult to persuade people to build as I wanted them to," Sassi says. "They wanted to do things their way." This meant that she had to work hard to achieve her goals, and did not always succeed. The ground-works contractor was instructed to keep any concrete from demolition work on site to be re-used on the new front drive. But, much to Sassi's annoyance, he had it taken away to landfill.

Sassi's is a timber-frame house, with all the timber sourced from sustainable forests. A typical UK timber-frame will use 100mm or maybe 140mm timbers. Sassi wanted 200mm and the manufacturer who was initially given the contract pulled out. A second supplier then had to be found, and a way of manufacturing, delivering and erecting the frames decided on. All of this added delays and cost to the project, so much so that Sassi and her partner finally decided to take up tools and do the internal finishing work themselves.

Was it worth it? Sassi is, quite rightly, proud of her achievement. "I knew going in that it would be tough, and really I took on too much", she says. "But the house I have built was worth it."

The project should have taken nine months but actually took 14. It should have come in at £1,200 per sq m but came in at £1,400. Expensive perhaps, but not as expensive as might be expected. The finished house is comfortable, inviting and worth at least what it cost to build. With a fair wind it could even realise a small profit.

Her advice to anyone contemplating a similar project is to stay hands-off. "I did too much. I was architect, I sourced all the materials, selected and engaged the contractors, did the project management and even went on the tools. It is too much." Kevin McCloud said that building your own house is the last great adventure. Sassi says: "Building a house like this is rewarding. It is sunny, warm, a comfortable living and working space. It costs very little to run, and I built it."

The Passivhaus standard has been around for about 12 years and it is likely to be the way we will build all houses in the future. The Government has called for all new houses in England to be "zero carbon" from 2016 (2011 in Wales) and has introduced the Code for Sustainable Homes.

Sassi's was a personal project. However, Kingspan, the leading UK insulation manufacturer, have developed something similar with their Lighthouse, a zero carbon, 2-bedroom home, the first to meet the level six requirements of the Code for Sustainable Homes. So now it seems that there is no reason why what Sassi has done could not be taken up by the big house-building companies and made profitable – for themselves and the planet.