The fundamental principle of Passivhaus is simple: reduce the heat losses of a building to such an extent that it hardly needs any heating at all. According to the Passivhaus Trust, the sun, human occupants, household appliances, household appliances and the warmth from extracted air cover a large part of a house’s heating demand. Passivhaus construction requirements include enhanced levels of insulation with minimal thermal bridges and well-considered use of solar and internal heat gains. As these houses are highly airtight, whole house mechanical heat and ventilation recovery (MHVR) systems are necessary to provide highly efficient heat recovery and excellent internal air quality. Any shortfall in heating is usually met by a small unit, such as a woodburning stove.
Reassuringly, the Passivhaus standard does not necessitate the use of repetitive designs or serially produced construction processes/systems, leaving the individual layout and look of a house to its owners and architects. And it is not only suitable for detached houses in rural situations: increasingly, Passivhaus projects are emerging in high density urban areas.
An excellent example of the former is Kirsty Maguire Architect Ltd’s design for the Hayshed Farmhouse in East Ayrshire, close to Scotland’s wet and windy west coast. The house was conceived to complement the existing farm cluster and thereby root it to its agricultural context whilst at the same time improving and refining the Scottish tradition of timber construction to exceed Passivhaus standards.
The results are impressive: the structure is a combination of timber frame walls made with I-joists to give the additional depth necessary to accommodate increased levels of thermal insulation, with glulam (glued laminated timber) beams used to create the roof’s curved agricultural barn form. The whole building is clad with recyclable seamless zinc sheet coloured to match the local red earth.
With U-values for the floor, walls, roof and windows of 0.12, 0.11, 0.11 and 0.84 W/m2K respectively, a primary heat demand of 101 kWh/m2/year, a space heating load of 9W/m2, an air-to-air heat pump for hot water and an air change rate of 0.22 @ 50 Pa, the overall achievement is impressive, especially as there are plans to add a domestic turbine to harness the local wind and enable the house to be powered entirely from the site itself. With a total floor area of 162.5m2, and a construction cost of £1750/m2 (including internal fit out, garage and landscaping), the house is certainly not over-expensive, but it is its energy performance that is remarkable: better than originally modelled, the energy bills are over 90% less than those of the old farmhouse with an overall aim for the building to generate significantly more energy than it uses, also making it a ‘PlusEnergy’ and ‘zero carbon’ house.