Note: You are using Internet Explorer 11, which may have display errors, we recommend using a browser such as Mozilla Firefox, Google Chrome, Microsoft Edge or Safari


Underhill House

In 2008, architect Helen Seymour-Smith contacted Sto after seeing StoLotusan Color facade paint demonstrated on an episode of Grand Designs. Helen was impressed and enquired if we could supply products and systems suitable for PassivHaus construction. Since then, we have worked hand-in-hand to help realise her own grand design.

The Story Behind Underhill House

Helen had been living and working in North London with her husband Chris. Together they dreamed of moving to the country with their young family and to start a new business venture: specialising in PassivHaus architecture. Helen's father presented them with a golden opportunity, if they were brave enough to take up the challenge.

Helen's father owned and farmed land in Gloucestershire. One particular plot of land had been refused planning permission and sat unused in an area of outstanding natural beauty. The planning authorities did not want any development work that would spoil the view of the fields and the ruins of a disused barn near the centre. All development proposals to date had been refused.

Helen's father offered the land to Helen if she could find a use for it and the couple jumped at the opportunity with a unique solution. Rather than building on the land, they would build under it, stabilising the foundations of the old barn and developing a large living area under it, while converting the barn itself into an office space for their new business venture.

To make the already-ambitious project more complex, Helen wanted the project to act as proof-of-concept for PassivHaus design, and set herself the goal of meeting the stringent criteria. Whether or not they were successful could make or break their new business, so the stakes were very high.

The planning authorities took some persuading that the views would remain unobstructed. To alleviate their concerns, Helen prepared visuals that displayed the proposed development from all conceivable angles. This was one design proposal the authorities could not refuse and they agreed to the solution as the proposed design would be near invisible on the landscape.

Meeting the PassivHaus Requirements

Having secured planning permission, Helen contacted Sto asking for advice on suitable facade systems for the project, and for help on the more complicated aspects of the design.

The proposal was to construct the supporting walls of exposed concrete, which could negatively impact the U Value requirements for PassivHaus construction. After meeting the Sto technical team, the StoTherm Classic external wall insulation system with Sto-Turbofix foam adhesive was selected. The system was fast to install, helped to meet the thermal insulation requirements and minimised any potential cold bridges.

The system was finished in the unique self-cleaning facade render StoLotusan.

Interior and Acoustic Solutions

Helen is an advocate of exposed concrete as a construction material, both for its aesthetic properties and as an excellent heat regulator. However, the hard, reflective surface of exposed concrete creates sound reverberation, leading to poor interior acoustics and making for an uncomfortable living environment.

Helen turned to Sto for a solution. The StoSilent Modular suspended acoustic system was specified, dramatically reducing reverberation times. Individual acoustic panels are suspended from the ceiling, providing a huge scope for creativity. The panels can be shaped, suspended at different heights and tinted in a range of colours to great visual effect. Helen designed a series of curved panels to draw the eye away from the angularity of the building, creating a more welcoming space.

The major benefit of a modular system to Helen was that it maintained maximum practical exposed surfaces for thermal mass benefits.

As the internal structure of the building was to remain exposed, the precast concrete ceiling panels were finished with interior spray plaster StoLevell In C, providing for a flat surface that complemented the suspended acoustic panels.

Links & More Information

For more information on Underhill House, Helen Seymour-Smith maintained a website and blog about the project at The AI PassivHaus.

For information on PassivHaus design, visit