Holiday Home at Rock North Cornwall

High thermal mass, the control of solar gain and water heating by a combination of solar and ground source energy are featured on this new build project in south-west England.

By English standards Radoon, a two-storey house overlooking the estuary of the River Camel in Cornwall, is a large dwelling, with six double bedrooms in its 323 m2 of floor area. The original Radoon was built in the 1920s using concrete blocks made with sand from the local beach. By the turn of the century the concrete was crumbling and demolition was the only option. London-based ECD Architects were tasked with the design of a replacement, and came up with an 'upside down' concept, with living accommodation above the bedrooms and superb views across the estuary and surrounding countryside. Sustainability was also central to the concept, and a number of approaches were combined to reach the best solution.

Masonry construction to provide high thermal mass was the key decision. Outer walls are made up of an outer skin of externally rendered 100 mm thick ' Thermalite' (lightweight autoclaved aerated concrete block) and an inner skin of 140 mm thick high strength Thermalite 7N block. Inside the 140 mm cavity is 90 mm of Celotex polyisocya-nurate (PIR) foam insulation. Overall U-value is just 0.2 Wm2K, well below current regulatory requirements.

First floor construction is also comparatively massive, being a combination of 150 mm deep precast 'inverted T' concrete beams at 450 mm centres infilled with standard dense concrete blocks. Curved steel beams support the zinc-clad roof. Insulation here is 300 mm mineral wool, which produces a U-value of 0.15 Wm2K.

Roof design is critical. With the site sloping up towards the north and the best views lying to the south, most windows face the sun for most of the day. Solar gain through the large south-facing living area windows in particular could be a problem in summer - and a bonus in winter. The answer is to extend the roof some 1.8 m out from the building line to act as a summer sunshade. A similar function is performed by the balcony, which runs across the entire south-facing façade and shades the bedrooms below. Summer cooling can be boosted by high-level vents over the stairwell, which takes advantage of the stack effect.

Thermal Mass Vented
An 'upside-down' layout and high thermal mass shape the architecture of this holiday home (Reproduced with permission from Clive Boursnell)

Cornwall is noted for its equable climate, which could be even milder as global warming really makes itself felt; already olive groves are being planted in the county. Even in the depths of winter the demand for space heating will be significantly less than in the rest of the UK, especially with such high degrees of insulation. However, solar energy absorbed by the building fabric from the low winter sun via the high-performance windows should be a major contributor to occupant comfort. There is still the chance, however, of chilly winter mornings following days when the sun has failed to shine, or even long periods of inclement weather. For quick warm-up there is a wood-burning stove - although the oil-burning Aga cooker should also make a significant contribution to space heating (when used). For longer periods there is low temperature underfloor heating supplied mainly by a ground source heat pump.

To reduce the extent and depth of the excavations needed to install an appropriate ground loop for the system, Swedish-developed ' compact collectors' from Oxfordshire-based ICE Energy Heat Pumps are used, installed horizontally at a depth of 1.5 m in the sandy ground. Heat extracted from the ground is stored in the 160 l capacity outer shell of an ICE-supplied unvented thermal store, which is wrapped around a 300 l inner tank used to supply both DHW and space heating. Heat pump coefficient of performance is 4.0; maximum temperature inside the thermal store 51°C, and the space heating is designed to maintain internal temperature at 20°C.

Before entering the main thermal store, mains water passes first through a heat exchanger in a booster thermal store supplied by a solar water heating installation. This is an unusual system, which has won several awards for innovation for its manufacturer Solartwin of the UK. All pipework outside the thermal store is flexible microbore silicon rubber tubing that is unaffected by freezing temperatures, so no antifreeze is needed. A small solar photovoltaic array drives the circulation pump. This circulates the water through the flat plate collectors as long as the sun is shining: there is no temperature control, and it is claimed that the system is immune to the normal risk of dangerously high stagnation temperatures. In optimum conditions it is calculated that the solar collectors will raise the temperature of the water entering the main thermal store to 40°C.

Low Energy House Strategies

slole_ ICE store pump

"indirect"

model slole_ ICE store pump

"indirect"

model

The heating system integrates ground source and solar energy (Reproduced with permission from ECD Architects)

Although the original house drew its water supplies from a well on the site this is now capped off. Rainwater is now harvested from the roof and stored in a 3,000 l tank located below ground at the rear of the building. From there it is pumped to a header tank and used to flush toilets. It is estimated that this reduces mains water consumption by 25%.

Construction began in October 2005 and was completed in August the following year. Building cost was £500,000.

Client: Claire Lloyd and Tessa Chamberlain Architect: ECD Architects

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