We Want Airtightness!

by Paula Hugens, eZED Ltd | 21 July 2014

One of the problems with older homes is not just the lack of insulation, it’s the air infiltration, what is commonly known as a draft. There is probably a 50:50 split between the envelope losses and air infiltration losses in these homes, so it shouldn’t be ignored if you want to be warm and comfortable. Air infiltration is an uncontrolled mechanism for heat loss (or heat gain in warm climates) and not a form of fresh air delivery.

Hot climates benefit from airtight construction as much as cold climates, in fact cooling a house through air conditioning is more energy intensive. If you prevent warm air permeating into house it will remain cooler.

By ignoring airtightness a significant opportunity for energy savings is being lost.

There would be very few places in the world, if any, that have a perfect climate of near 20 ºC day and night all year round. Air infiltration will therefore always cause a loss of comfort and an increase in the energy bills. High levels of airtightness are essential if you want to ensure a constant comfortable indoor environment all year round in an energy efficient manner.

You should consider that an airtight envelope will : –

  • Eliminate airborne pollutants from entering the home through gaps in the building envelope.
  • Dramatically reduce risk of condensation in the building fabric, which in turn reduces the risk of interstitial mould and decay*
  • Allows ducted heating and/or ventilation systems to perform at much lower velocities making them quieter, cheaper to run and more comfortable.
  • Enables full potential of energy efficiency savings to be realized.
  • Improved acoustic performance as airborne noise will be obstructed
  • Create a wonderfully comfortable environment

The airtightness envelope should always be installed on the inner face of the insulation.

* Building wraps are installed to provide windtightness and some weather-tightness  These require some back diffusion capability otherwise moisture vapour will become trapped inside the wall or roof construction. Being installed on the inside face, the airtightness envelope can prevent moisture vapour entering the construction and reduce the risk of interstitial condensation and mould depending on it’s diffusion resistance.

With some care a new house in New Zealand can achieve an n50 value of 2 ach, this would greatly improve the thermal performance of a code minimum house. For a house to achieve better thermal performance both insulation and airtightness should be considered as they are similar in terms of economic investment.

Are 0.6 Air Changes Relevant to the New Zealand Climate?

A very common question being asked is whether the Certified Passive House airtightness performance requirement of n50 0.6 air changes per hour is relevant to the New Zealand climate.

Airtightness is a performance requirement that is independent of climate. If the level of Infiltration is not minimized it is very difficult to achieve the highly energy efficient maximum allowable heat load of 10w/m² per annum that underpins the performance of a Passive House. Allowing a higher n50 value would require more insulation to compensate for the additional energy demands.

Having a high level of airtightness does not prevent proper controlled ventilation. Like all homes, Certified Passive Houses still have windows that open to provide good cross-flow ventilation and fresh air requirements when desired. When outdoor temperatures start to cause excessive cooling or warming of the home the windows can be closed. Instead of relying on polluted infiltrated air (such as air that has passed treated timber, fibrous insulation, dust and mould) to provide the necessary fresh air volume when windows are closed, a Passive House uses a mechanical ventilation system with heat recovery, this is the final step in achieving high thermal performance and energy efficiency.

Copyright eZED Ltd 2014