HOW IS A PASSIVE HOUSE DESIGNED?

A Passive house is designed with efficient floor area ratios, using 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.

CERTIFIED PASSIVE HOUSE

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 a Passive House)

  • Highly insulated foundations with a U-value that does not exceed 0.25 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).
  • Eliminate 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 by 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

OVERHEATING

As Passive Houses are well insulated, & have good air-tightness,  over-heating can be a problem.  More so in contemporary passive houses with lots of glass.  The Passive House calculations (PHPP) look at the possibilities of overheating and flag up any issues.

At Marshall McCann Architects, with our experience of living in a Passive House, we understand the need to prevent overheating, and the need to cool the house quickly.

Our designs look at orientation, solar shading, & reduction in glazing on key aspects.  We often use the site characteristics to help (eg. deciduous trees shade the house in the summer, but allow solar gain in the winter).

We also cultivate a natural ventilation strategy, to help cool the house when needed.