by Jakob Schoof | Photography by Torben Eskerod

In the small town of Montfoort, the first ten of millions of Dutch terraced houses have been converted into Active Houses. Their roofs harness the sun’s energy in three ways: to generate power, to supply heat, and as a source of light that significantly enhances the comfortable living conditions in these spacious but very deep houses.

Between 1945 and 1975, 2.5 million houses were built in the Netherlands, about 1.3 million of which were terraced houses. Many of these are now waiting their turn to be renovated, fit for the 21st century. For the houses at 29–47 Poorterstraat in Montfoort, this future has already begun. In the last few months they have been converted into the first ‘active houses’ in the country, and now achieve an A++ energy label for energy performance. The term ‘active house’ (See indicates that the houses use only a fraction of the energy that new-build housing in the Netherlands would normally consume. Furthermore, and possibly even more important, there are high levels of daylight, fresh air, a good indoor climate, and healthy materials. The ten active houses are part of a larger estate of terraced houses in which the same owner, the GroenWest housing association, has recently renovated a further 82 houses. The principal difference between the buildings in Poorterstraat and their neighbours is the extended top floor, literally every square centimeter of which makes use of the light of the sun.

A prism for more daylight –the conversion concept

The architects’ strategy for the conversion was to open up the centre of the houses, and bring in daylight from above. Previously, the attic had served simply as a storage space; accessible only via folding stairs. Now it has been significantly enlarged and turned into a living area, with a new permanent staircase leading up from the first floor. The increase in space is also visible from the exterior, most noticeably at the gables of the houses. The architects have added a prismatic element covered with large expanses of glass, solar panels and zinc sheeting onto the existing asymmetrical roofs. This new rooftop extension channels daylight from both sides into the attic. So if the residents have their bedroom on this floor, they are not only woken up by the morning sun streaming in from the roof terrace in the east – they can also enjoy the evening sun that enters through the high roof windows in the west and is reflected back into the room from the white-plastered inside of the opposite roof.

The size of the lower two floors was not altered, neither was the size of the facade windows; the old, poorly insulated windows with their thin aluminium frames were merely replaced by new, triple-glazed windows with timber frames. Nonetheless, the rooms have become perceptibly brighter, as the new open staircase now allows daylight to penetrate right down to the ground floor and into the spaces that surround it. On the first floor, the timber and glass partitions that previously surrounded the staircase were replaced with a low balustrade, so that natural light now also floods the adjacent corridor.

“The overall change that the refurbishment has brought is tremendous. No more flaking paint, no more draughty or leaking windows. The new paint on the walls feels much sunnier now, and of course the attic is bright. I also like the views from up there – they make you feel as though you are at the same level as the birds. We will use the additional space as a study for my wife and me, because we both work quite a bit from home”

Gerard Michels lives in Poorterstraat 41 together with his wife and two small children.

On the ground floor there is an open-plan kitchen/dining/living area of around 45 square meters, stretching from the entrance side right back to the garden. In order to keep this space as open as possible, the stairs consist only of treads, so that whoever enters the building can see right through horizontally. The surfaces in the rooms further contribute to better daylight provision, as the old light-absorbing spray plaster was replaced with smooth, white walls and ceilings that reflect the light. Next to the stairs, a white wall was added in order to reflect daylight into the living room.

“The refurbishment has brought a magnificent change to our house! Before, it was difficult to get it to a comfortable temperature due to the lack of insulation, and the windows were draughty. If we wanted to have a temperature of 20°C inside our home, we had to set the thermostats to 23°C. Still, we liked the house because it is one of the largest rental homes in Montfoort, and also has an unusually spacious back garden, which you don’t find in newer rental homes any more.”

Edwin Hamelink lives in Poorterstraat 33 with his wife and four children

The natural ventilation also benefits from the permeability between ground floor and attic. Opening the roof windows on the top floor creates a chimney effect around the stairwell that channels the used air up from both the floors below, and allows it to escape through the roof into the open. This strategy ensures, above all, that there is some welcome cooling in the houses in the summer. In winter, a mechanical ventilation system with CO2 sensors guarantees a comfortable and healthy circulation of air.

Social housing made attractive – the choice of materials

The roof structure and the facades of the houses were almost totally revamped. In this way, it was possible to attain the ambitious energy targets in the most efficient manner, because these require good insulation and good solar gain from the windows.

The choice of materials used for the facade and roof was important for another reason. The ten houses are part of the social housing stock – and in the Netherlands there is an unwritten law that these should not look any different from other houses. The facades were fitted with a new brick facing and dark weatherboarding. From a distance, this range of materials is reminiscent of the pre-conversion situation. But the new bricks are only half as thick as the old ones, which means that the wall cavity (and thus the space for thermal insulation) became wider without the need to increase the width of the foundations. The floor was insulated from underneath too, with extra insulation being installed in a 50 cm deep, existing crawl space underneath the floor plate.

A complete solar solution – the energy concept

Key to the energy concept is the new roof that was developed as an all-in-one ‘Solar Solution’ by VELUX and Danfoss. This element combines numerous functions: added space, the supply of daylight, ventilation and the generation of solar energy. The orientation of the houses dictated how much solar energy could be ‘harvested’ from the roofs.

On the side of the entrance, which faces west, each roof has a 21 square metre photovoltaic installation. On the eastern side, right up against the roof ridge, is a row of solar panels that provide the houses with hot water. Positioning both systems on the side of the entrance might have improved the energy balance by another few kilowatt hours. But the planners considered that exposing the attic floor to daylight from both sides was more important than unilateral maximisation of energy yields.

The architects reckon that the houses will eventually use between 80 and 90 per cent less energy than before the conversion. In addition, the buildings have been turned into all-electric houses that need no gas boilers or chimneys. Heating is supplied by a ground source heat pump and two ground source heat probes in each house. The heat pump takes its power from the photovoltaic modules on the roof. All the essential technological installations – heat pump, hot water tank and inverters for the PV system – are concentrated in a utility room in the annex on the street side.

The triple benefit – less cost, more space, a better image

Replacing the old gas boilers in the houses by a stand-alone system that relies purely on renewable energies was a daring but forward-looking move in a country like the Netherlands, where gas prices are traditionally low. However the client and the architects expect that with rising prices for fossil fuels, this move will eventually pay off. According to calculations by BouwhulpGroep, the energy costs for the residents will be reduced by around 130 Euros per month as a result of the conversion. Tenants pay around 115 Euros more rent per month for their houses, which, in turn, have been enlarged by 17 square meters each.

The increased revenue from the houses will help the GroenWest housing association to pay back the costs of the conversion – about 130,000 Euros per house. Only a fraction of this sum however, was incurred by the Active House standard – the largest part went to standard maintenance and refurbishment. Another benefit of the conversion project is that, being a research and development investment, it will help the company to future-proof the thousands of other existing homes in their possession. As Peter Korzelius, former Chairman of GroenWest said before the start of the works, “We are faced with a situation in which we will have to renovate one third of our housing stock – in other words 4,000 residential units – within the next five to seven years. This is an enormous task. By running this project, we wanted to gauge what it means to really push back the boundaries of what is technically feasible: as much incidence of daylight as possible, coupled with as little energy consumption as possible.”

For the tenants, the conversion will result in a net saving – and, more importantly, a distinctly improved quality of living both in their houses and in the neighbourhood. Poorterstraat used to be a socially-underprivileged part of Montfoort: it is now a flagship district where residents count themselves lucky to be able to call some of the most extraordinary social housing in the Netherlands their own.

Section through one house showing the distribution of luminance levels under overcast sky conditions in photo-realistic (above) and false colour (below) views.

For a long time, Poorterstraat in Montfoort, a small town west of Utrecht, looked like hundreds of other estates of terraced houses in the Netherlands. Two-storey houses, built in 1976, with tiled roofs, brick facades, and weathered, light blue timber cladding, lined the street. The row of houses already had a striking asymmetrical roof shape before the renovation: on the entrance side, a string of almost windowless extensions stretched right to the pavement, containing cloakrooms, toilets and storerooms. While on the garden side, flat roofs extended over half the depth of the building. When asked what they liked most about their houses, residents would typically reply, “The size. Otherwise there’s nothing special.” The terraced houses offered more space than today´s new building, but they were badly insulated and dark. The ceilings were low, the steep pitch of the roofs did nothing to contribute to the light in the rooms, and the entire attic was only used as storage space. In addition, the extensions on the street side cast shade on the entrance facades. Most of all, the residents were not too happy about the reputation of Poorterstraat. “I am really not proud of my address,” admitted one resident before the conversion. The renovation of the ten social housing units therefore also became a social concern for the GroenWest association. As former Chairman of the association, Peter Korzelius said, “It is good that this of all streets has now become a flagship project.”

Before the conversion, the coarse plaster and the wall claddings that the inhabitants had added took away a lot of daylight from the rooms. They have now been replaced by white, smooth render that evenly reflects the daylight coming in through the windows.

“The Active House principles are all about designing buildings that are suited to their occupants. In our vision for the houses in Montfoort, we translated this concept into light, air and space. These three aspects were the key to our design, whether it was in rooftop extension, the column of light in the staircase or our focus on the indoor air quality. The focus on people is also what makes our project in Montfoort stand out from other approaches to energy-efficient refurbishment, including so-called ‘passive house’ refurbishments. In passive houses the building plays the most important role, whereas in an active house, the user is at the centre. An active house therefore ought to emphasise the active role of the occupants. The Active House specifications are just a reminder of how well the project was executed, and not a goal on their own.”

Yuri van Bergen and Haico van Nunen, BouwhulpGroep

This article is featured in D/A magazine #19, for more information visit