Considerable attention has been lavished on Crossway, a high-performance home designed by British architect Richard Hawkes and completed a couple years ago on a spacious lot in county Kent, in southeast England. It wasn’t until July of this year, however, that the house, which has become as well known for its parabolic roof as its energy efficiency, was finally certified for Passive House performance, becoming one of the first new homes in the country to meet the standard.

Hawkes, who tracked the project’s progress in a blog, and his wife, Sophie, occupied the four-bedroom 3,000-sq.-ft. house for about a year before hiring a service in February to perform a blower-door test, which showed 0.56 air changes per hour at 50 Pascals. Later that month, the house also earned an A-A rating, the highest achievable, for energy efficiency and CO2 emissions in government-mandated Energy Performance Certificate tests, with a 93% energy efficiency score, and 103 out of 103 points on the “environmental impact” CO2-emissions rating.

Adding a thermal buffer

Other than the parabolic roof – a Catalan vault constructed of 26,000 locally made clay tiles arranged in three layers – the house is equipped with a combined photovoltaic and solar thermal system and phase-change material thermal store, with a 4 kW heat register linked to the building’s heat recovery ventilation system. A biomass boiler has been installed as backup, but has yet to be used.

For the exterior walls, in between their cellulose insulation and interior-facing layer of plasterboard, Hawkes installed 5mm-thick DuPont Energain panels, which are designed to absorb ambient heat as room temperature rises (starting at about 72 degrees), store it until the temperature drops (at around 64 degrees), and then release it back into the room.

After some adjustments, the home’s energy efficiency systems seem to be performing as expected, Hawkes told the Scottish Passive House Centre (SPHC), a consultancy and certification group serving the U.K. He added that the house has had 100% free hot water since March and its PV/solar thermal system has generated almost 700 kWh of electricity since the end of June. SPHC handled the building’s Passive House certification, which was awarded on July 10.

Clay soil and the greening of a roof

Though the region’s clay soil is terrific material for tile and brick, it did mean that the builder had to sink a series of 36-ft. pilings into the ground to guarantee stability for the foundation. The clay also is doing duty as a native-plant substrate in a center channel built into roof’s porous tiles, which were dressed with metal mesh and then filled with gravel and clay. With the support of an inexpensive irrigation system, grasses and flowers have finally taken root.

None of this came cheaply. Although we haven’t gotten word on the final cost of construction and materials, estimates were mentioned in an overview of the project for “Grand Designs,” a TV series presented by Britain’s Channel 4 Television that focuses on architecturally unusual residential construction projects. The initial budget of about $473,000 for Crossway grew to $630,700 as the project got underway, and the cost of the parabolic roof, originally pegged at $134,000, had drifted to about $165,000.

Judging from Hawkes’ comments about the results, though, it doesn’t seem as if he or his wife have regrets about the endeavor, and the attention it has attracted probably hasn’t hurt either.