How a Brooklyn brownstone became a low-energy passive house

With the development of its waterfront as an industrial hub and the building of factories in the 19th and 20th centuries, Brooklyn became known for its gritty aesthetic. So much so that in the 1950s Truman Capote described it as “an uninviting community. A veritable veldt of tawdriness …”.

But today neighbourhoods such as Park Slope, in northwest Brooklyn, are highly desirable. Among a row of brownstone houses here, one is significantly different from the rest – albeit invisibly so to the human eye. A thermal image of the street in winter shows a row of brownstones all with fiery colours, while one house is deep blue, showing that no heat is escaping.

The property is the first in New York City to be certified as a passive house (or Passivhaus) – an international low-energy standard developed in Germany in the 1990s that typically results in heating and cooling bills that are 80 to 90 per cent lower than those for conventional buildings.

“A passive house is a building that is very well insulated, virtually airtight and primarily heated by the sun,” says Julie Torres Moskovitz, the home’s architectural designer and author of The Greenest Home: Superinsulated and Passive House Design.

Passive house buildings – they needn’t just be houses – are designed to capture the warmth of winter sunlight and to be protected from high summer sun. The concept has been evolving for more than a century.

“The Fram, Fridtjof Nansen’s extraordinary north polar explorer ship [built in the 1890s], was thermally a ‘passive ship’, using super insulation, triple-glazed windows and heat recovery,” says Professor Wolfgang Feist, director of Germany’s Passive House Institute.

It is, claims the institute, the fastest growing energy performance standard in the world. Currently there are about 50,000 buildings constructed using the principles, 7,000 of which are certified. Most are in German-speaking Europe and are newbuilds. The retrofit of a 110-year-old brownstone in Brooklyn is “a great example, showing how energy efficient technology can be applied to an existing structure,” says Feist. “It shows we can create a sustainable future while taking care of existing traditions.”

On a recent winter morning, the temperature on the tree-lined street was 10C. Inside, the property was light, airy and a cosy 22C. “We haven’t put the heating on yet,” says one of the owners, who is in his thirties. When he and his wife bought the three-storey property in November 2009 it was derelict. “We weren’t interested in ‘olde worlde charm’,” he says. “We wanted a building we could gut.”

His major concern was that, once renovated, their home should be energy efficient. Online, he “stumbled upon” the idea of passive houses. The concept appealed for its “precise prescription and procedures,” he says. Architects use a software package developed by the Passive House Institute that models climate, latitude, orientation and component parts to test whether the finished building will be energy efficient – needing a maximum of 15KWh/sq metre/year for heating.

For various reasons – a history of relatively low oil and gas prices, conservatism and a consumer culture that Torres Moskovitz scathingly dismisses as “a cushy wasteful lifestyle” – the passive house standard has been slow to take off in the US, where there are fewer than 100 certified properties. So perhaps it is not too surprising that she is a recent convert. It was her clients that introduced her to the concept.

“We wanted to give a talented young architect the chance to develop her career,” says the owner. Torres Moskovitz, then in her thirties, enrolled on passive house training and invited a passive house consultant, Jordan Goldman of Boston-based ZeroEnergy Design, to join her team.

We weren’t interested in ‘olde worlde charm’. We wanted a building we could gut

By the end of 2012, the renovation was complete. After the first year of occupancy, the family of four’s annual bills for heating and cooling ($518) are almost a fifth that of New York homes of a similar size (310 sq metres), according to the government’s residential energy consumption survey. An estimated 16 tonnes of CO2 has been saved, which is, says Torres Moskovitz, “about the same as taking four cars off the road”.

A Mitsubishi air source heat pump provides the limited amount of “active” heating and cooling required. In this first year it used 2,462KWh – less than a single small radiator left on for two months. Furthermore, this is higher than the long-term modelled power consumption as construction materials are still drying out, requiring more heat in the first year.

Torres Moskovitz calculates that building to passive house standards added a 5 per cent premium – about $63,000 – to the major renovation project. This is forecast to be repaid within 10 years through savings on energy bills. A passive house retrofit, without such major architectural work, would cost in the region of $200 to $500 per sq ft with a similar payback period, she estimates.

So what are the main features of the home that architects have since dubbed “the Tighthouse” due to its airtight properties? Via Skype, Torres Moskovitz takes me on a guided tour of the house.

First, she taps the exterior wall. It sounds hollow. The brownstone veneer was removed and 10cm of exterior foam insulation was placed over the masonry with an equivalent thickness of insulation inside. “Because New York has extremes of temperature, insulating from inside alone can cause a freeze-thaw that harms the masonry wall,” she says. The thick insulation sandwich also reduces street noise.

She points out traditional window cornices in keeping with the building’s period character. “These are fibreglass rather than original wood,” she says. “The owners see continual maintenance as unsustainable.” The modern windows, however, have caused controversy. “Some people love the look of double-hung (sash) windows,” she says. “It’s possible to get windows with a horizontal divider mimicking traditional windows. If the street were landmarked [a preservation order], we would do that.”

We enter the house at garden level, through a door of triple-glazed glass filled with inert gas. Components such as windows and doors that meet exacting standards of very low U values (meaning highly insulating) are mostly shipped from Europe, which somewhat negates environmental benefits, but their manufacture in the US is gaining momentum. They are more expensive than regular windows and doors but far more efficient. Meanwhile, European-made components are dropping in price because more manufacturers are making them.

Inside, a double-height art studio transforms the basement into a light and liveable space. The floor had to be dug up and rebuilt to meet airtightness and insulation standards and prevent water from penetrating the “building envelope”. “The result is that the basement is as insulated and dry as the rest of the house,” says Torres Moskovitz.

Staircases that have been rerouted are made from perforated steel, allowing daylight to filter through the mid-terrace house, as do internal windows. Low energy LEDs provide supplementary lighting.

As for the aesthetic, it is all minimal and monochrome. Floors are made from recycled grey timber.

“The owner’s not into ‘organic’. He likes steel and glass and the colour grey.” There’s just one period interior feature: an exposed partition wall of original brick, but like the fibreglass cornices, this isn’t what it seems. It was built from bricks reclaimed from the chimney flues in front of a partition wall, and then treated with an airtight seal.

“Making the house airtight was the biggest challenge,” says Goldman. “One of the requirements for a passive house is rigorous airtightness, stringently tested with a machine called a blower door. There haven’t even been any new houses built in New York City that are this airtight.”

Although all windows can be opened, airtightness is vital for a passive house’s most efficient functioning. “It’s a ‘machine for living’,” says Torres Moskovitz, quoting Le Corbusier. “The air within the house changes once every three hours with the ventilation system.” A “mechanical room” houses a system of tubes. Some duct fresh filtered air into the house from roof level, others extract air and moisture. A heat exchanger captures waste heat – from occupants and domestic appliances – in winter and cools incoming air in summer.

Torres Moskovitz opens a glass door to the rear and shows me a freestanding steel staircase to the garden. She points out that it doesn’t quite touch the wall of the house, not even with a bolt. “This prevents ‘thermal bridging’ [loss of heat by conduction],” she says. This is another principle of passive house design not common practice in everyday construction.

It’s a ‘machine for living’. The air within the house changes once every three hours with the ventilation system

We finish our tour in the master bedroom and terrace – a new fourth-floor, rooftop extension with a roof oriented and tilted to improve efficiency of solar (PV and thermal) panels. There are views to Manhattan across countless houses. Soon the colour blue on a thermal image won’t be so unusual in the city.

“There are three energy efficient retrofits under way in this street right now,” says Torres Moskovitz, “and others across the city.”

Justin Bere, a British architect who completed London’s first certified passive house in 2010, praises the Tighthouse as an example of “architecture that combines the qualities of attractive design, high performance, low energy use and great comfort for the occupants. To achieve such a high design quality in a retrofit of an NYC row house that’s over 100 years old makes it all the more of a triumph”.

In the UK, there are 100 certified passive house buildings, including a retrofit of a 19th century cottage in Hereford and a new-build in Inverness. Bere talks passionately about how the standard could alleviate fuel poverty. “Deals are being made to spend vast fortunes on power stations to meet unchecked energy use. These deals will leave people at the mercy of rising prices from big energy providers and struggling to stay warm,” he says.

“How much better it would be to spend money on ways of massively reducing energy consumption for generations ahead?” At present, according to the government, domestic buildings are estimated to consume more than a quarter of the UK’s energy, more than road transport or industry.

Back in Brooklyn, the Tighthouse appears to have withstood recent wintry conditions. “During the extreme weather of the polar vortex, the house performed great,” says one of its inhabitants. “We did not have to turn up the thermostat at all.”