A building beneath a 300-year old derelict barn in the heart of the English Cotswolds has today (1st February) become the first certified Passivhaus in England.


Architect, Helen Seymour-Smith of Seymour Smith Architects, developed Underhill House which, completely invisible from the surrounding countryside, has involved the house being constructed underneath and adjacent to the existing carefully restored Hill Barn.


A juxtaposition of sensitive refurbishment and modern design within an Area of Outstanding Natural Beauty (AONB), Underhill House has been created for minimal visual and environmental impact and will feature on Channel 4’s Grand Designs in the autumn. The project has been completed with the support of principal partner Aggregate Industries and insulation providers, Dow Building Solutions in addition to a range of other suppliers.

Air tightness tests on the building - a key requirement for a structure to qualify for Passivhaus certification – saw Underhill House achieving an impressive n50 result of 0.2198 air changes (ac/h) an hour at 50 pascals (pa), compared to the demanding Passivhaus requirement of 0.6 ac/h @ 50pa. The UK equivalent q50 result was 0.23 m3/h/m2, and current building regulations in the UK are 10 m3/h/m2.

Key sustainable elements leading to successful Passivhaus certification are:

  • An entirely concrete structure which is mainly pre-cast and supplied in entirety by Aggregate Industries. Concrete brings major benefits to the passive eco design, in the exploitation of its thermal mass and the formation of a monolithic structure which is naturally airtight. The use of precast concrete delivers the benefits of off
  • site construction including reduced wastage but also achieves a superior finish, critical for its internal exposure.
  • STYROFOAMTM-A insulation from Dow Building Solutions encases the complete structure, with key product FLOORMATETM 300
  • A running beneath the entire floor slab. Unlike alternatives, FLOORMATETM 300-A is highly moisture-resistant meaning it copes with being outside the waterproof envelope. It also has high enough compressive strength to sit below the concrete structure, leaving the all-important thermal mass exposed internally.
  • A predominance of south-facing glazing to make best possible use of passive solar gain, and mechanical ventilation with heat recovery.

Helen Seymour-Smith, the project’s architect, said; “We are thrilled to have been awarded England’s first Passivhaus certificate. Passivhaus design achieves a staggering 90% energy saving compared to that of an average house, and we really believe it is the best standard available for achieving the reductions in carbon emissions that are so desperately needed.

“We set ourselves quite a challenge holding the existing barn in mid air, and digging a hole under it to slot the house into – but it has enabled us to create a bold design without impacting on our stunning AONB setting.”

“With much of the concrete structure exposed internally, the interior has been carefully designed to show that concrete can be sexy, and can play a positive part in environmentally friendly construction. Using the benefits of thermal mass alongside highly-durable insulation especially designed to perform in such demanding conditions has resulted in an energy efficient building that I hope will inspire others and set standards for the future.”

Bill Bolsover, Group Chief Executive of Aggregate Industries, the project’s principal partner said: “Our brief from Seymour-Smith Architects for Underhill House was simple. Was it possible to design, construct and install a large precast concrete framed house into the hillside and then build the hillside back onto the house to eliminate visual impact? As leaders of sustainability in our field, we took up the challenge and delivered through a company effort involving both our Concrete and Precast Solutions teams. We are delighted with the results.”

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