Air tightness in a home is the prevention of uncontrolled air movement throughout the building envelope. A draughty house leaks energy and is uncomfortable to live in, we call this infiltration.
Why is air tightness important?
Uncontrolled air movement contributes to heat loss in winter, heat gain in summer, presence of allergens and compromises the performance of insulation.
An airtight building minimises heat loss and therefore it is easy to maintain an even indoor temperature with the minimum amount of active heating.
This means that an MHRV* system can work effectively, saving up to 90% through heat recovery, and enjoying a healthy even temperature throughout the whole building. A combination of an airtight building with a continuous layer of insulation ensures that energy is kept within your home, rather than being wasted by escaping into the atmosphere.
*Mechanical Heat Recovery Ventilation
How is air tightness tested?
Measurement of air tightness is carried out using a ‘Blower Door Test’. It is basically a powerful fan which mounts into the frame of an exterior door. The fan has sensors that can accurately measure the amount of air it takes to maintain a constant pressure of 50 pascals (a fresh breeze). The test includes pressurising and depressurising the building, and the final result is an average of both readings. If significant air leakages are present, they can be detected by several methods.
The testing of a new building is normally carried out twice, once after an airtight seal has been installed (pre-line stage) and once at build completion. It is important to do the first test after the fabric of the building is complete and the windows are fitted as it is still possible to find any breaches and fix them.
In a Certified Passive house air tightness must be less than 0.6 air changes an hour. A Certified Low energy home must be less than 1.5 ac/hr. Minimum code-built homes average 4 to 10 ac/hr.
How is the performance of insulation (or the R-Value) affected by air tightness?
The R-value of a material is the materials resistance to conductive heat flow (a measure of its thermal resistance). The higher the R-value, the greater the insulating effectiveness.
The performance of insulation is reliant on no air movement through it, as it is trapped air that is the secret to its performance.
A 2011 study1 found that the R-Value of insulation decreases by a factor of 4.8 if there is a 1mm gap in air tightness (of the computer simulated area measured1), suggesting a significant decrease in performance.
How is moisture movement affected by air tightness?
Apart from leaking heat energy, a home that is not airtight can also suffer from moisture migration into the fabric of the building (Water Diffusion).
The adjacent diagram illustrates Moisture Migration Through the Building
Envelope where gaps in air tightness exist.
Water diffusion in this case is the movement of warm air into, and through, the exterior fabric of the building. Warm air always moves towards colder air where it is able to – in a non airtitgh home this can be through the building fabric.
The 2011 NZ study1 demonstrated that water diffusion increases by a factor of 1,600 when gaps in the fabric of the building exist. If moisture builds up within a wall cavity the integrity and effectiveness of the insulation will be compromised.
Additionally, as warm air flows through insulation to the exterior of the building at the point it finds a cold surface, condensation occurs. This typically occurs at 11-12 degrees C and is called the dew point. When this occurs it often leads to the formation of mould. Unfortunately, this can go undetected as it is not visible.
A Note on Ventilation
An airtight envelope can easily be ventilated with the opening of windows. However, this has found to be a very unreliable way to ensure the control of fresh air, humidity, and allergens, plus heat loss is uncontrolled.
In an energy efficient house, it is normal to fit a mechanical heat recovery ventilation system (MHRV). This system continually replaces the stale air inside with fresh air from outside through a balanced unit that is also able to capture the heat with an efficient heat exchanger and remove moisture. Incredibly the units are ultra-efficient and consume the equivalent power as one light bulb.
Windows can still be opened at any time, but when the windows are closed the MHRV can work at its most efficient – all air movement is controlled through this system, managing not only indoor air temperature but humidity and allergens.
Noise Control!
One of the major benefits of combining quality windows with airtight construction is a quiet home, which becomes especially important in urban areas with close neighbours. If you’re relying on open windows for ventilation, dealing with noise from traffic, neighbours, or even nighttime insects can be frustrating.
In the meantime, if you would like to find out more about a variety of ways to produce a healthier, more comfortable and more energy efficient home, including full passive certification, please feel free to grab our informative free guide here!
Don’t forget to contact us afterwards if you would like some more information! 03 3130103 or info@chattertonhomes.co.nz
1Study Reference: A New Zealand Based Study on Airtightness and Moisture Management, Proclima, 2011.
