Building Physics
The Building Physics model
ZEDstandard construction is summarized as follows.
- Super insulated building fabric. U value 0.1Wm-2K-1
- Triple glazing to have overall frame & glass U value 1.2Wm-2K-
- Sunspace window u value to be 1.6W/m2K overall
- Airtight construction tested to achieve 1.5 air changes per hour at 50 Pa (6.86m3/hr m2)
- Passive heat recovery ventilation using wind cowls
- Passive solar gain where possible
- Energy saving lighting throughout
- Maximized day lighting to all habitable rooms & workspaces
- Solar shading covering 100% of the glazed aperture at noon in summer
- Night-time purge ventilation
- A thermally massive ceiling walls, and floors with a minimum of 50mm dense concrete (or equivalent) on 75% of visible surface area.
Passive whole house ventilation with heat recovery
Ventilation is provided from a passive system that uses the wind and the stack effect to provide fresh air and extract stale air to and from every room in the house 100% of the time. The Wind Cowl as it’s best known, is a natural ventilation system that offers passive ventilation with heat recovery. It has been developed to harness natural wind currents to create air pressure sufficient to provide of a plentiful and healthy fresh air supply to buildings with no energy cost. The system also uses passive stack ventilation principles, and can operate in near windless conditions, although the main period of its use is in winter when there is normally more wind. The heat exchangers in the Wind Cowls recover up to 70% of heat that would normally be lost with the outgoing air from a building. If trickle-vent ventilation were used when building to ZED standards, this would account for 50% of heat losses within the building.
The Wind Cowl is divided into three main parts:
- The Top Unit consists of a rotating unit that funnels fresh supply air in and stale extract air out of the building. The air travels through a heat exchange unit, stale air pre-heats the incoming air in the winter, and cools the hot incoming air in the summer.
- The Base Unit connects the Wind Cowl to the building structure as well as operates as a transition unit between the Ductwork and the Top Unit above.
- The Ductwork directs Supply Air and Extract Air to and from the different parts of the building respectively. The ductwork consists of standard galvanised steel ducting. In addition there is a drain that collects condensation from the Wind Cowl and takes it to a Wash Basin Trap. Fan driven heat recovery uses about 500 kWhr/yr – about half the output of a wind turbine.
High air-tightness
Once super insulated the biggest heat losses from a home are due to ventilation. Ventilation is required for breathing, moisture control and odour control. Early super insulated houses attempted to control energy losses due to ventilation by just reducing ventilation; however they experienced problems with stale air and odours.
ZEDstandard houses control energy losses due to ventilation with airtight construction that then means all incoming and outgoing air can be controlled and passed through a heat exchanger. In this way, the interior space can have plentiful ventilation with very little energy loss. Air tightness is predominantly achieved by the use of wet applied plaster to the internal walls and careful robust detailing that inherently leads to an airtight envelope. Care is taken during construction of the walls that all perpends are fully filled and that plaster is applied right down to the screed. Careful use is made of skirting boards and architraves to cover joints where shrinkage cracks may occur. Full use of acrylic gap sealer (Painters Mate) is encouraged in the decoration package.
Thermal mass
This is the term used for all the exposed, uninsulated high-density surfaced areas inside the building, such as stone floors or masonry walls, which store large amounts of energy. Thermal mass is important because thermally massive buildings have the ability to absorb heat from times of high gain that can then be re-emitted at times of heat demand. This ability to absorb heat also keeps thermally massive buildings up to 10deg cooler than peak summer daytime temperature so the need for air conditioning is avoided. Without thermal mass a building has to closely control the temperature of its internal environment. This tends to lead to technologically complex control systems and powerful heating and cooling plant. Without thermal mass any heat loss or gain has to be replaced or removed by actively supplying heating or cooling. If an external door or window is opened, a thermally lightweight building will quickly reach the same temperature as the outside. Likewise if a thermally lightweight building has a large south-facing window through which the sun is shining, internal temperatures can quickly become uncomfortably high. This ends up in a large amount of energy being used to heat the building one-minute, and then cool it the next.
Thermally massive ZEDstandard construction can absorb heat from solar gain through a south facing window without the internal temperature of the space increasing uncomfortably, and then reemit the heat at a time when the heat is required. ZEDstandard construction has about 5 days worth of thermal storage. This means that if all heat gains were removed from a ZEDstandard building, it would take 5 days for that building to cool down to uncomfortable levels. No sophisticated control system is therefore needed and hence no ‘peak output’ heating and cooling systems are required. As heating and cooling is avoided the interiors benefit from a pleasant, healthy, stable internal environment without any scorched air and dryness problems that can be experienced in a traditionally heated home.
Solar orientation
South facing homes have lower winter fuel consumption than East West facing units. High performance windows ensure that large areas of glazing can be used to give excellent daylight and good solar gain (with shading on South facing windows to prevent summer overheating) With a southerly aspect incorporating a sunspace, it is possible to build solar homes that maximize the use of passive solar design to such an extent that no conventional heating system is required. Any glazing to the South, can achieve a net gain in energy over the year, whereas North facing glazing a net loss. Hence the emphasis on orientation for large glazed openings. On very warm days, the inner doors can be closed and the sunspace acts as a ventilated bufferzone to shield the home from extreme temperatures. ZEDfactory have built up considerable experience and expertise with passive solar design. Over heating has been over come by roof over hang and solar shading in conjunction with secure sunspace ventilation. Through building physics models and the evolution of passive solar design, our experience has shown that these spaces not only provide warmth, but considerable amenity value, and a substantial increase to the sales value.
Cool summers
There is a current trend towards lightweight fast-track building techniques such as lightweight steel, lightweight timber frame or SIPS panel type construction for building dwellings. These buildings, however well insulated, will require intensive air conditioning as summer temperatures increase, and summers like 2003 become normal. Solar orientation if not designed correctly can cause summer overheating. It is important to get solar gain into the base and heart of the building, and to prevent high angle summer sun from entering through sunspaces. Large South facing overhangs provide the optimum solar gain as they will prevent direct summer sunlight from reaching the interiors.
Retractable blinds will prevent direct sunlight from reaching the sunspaces when required in the summer. BDa have carried out extensive studies to optimise the solar potential of buildings to give a comfortable and easily controlled environment.
Passive solar design is one of the easiest and simplest forms of collecting renewable energy, removing one month from either side of the heating season.