Passivhaus is a standard for housing design developed by Professor Wolfgang Feist of the Passivhaus Iinstitute in Germany. The passivhaus standard sets out a way to reduce energy use for space heating and cooling by 90% compared to a standard house.
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 and solar gain. Touchwood Homes have our own PHPP expert who working with our designers ensures our projects meet the criteria
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. Also the total primary energy use for all appliances, domestic hot water and space heating and cooling is less than 120 kWh/m2/yr.
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.
How are Touchwood Homes built to acheive this?
Reducing the space heating energy demand of the house is always a priority in our designs – so our houses are always designed with very thick walls so masses of insulation can be installed – 300mm of insulation is present in the walls as a minimum, along with 400mm in the roof. This void is pumped solid with Warmcel insulation, thus providing excellent insulation and so minimal heat loss.
Building Technique
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 in the building fabric. Good detailing around windows and doors is also vital to eliminate draughts - this being a common location for air leakage . Small breaks in the external air-tight layer means cold air can pass straight through the insulation making it ineffective. Our on-site construction teams excel at these aspects, always producing homes with an air-tightness figure below 1.0 – where building regulations state a figure of 10.0.
Cold-Bridging
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 in mind.
Air-Tightness
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’.
Air Movement
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 cannot be achieved with conventional light-weight insulation.
We always use solid sarking across all walls and the roof which is mastic sealed at every join. This provides a proper, long term robust air tight layer which cannot be achieved with the conventional tape and membrane solution applied to all external surfaces. Tapes will eventually dry out and come unstuck forming air leakage zones, and without solid sarking on the roof, forming an air tight membrane seal between rafters is fraught with difficulties and prone to failure. Our practical experience forming an air tight layer sets us apart, and allows us to guarantee passivhaus levels of air tightness.
Successful Results
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.