An environmental strategy explored the location, exposure, orientation and topography. A series of strategic options aimed at satisfying the thermal and visual requirements of the occupants were developed by WSP Environmental in conjunction with the architect.
Computer simulations determined the performance of the heating and cooling demands of the house as well as thermal comfort for a typical year. Dynamic thermal modelling was used to predict the annual thermal performance, and daylight analysis helped to estimate the contribution made by the glazing.
The scheme comprises a well-insulated building envelope, minimising fabric losses and benefiting from useful solar gains in winter. Partly buried, the floor and a large proportion of the perimeter walls are in direct contact with the ground, using the thermal stability of the soil and ensuring that fabric heat loss is minimised but also that heat gains through those surfaces are avoided. The south facade is exposed and contains full-height glass surfaces that capture solar gain in winter in conjunction with a labyrinthine underfloor cooling system to aerate the building and remove heat build-up.
The green roof increases the insulation of the roof slab and contributes to the microclimate of the site by minimising warm surfaces in the summer. This microclimate is aided by a small pool - or moat - on the south side, which provides cooling breezes and doubles as an architectural feature, creating reflections within the building and bouncing light further into the interior.
Model assumptions The thermal analysis was carried out using the dynamic thermal modelling software TAS (Thermal Analysis Simulation), designed by EDSL. The results showed that the annual heating loads can be substantially reduced by preheating the fresh air through the underfloor labyrinth (from 1,750 to 1,137kWh/yr). The labyrinth is also highly beneficial in precooling the inlet air in summer so that the number of hours above 26 ¦C in the lounge space drops from 13.9 per cent to 0.1 per cent. Crossventilation in spring and summer promotes air movement. When the temperature outside is lower than the inside, the incoming air will be beneficial to the removal of internal gains. When the ambient temperatures are greater than the indoor temperature, the intake of air from outside should be reduced to prevent overheating. However, in the hottest part of the summer it might be desirable to open the front elevation doors in order to enjoy the outer space. Despite an increase in temperature, occupants will benefit from the resulting air movement. The introduction of passively preheated/ cooled air from an underground labyrinth and the reduction of air intake directly from the outside in summer considerably reduces the heating demand while maintaining comfortable conditions throughout the year. This option benefits from the high thermal stability of the ground to preheat/ precool the incoming air as required.
The average daylight factor is 5.1 per cent, which indicates that this is mainly a well day-lit space with bilateral lighting coming from both the south glazed facade and the rooflight. However, the daylight uniformity ratio is 0.12, which indicates uneven daylight distribution in the house.
Controls Approximately 19m 2 of monocrystalline photovoltaic cells by BP Solar are laid at 30 ¦ at the rear of the property. (Plans for a laminate PV canopy doubling as a brise-soleil providing an additional 30m 2 were abandoned early in the cost planning stage. ) The PV system, as installed, comprises 125 x 125mm modules spaced 6mm apart to create the potential for about 2kW of power, distributed down to the main switchboard in the utility space. A solar hot-water system comprises 5m 2 of LaZer2 vacuum tube collectors within an anodised aluminium frame. The absorbed energy is trapped inside the vacuum and transferred to an internal manifold that directly heats the water, thus minimising energy losses by operating at up to 93 per cent efficiency independent of the external air temperature. Water is heated to 60 ¦C. At this temperature, the vacuum tube is still 90 per cent efficient whereas, allegedly, flate-plate systems are around 43 per cent efficient. The hot-water system digital controller senses when the temperature of the solar panel is 3 ¦C warmer than the water in the cylinder, and the pump is turned on to circulate water and heat through the system. Blender taps have been fitted to avoid the risk of scalding. There is also an immersion heater back-up.
Underfloor heating throughout the living, dining and sleeping quarters comprises cross-linked polyethylene pipes with an integral oxygen diffusion barrier to meet the performance specification of around 3.3kW.
Rainwater Rainwater drained from the roof is directed into the pool at the front of the house, although it is not used for grey water internal use.
Glazing Low-emissivity double glazing (6-12-6mm) has been used throughout.