Airtightness
The Challenges:
Many NZ homes are compromised by a lack of airtightness, with draughts and gaps – even small cracks at structural joins – allowing the continual uncontrolled leakage and loss of air and heat. This results in reduced efficiency of the heating and ventilation systems, reduced comfort and health, and higher operating costs:
“Up to 90% of heating and cooling can be lost through leaks… Heating and ventilation work harder in leaky homes – like running your car’s AC with the windows down.”
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Increasingly in recent years NZ architects, designers and builders are conscious of the importance of airtightness, and are purposefully integrating sealing systems, improved materials, and innovative techniques to address this problem. The benefits of airtightness include:
Energy efficiency – reducing uncontrolled air flows through the structure helps to maintain consistent temperatures in the home, improves the effectiveness of the insulation, and supports the smooth operation of the heating and cooling systems
Indoor air quality – reducing the external pollutants such as dust and pollen, and invisible pollution such as volatile organic compounds (VOCs), that can impact health and wellbeing
Comfort – minimising problems such as cold corners and draughty areas in living spaces
A key factor with improved airtightness is the need for effective ventilation to ensure a steady flow of fresh air through the home, and avoid stale air, mustiness and the build up of CO2.
BRANZ advice:
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Many older homes such as turn-of-the-century villas were well-known for their draughtiness due to gaps and lack of sealing.
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However this constant air movement through older houses also helped to minimise moisture accumulation inside the home and within the roof space and walls. With better airtightness in today’s homes comes an increased risk of internal moisture and dampness:
Causing mould growth which is harmful to health
Damaging the materials and structures of the home with rot and corrosion.
So airtightness, ventilation and internal moisture control are all closely interconnected. Intelligent design both for new builds and renovations should integrate these three dimensions for optimal results.
How to achieve Airtightness
There are a range of inter-related methods to support a more airtight home:
Air vapour control layers, wraps or membranes to establish a continuous air barrier and support internal moisture control
Sealing junctions, particularly around the joins between the floor and the walls (skirting boards), walls and ceiling, attic hatches, and penetrations (plumbing, ducts, electrical outlets, downlights) – using specialised seals and tapes
Sealing around windows, skylights and door frames – including internal access doors between the home and the garage
Installing a mechanical heat recovery ventilation system
Using structural insulated panels (SIPs)
Stopping up unused chimneys in older homes
Blower Door Testing
Airtightness is measured by a Blower Door Test. A high tech fan is set in to an external door, which gently pressurises and depressurises the building to assess air leakage and record the air changes per hour through the structure (ACH).
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A Blower Door Test should be undertaken firstly in the construction phase, when the fabric of the building is complete, the windows installed and all openings are sealed, in order to identify and fix any leaks – and again on the completion of the build to verify the airtightness performance of the structure.
Blower Door Testing is a complicated technical procedure and must be carried out by a certified professional with the necessary equipment:
Pro Clima technical info:
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Aero Solutions:
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BRANZ advice:
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Airtightness Requirements
The current Building Code has no specific provisions requiring airtightness in homes, and recent initiatives by some construction sector groups have promoted the introduction of new policy and measures to address this:
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“Today in many parts of the US, UK and Ireland, a blower door test is required for new buildings, and it’s common practice across much of Europe. In Australia, air infiltration testing is included as a compliance pathway in the National Construction Code.”
NZ Green Building Council:
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BRANZ advice is that 3 air changes per hour (ACH) is a reasonable target for airtightness:
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Superhome Certification requires airtightness to be achieved through a range of methods:
BASE: Less than 3 air changes per hour (ACH) tested with a Blower Door Test – airtight design strategies including a rigid air barrier with taped joints, seals between bottom plates and floor, and avoiding or sealing all penetrations
BETTER: Less than 2 ACH tested on site with a Blower Door Test – internal airtightness layer, airtightness wrap or other airtightness strategy eg rigid sheet material
BEST: Less than 1 ACH tested on site with a Blower Door Test – internal airtightness layer, airtightness wrap or other airtightness strategy eg rigid sheet material
Other high-performance home certification systems such as NZ Green Building Council Homestar and Passive House also include airtightness incentives and requirements:
“It is good to make sure that your home, your asset, is warm and dry. Iike to go around and seal up as much as you I through the walls, the floor, the roof, the ceiling, anywhere and everywhere, because you know if you stop that air flow it’s not going to escape.”
Clayton Mitchell: Statement Homes
“In certain situations airtightness is appropriate – you do have difficult sites where they might not get direct sunlight, or you might be in an urban setting where noise is an issue, there’s pollutants – and that is the best way to deal with the situation is airtightness and some kind of mechanical ventilation system. There’s always different ways of doing things, it’s relative to the person, site conditions, budget.”
Duncan Firth: Solarei Architecture
“With our own home we achieved 2 air movements per hour... in a renovation that’s going to be quite a challenge, as it will require an airtight membrane, specific attention around light switches and openings, bottom plate fixings, sealing connections and you need a building company who is committed to that level of detail.”
Guy Gillespir: Character Builders
