Underhill House, Gloucester

A building beneath a 300-year old derelict barn in the heart of the English Cotswolds became the first certified Passivhaus in England. Underhill House is completely invisible from the surrounding countryside, and work 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 National beauty (AONB), Underhill House has been created for minimal visual and environmental impact.

One of the key sustainability elements which lead to successful Passivhaus certification was the entirely concrete structure which is mainly precast. Concrete brings many 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.

The concrete used contains FA and GGBS as cement replacements. External paving uses a by-product of the china clay industry, the screed is made of crushed glass bottles, and internal blockwork uses recycled sawdust.

Airtightness 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 requirements 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.