Air Leakage and Ventilation
What’s the difference?
‘Ventilation’ is an operable means of controlling the flow of outside air into a room, by means of windows that can be opened or mechanical ventilation systems.
‘Air leakage’ is the uncontrollable leakage of internal or external air through cumulative small cracks and holes in a building.
Why does it matter?
In recent years, awareness about the benefits of improving airtightness has increased. Air leakage is tested using a blower door, which creates a pressure difference between the inside and outside of the house and calculates a rate of leakage measured in air changes per hour (ACH) or as permeability in m3/m2/h at a pressure of 50 pascals.
Increasing bodies of data suggest that the existing freestanding houses in Australia have air leakage rates of 15–30 ACH@50Pa (Edwards, 2018)
The German Passivhaus construction standard stipulates an air leakage rating of less than 0.6 ACH@50Pa.
Mechanical ventilation is generally recommended to maintain indoor air quality in homes if they are tighter than about 3 ACH@50Pa.
Experts in Australia are increasingly suggesting that in cooler climates, air leakage ratings of 4-8ACH@50Pa can be achieved using conventional construction methods and that this may be the current market’s sweet spot for cost-effectiveness. (Edwards, 2018)
Cooee’s approach
Our approach is to use standard construction methods to achieve an air leakage rating of less than 7ACH@50Pa. We do this by focusing on the plasterboard/internal face of the house as the air barrier, rather than implementing non-conventional wall systems and wraps to control leakage. By controlling air leakage at the internal face, you also focus on limiting vapour in the wall and roof cavities. When using the plasterboard/internal face as the air barrier, we focus on:
Minimising downlights, and ensuring all downlights are sealed and IC-F rated (so insulation can go over them)
Avoiding ducted heating or cooling systems
Specifying that architraves around windows and doors, and skirting/floor junctions are gap filled
Specifying exhaust fans and rangehoods with backdraft dampers
Working with builders to ensure trades understand the importance of minimising and taping holes in plasterboard, even if they are behind joinery
Specifying good quality well-sealed windows and doors
Detailing draft-stoppers to external doors and manholes, including pet doors
Installing a barrier around cavity sliding door pockets
Draft-sealing is the single most cost-effective way to reduce energy bills and experience increased comfort in both retrofits, renovations and new builds.
But isn’t ventilation good?
There is no industry consensus on what level of air-tightness is ‘too much’, and this makes sense given that occupant use can be so varied. We suggest all clients install an intelligent home meter that measures indoor and outdoor temperature, as well as a carbon dioxide meter. If we’re confident that excellent air-tightness will be achieved on your project (due to design and builder experience) we will include a form of heat-recovery ventilation which is a cost-effective and highly energy-efficient form of filtered trickle ventilation. These are also very easily retrofitted if you have concerns about carbon dioxide levels after you’ve moved in.
Like many parts of what we do, ventilation and draught-proofing are about striking an optimal balance between sustainability, intelligence and simplicity.
Where to find more
The information above is extracted from my book, 101 Things I Didn't Learn in Architecture School; And wish I'd known before my first job.
While the book is written for students and graduates of architecture, it is also an excellent handbook for anyone embarking on their own project. We provide a free copy with every on-site briefing and feasibility meeting, or you can purchase your own copy here.
My knowledge of this subject comes from years of working under Jenny Edwards, building scientist and Director at Light House Architecture and Science. Her website includes further excellent resources and case studies on this topic. See below for two of Edwards’ articles, referenced above.
Another great resource on basic principles in Australia is the YourHome.gov website which is free to access. Sustainability Victoria Zero Net Carbon Homes also has some great videos on DIY draft proofing and the basics of energy-efficient construction best practice. (This post’s image comes from the latter.)
Green It Yourself is a very useful resource for DIY draft-sealing and understanding the mechanics of where leaks occur.