The Portman Estate has created its very first Passivhaus—an environmentally friendly home that recovers and circulates the heat generated by its occupants

Words: Viel Richardson

The Portman Estate’s efforts to reduce the environmental impact of its property portfolio have entered a striking new phase, with the completion earlier this year of the refurbishment of 2 Gloucester Place Mews. While to the untrained eye the new house may look like just another tastefully modernised period property, behind the scenes it represents a major advance, being the first of the Estate’s properties to meet what is known as the Passivhaus standard.

“In essence, a Passivhaus is a very simple concept,” says Mark Hopkins, senior project manager at the Estate. “The idea is to construct a building in such a way that as little heat as possible is lost through gaps in the building’s fabric.” The Passivhaus standard, which was developed in Germany in the early 1990s by Professors Bo Adamson and Wolfgang Feist, has gone on to become a widely respected measure of environmental performance—and one that, while simple in theory, is fiendishly difficult to achieve.

Houses generate significant amounts of heat as a result of the occupants’ daily routines. Cooking, cleaning, lighting, washing, not to mention the human bodies themselves—all generate heat. But with traditional houses acting like giant thermal sieves, much of this is lost to the atmosphere and has to be replaced by energy-hungry heating devices.

Extremely efficient
“A Passivhaus does not depend on heat-generating systems to stay warm, but instead uses the energy that the house itself produces,” Mark explains. The core technology that makes a Passivhaus possible is insulation. “You apply extremely efficient insulation on the inside of the external walls. The type we have used here was developed in the aerospace industry. It is only about 40mm thick but has the insulation value of something four or five times the depth.”

The team have installed a membrane between the internal and external walls, which stops drafts coming into the building through the brickwork. “This is an intelligent membrane that still allows the wall to breathe,” says Mark. “That needs to happen in these older buildings in order to prevent moisture build up and condensation in between the walls.”

Heat retention properties have been designed into the structure of the building, which uses materials with known thermal qualities. Components such as the triple-glazed skylight and the secondary glazing built into the windows are all Passivhaus certified. Any thermal bridges—structural pathways through which heat can travel from the interior of the house to the exterior—have been identified and re-designed to mitigate heat transmission. So, for example, any steel connections from interior structures that terminate in the external walls have been thoroughly insulated around the area of contact.

A key tenet of the Passivhaus standard is air-tightness—there has to be as little leakage from the house as possible, as this is where a great deal of heat can be lost. But as the occupants do still need to breathe, an efficient ventilation system is required. “Fresh air is drawn into the building by a high performance fan unit,” explains Mark. “This fresh air goes into a heat exchange unit, where it picks up heat from the warm air that has circulated around the house and is on its way out. The fresh air then flows into the house through a series of discrete slots installed throughout the property.”

One extra benefit of the ventilation system is that the air quality in the Passivhaus is extremely good. As it is drawn into the house, the air is filtered to remove dust and pollutants. As a result, it is ideal for people with respiratory issues such as asthma or hay fever. Changing these filters is the most technical thing an occupant will have to do, but Mark assures me that this is an extremely simple, easily accessible job.

Keeping things simple and unobtrusive for the occupants was an important consideration. “We wanted to make sure that everything was happening in the background,” says Mark. “There is a simple control panel from which the occupant can set the temperature or the rate at which the ventilation system draws air in. After that, the systems take care of everything.”

Listed building
Putting all this together involved some major changes to a Grade II listed building—not always the easiest thing to get through planning. However, the Westminster planners were very much on board with what The Portman Estate wanted to achieve here. Not only was it rejuvenating a slightly tired property, but the Passivhaus approach was very much in line with the borough’s long-term carbon reduction aims.

There were no internal decorative features that needed to be preserved, so Mark and the team were able to take the internal construction right back to the external walls. The main planning stipulation was that the original staircase had to be retained, so the team removed it, insulated the wall behind where it stood, then reinstalled it in its original position.

Passivhaus standards come in a number of different forms. The one that this building has achieved is known, less than snappily, as the Passivhaus EnerPHit standard—the highest level of certification achievable when an existing building is being converted rather than a new one constructed from scratch. “It is designed to consider some of the constraints this causes,” Mark says. Even so the specification is still very high. “To give you an example, the air-tightness of a building is measured in the number of changes of air per hour inside a property with all the doors and windows closed. The UK building regulations for new properties state this needs to be below seven changes per hour; to meet the EnerPHit standard it has to be less than one.”

Real trust
The exacting nature of the requirements meant the choice of contractor was very important. “Companies with the skill to do this type of work are rare in the UK; it is a big step-change for contractors because the standards are so much higher than the norm,” Mark reveals. “We chose a contractor that we have worked with for many years and had real trust in. They had some experience in air-tightness work but had never built to Passivhaus standards before.”

The building work demanded painstaking rigour, as a slip as simple as a poorly fitted door or window or a small hole in a membrane could mean missing out on the certification. “It meant that the contractors had to pay close attention to every detail in all aspects of the build throughout construction.” The building was inspected regularly by an independent company during the construction phase, with the details photographed and checked by the designers. The contractor even had to provide receipts to show that only specific materials sourced from certified traders had been used.

Once complete, the property had to be tested. Air was pumped into the house until it reached a specified level above the outside air pressure, then the pumps were turned off and sensors inside the house recorded the rate at which the air pressure fell—this allowed the changes of air per hour to be accurately assessed. Independent inspectors also carried out a thermal imaging survey from outside, looking for areas of heat loss or any thermal bridges.

Diligence, expertise and hard work
“The results of these tests, along with the documentation the contractor had to gather during the build were all packaged up and sent to the Passivhaus Institute in Germany where they were examined. Only if everything reaches the required standard, is it certified as a Passivhaus,” Mark explains. “Due to everyone’s diligence, expertise and hard work this project gained full certification.”

For a developer, this is clearly not the quickest or cheapest option for upgrading an old property, so taking this path shows how seriously The Portman Estate takes its environmental responsibilities. The Estate is about to embark on a 10-year carbon reduction programme and many of the lessons learnt here will be applied elsewhere. While not all properties can be raised to Passivhaus standard, techniques such as minimising thermal bridges and improving the air tightness may be applied to future refurbishment projects.

Mark and his team are rightly proud of the finished property. “I like the fact that it blends the historic habitat along with the modern finishes while incorporating the best technology to create a low energy house,” Mark says. “I like the fact that the Passivhaus features have in no way detracted from the building. The quietness of the building, sitting as it does next to one of the busiest roads in central London always surprises me. But most of all I like the fact that we have created a really special place for someone to live.”

The Portman Estate

SpaceMark Riddaway