The National Construction Code (NCC) prescribes requirements for energy efficiency in buildings, including how the building envelope should be sealed to prevent air leakage. The elemental provisions of the NCC describe how walls, floors, ceilings, windows, exhaust fans, etc should be constructed or sealed. Similarly, house energy rating software includes certain building sealing measures as part of the thermal performance assessment. Items such as sealed downlights and door seals are checked off to achieve the required 6-star rating. Yet there are currently no mandatory inspections or tests to quantify the extent or quality of installed building sealing in Australia.
Natural ventilation is good isn't it? The answer is yes, but only when controlled. Uncontrolled ventilation is never good. The days of fixed wall and ceiling vents are long gone. Considered necessary to maintain fresh air while solid or liquid fueled heaters emitted dangerous carbon monoxide, these fixed vents are now outdated and counterproductive. No longer are we content to huddle around an open fire in the lounge with a door snake preventing the draft from coming under the door and a blanket over our shoulders. The adage; "Seal it tight and ventilate right" says it all.
We now expect to condition our whole house to a comfortable temperature year round, so with escalating energy costs and concerns about CO2 emissions contributing to climate change, we need to build to a higher performance standard. The increasing average size of houses in Australia makes it even more important to reduce uncontrolled losses. House energy rating (HERS) software assumes that the building envelope is sealed, but even a 6-star energy-rated house doesn't meet the minimum energy efficiency standard if building sealing isn't properly carried out on site. Constructed houses are rarely tested, so we usually don't know how they perform until the energy bill is received. Some houses have been tested and have shown that energy usage is much higher than predicted by the HERS assessment due to excessive air-leakage through the building fabric, HVAC, light fittings, etc.
Air leakage can be measured by conducting a blower door test (common in the northern hemisphere but hardly heard of down under). A fan device is installed in place of an external door and the house is depressurised. The fan pressure is monitored by computer and readouts show the amount of air leakage at a given pressure - usually 50 Pa. This is expressed as "air changes per hour at 50 Pa", or "ACH50". That is how many times the entire air volume of the house is exchanged in an hour at 50 Pa pressure difference between inside and outside. The pressure difference simulates the effect of wind, which causes negative pressure on the leeward side as it passes over a house, but is exaggerated to give a meaningful reading. 'Natural pressure' is generally considered to be X ACH50 divided by a factor of 20. So a reading of 20ACH50 equates to twenty air changes per hour at 50 Pa, or one whole house air change per hour at so-called 'natural atmospheric conditions'.
When air-tight building is discussed, often people think of the Passive House standard. Passive House (or German Passivhaus) is a rigorous, voluntary standard for energy efficiency in a building, reducing its ecological footprint. It results in ultra-low energy buildings that require little energy for space heating or cooling. One of the key principles of Passive House is sealing of the building envelope. The Passive House standard is a very low 0.6ACH50, which requires a balanced mechanical ventilation system with a heat exchanger to keep the inside air fresh. Balanced ventilation systems are recommended for 2ACH50 or lower and some sort of ventilation system may be required for anything less than 10ACH50 to prevent moisture build-up.
A CSIRO sample of 20 typical existing (presumably modern code compliant) Australian houses achieved on average 19.9 ACH50. To put this into perspective, American building codes require 2-3ACH50 in cold climates and 5ACH50 in the warmer states. In the United Kingdom a minimum standard of 4ACH50 is required and Europe building codes require between 2 and 3 air changes per hour at 50 Pa.
To test energy efficiency measures CSR constructed a test house in Schofields NSW. The CSR house achieved 9.3ACH50, but was further improved with some temporary fixes to achieve 6.5ACH50. After testing CSR recommended a target of between 7 and 10ACH50 for energy efficient houses in Australia.
On a couple of occasions clients have asked us to specify a higher standard of building sealing for improved energy efficiency (or to at least deliver that promised by the HERS rating). Blower door tests were done to quantify the standard achieved. Due to diligent attention to detail by the builders, both achieved lower than 10ACH50. The house at Scott Parade achieved an air exchange rate of 6.79ACH50; equivalent to one complete air change every two hours at natural atmospheric conditions (or 0.49ACH2.5). This was further reduced (but not tested) after some minor remedial work. This house was fitted with heat recovery ventilation to keep the air fresh and requires very little supplementary heating and no artificial cooling.
Angus Kell of CSR delivered a very useful presentation at the CSR Building Knowledge Seminar which I attended in Geelong back in 2015. You can view Angus Kell's presentation here: https://youtu.be/8Gii-PXGve8
Darren Bowman - Building Designer & Building Thermal Performance Assessor.