One of the major factors of a passive home is its airtight building envelope. Air tightness together with an efficient mechanical heat recovery ventilation (MHRV) system will enable a house to breathe without compromising the integrity of the insulation or the air quality, and allows for extremely low heating and cooling costs.
In general an airtight building envelope ensures the ventilation system works optimally, but more importantly is key in preventing moisture damage and mould growth, a major issue affecting the health of today’s population.
The information below details why airtightness is so important and why we should be considering it when building. There is no better time to make a huge difference to our living environments and homes performance.
If you want to find out more please contact us at our office, or download our informative brochure on building for health and comfort which covers a number of things to consider when building for the performance and comfort of your home.
What is airtightness?
It is basically prevention of uncontrolled air movement throughout the building envelope. A drafty house leaks energy and is uncomfortable to live in, we call this infiltration.
Why is it 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 savings of up to 90% on heating can be made whilst 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.
New Zealand MBIE building regulations state that designers, ‘…. have to take air tightness into consideration’. However, there is currently no guidance on how this is achieved, no measures in place and no requirements that document if it has been achieved. So, to gain guidance and advice we can look at examples overseas and also the research that has been carried out here in New Zealand.
How is Airtightness 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 are able to accurately measure the amount of air it takes to maintain a constant pressure of 50 pascals, this is equivalent to a wind speed of 5 on the beaufort scale or a fresh breeze. The test includes pressurising the building and also 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 from feeling airflow by hand, use of a ‘Thermoanemometer’, hand held smoke puffer or an infrared camera. Source: Proclima.co.nz.
The testing of a new building is normally carried out twice, once after an airtight seal has been installed (i.e Intello wrap) 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.
So, the next bit looks a bit technical, but bear with me …..
N50 (measure of airtightness) = Volume of air transported by the Blower Door
Net Volume of the Building
In a Passive House, for example, the air tightness measure (n50) should be ≤ 0.6.
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. In fact they have found through research 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 measured*), suggesting a significant decrease in performance.
An insulation value of R 3.3 without a gap reduced to an R-value of 0.7 with a 1mm gap!
In basic terms, the research suggests that for insulation to work effectively you require air tightness.
How is moisture movement affected by air tightness?
Water diffusion in this case is the movement of warm air into, and through, the exterior construction of the building.
Research has shown that water diffusion increases by a factor of 1,600 when gaps in the fabric of the building exist.
Without a gap (in other words airtight) water diffusion was measured at 0.5g/m2 over a 24 hour period.
With a 1mm gap the same computer simulated area* was measured at 800g/m2 over 24 hours.
In basic terms, if moisture builds up within a wall cavity the integrity 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. This can occur readily within the construction where the dew point is found. 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 but this has found to be a very unreliable way to ensure the control of fresh air and humidity. In an energy efficient house it is normal to fit a mechanical heat recovery ventilation system (MHRV). This system replaces the stale air inside with fresh air from outside through a balanced unit that is also able to capture the heat in winter with an efficient heat exchanger. Incredibly the units are ultra efficient and consume only 30 -40 watts of electricity an hour which is equivalent to one light bulb.
It is a myth that you can not open windows in a Passivehaus, you can when ever you like. When they are closed they become part of the airtight envelope which delivers optimal thermal performance and comfort. The mechanical ventilation system will constantly remove excess moisture that may have entered through open windows especially in our humid northern climate, again creating a warm healthy mould free environment. In fact, Passivehaus quality assurance requires that there be at least one openable window in every habitable room.
Extra Plus….Noise Control!
One of the big pluses that the combination of quality windows and air tightness delivers is a quiet house. This becomes more important in cities with neighbours. If you are forced into opening windows for ventilation and you have a noisy road or neighbours or bugs at night this can be really annoying!
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