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27 July 2015

Airtight Envelope Requirements for Passive Houses

Written by Jon Davies
Technical Sales Support and Education Manager, Pro Clima New Zealand

An older villa leaks air at a rate of 20-25 air changes an hour when tested at a pressure of 50 pascals (n50). A new standard house will leak between 5-10 ACH. When heating a house that leaks, even if it has good insulation, the warm air will escape. This is why, especially in older houses, people would try and heat one or two rooms because the cost of heating the whole house would be very high. Even in a new house the uncontrolled movement of air means the rooms in the house will vary in temperature and will be expensive to heat evenly.

Most European buildings now include airtightness as a part of their building code requirements. By law they are required to have new houses tested for airtightness as it has been recognised that it has such an impact on the efficiency and comfort of a house. The Passive House standard has a requirement for a house to leak no more than 0.6 air changes n50 which is up to 15 times more airtight than a typical new home. However, this doesn't mean that windows and doors can't be opened in a PH. Many areas of New Zealand have a beautiful climate for much of the year and the indoor-outdoor flow is a great feature of many homes. So when the weather is favourable, users can definitely open up the windows and doors. But when the bugs come out, the temperature drops or homeowners want to shut out the road noise and close the windows and doors, fresh air will continue to flow freely with the use of a balanced pressure mechanical ventilation system. This is controlled ventilation and recovers far more energy than it consumes. This also ensures even temperatures and a relative humidity that is just right for healthy living.

Airtightness does not happen by accident, the building design must show how the airtightness is achieved and the joinery must be airtight. In one particular construction, INTELLO, a humidity-variable moisture control and airtightness membrane maunfactured by pro clima, is used internally between the insulation and the plasterboard.

Windows will commonly have a double seal arrangement and every electrical cable or pipe is sealed as it enters or leaves the building. The house is later tested with a Minneapolis Blower Door which pressurises and depressurises the building while calculating the leak rate.

Once a building has passed the Blower Door test the application for Passive House Certification can be placed with the Passive House Institute, as all the other design criteria have already been met.

Written by Jon Davies
Technical Sales Support and Education Manager, Pro Clima New Zealand

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