HOW IS A PASSIVE HOUSE DESIGNED?
A Passive house is designed with efficient floor area ratios, utilizing the building’s orientation: maximizing winter solar gains and minimizing summer solar gains. This is done by:
- Maximizing winter solar gain with large south facing windows, whilst minimizing over shadowing from other objects and buildings
- Minimizing northern facing windows.
- Minimizing summer solar gain with solar shading to stop overheating (deciduous trees are great for this, as they lose their leaves in winter).
- Designing in passive cross, or stack, ventilation; keeping the building cool in the summer.
- Compact design, with low volume to floor area ratio and a low floor area to external surface ratio.
To be a Certified Passive House the building has to be designed to PHPP and has to meet the following criteria: (if it is not a certified passive house, it can not be called passive house)
- Highly insulated foundations with a U-value that does not exceed 0.12 W/(m2K) – the lower the figure the better
- Highly insulated walls with a U-value that does not exceed 0.15 W/(m2K)
- Highly insulated roof with a U-value that does not exceed 0.15 W/(m2K).
- Thermally broken insulated frames with triple glazed windows that achieve a U-factor that does not exceed 0.8 W/(m2K).
- Eliminated thermal bridging – A thermal bridge occurs, generally at junctions, where the u-value is not as good, allowing a greater amount of heat to flow through this point. In traditional buildings, thermal bridges are normally found at junctions; such as window reveals, where block walls penetrate through the ground floor insulation. Modern, highly insulated buildings, built with traditional details can lose up to 50% of their heat through thermal bridging.
- Air Tightness – which is defined as air loss from unsealed joints throughout the entire house, should not exceed 0.6 times the house volume per hour at 50 Pa.
- Mechanical Heat Recovery Ventilation System – This system recovers the heat from the warm stale air exiting a building, heating fresh air entering a building. The intake and extract air do not cross; so fresh air is not contaminated by indoor pollutants.
- Hot water is supplied through renewable sources such as solar collectors or heat pumps, depending on personal choice.
- Low energy appliances and lighting are necessary in a Passive house.
- An annual Specific Space Heating Demand of below 15kWh/(m2a), or an annual Heating Load of below 10W/m2.