To meet the standard, software called the Passivhaus Planning Package (PHPP) is used to enter full details of the project in question – such as U values of walls, cold bridging details, solar gain etc. Touchwood Homes have our own PHPP expert who working with our designers ensures our projects meet the criteria. 

The principle minimises heat loss in these main ways:

• Excellent levels of insulation
• Minimal cold bridging in the external frame
• Excellent levels of air tightness
• Controlled internal air quality by mechanical ventilation system
• Clever design of solar gain and summer shading to maximise winter heat gain from the sun.
• Use of low energy household appliances and lighting.

Formally the principle is defined as the total energy demand for space heating and cooling is less than 15kWh/m2/yr of treated floor area - a standard house uses around 110kWh/m2/yr. The total primary energy use for all appliances, domestic hot water and space heating and cooling is less than 120 kWh/m2/yr.

Construction Issues

A particularly important aspect to meeting the standard is extreme attention to detail on site during construction. For example to achieve the air tightness specification demands no worker on site cuts corners – inspection of work and attention to detail are critical to avoid breaks in the air-tight layer. Good detailing around windows are doors is also vital to avoid drafts going right through the building fabric. Small breaks in the external air tight layer means cold air can pass straight through the insulation making it ineffective.

Recent research by Mark Siddel published in the green building magazine has shown how important an external air tight layer is -  a 300mm thick wall packed with insulation exposed to a 2.5m/s wind speed (very low) would make the insulation 35% less effective. The key to making insulation work effectively is to stop any air movement through the insulation, which can occur in two forms, ‘open loop’ and ‘closed loop’.

Open loop air movement occurs when air from outside can travel straight through the insulation – a movement type successfully stopped through the use of an air tight layer. Closed loop air movement is when the air moves within the insulation through convection currents. This also makes the insulation in-effective since the air movement transports heat through the insulation. Our solution to closed loop convection currents is to insulate with Warmcel insulation, installed to a density of 50kg/m3. At this density air movement no longer occurs within the insulation layer – a solution which is impossible to achieve with conventional Rockwool type insulation.

Cold bridging is another aspect that has to be on the mind of anyone working on the timber frame during construction – otherwise mistakes will be made such as using metal brackets which could conduct heat straight through the building fabric. It is practical solutions such as these that make our houses perform well – everything has to be installed with attention to detail.

Tests on our houses have shown the success of our approach – our recent project in Essex achieving an air tightness figure of 0.47 m3/m2.hr @50Pa (passivhaus stipulates a figure less than 1.0). When this level is achieved along with super insulation, there is no longer a need for a conventional heating system - the minimal heating demand can be met by a system integrated into the house ventilation unit.