CASE STUDY: Dig Architects - Hillside House

Planning departments around the UK are now starting to place energy requirements (space heating demand / total energy use / renewable energy production) on new homes, in a drive towards net zero operational demand. In advance of this policy being implemented in Cornwall (June 2024), Dig Architect’s embarked on their ‘Hillside’ house as a case study project, completed in 2022 in Millbrook, South-East Cornwall.

Dig Architect’s founder Daniel Mattholie recognises that while designing for full Passivhaus specification does result in extremely efficient buildings, it can also carry a significant construction cost uplift (estimates range from an additional 10-14% for net zero homes, and potentially up to an additional 25% for full Passivhaus).

For ‘Hillside’, Dig Architects selected a combination of the most energy efficient and cost-effective elements of Passivhaus, creating a system that achieves net zero in the Cornish climate without the additional costs – omitting the unnecessary / marginal performance gains. The result was a nonstandard ‘low tech, high performance’ approach - to drive down costs (both ‘up front’ and future maintenance / replacement) and energy demand in use.

The design of ‘Hillside’ followed these design principles:

• Orientation of windows: capture and control south facing solar (sun) gains and minimise loses through north elevations.

• Form factor: keep external envelope small, relative to internal floor area.

• Insulation: reduce heat loses, and therefore space heating demand, through high levels of insulation and glazing spec.

• Thermal bridges: Minimise gaps in the insulation layers, achieved by the unique approach to insulating 100% outboard of the timber structure, rather than amongst it as it commonly done. (this also has numerous installation / longevity of frame benefits).

• Airtightness: seal the building ‘envelope’, backed up by heat recovery ventilation system to ensure superior air quality levels.

• On-site renewable energy production - favouring solar panels (10kW array) rather than air source heat pumps (better suited to low performance buildings).

• Low tech energy storage - Standard water cylinders (2x 300l) acting as a low cost, longer/cleaner life battery –effectively storing excess solar panel energy in the cylinders as hot water (supplying the home for a number of days, to cover bad weather spells).

In its first year of operation ‘Hillside’ – an all electric house (no oil or gas) - exported (sold back to grid) nearly three times the energy it imported (paid for, from the grid), it generates roughly double the energy it uses, and makes a small annual profit on electricity bills. This will be quite a saving over the course of the building’s life.

‘Hillside’ was built at below average build cost. Dig Architects have proven that high performance, net zero operational homes can be beautiful, uplifting spaces, without the extra cost.