Building Performance – Lesson 4

Building Performance – Lesson 4

4. Air out equals air in.

This is a principle that is not climate related. In physics, we know that for every litre of air we push out of a building, another litre of air must enter to replace it.

When we turn on a kitchen exhaust fan and push air out of the house, an equal amount of air must find its way back inside. This air comes back in through what is called, ‘the path of least resistance.’ The bigger the hole the easier the path, and the greater the quantity of flow from this area.

The problem we have is that in an un-sealed house the path of least resistance is commonly at but not limited to:
– Recessed light fittings
– ducted heating outlets
– Wall mounted power points & light switches.
– Plumbing penetrations through walls and floor
– Edges of windows
– External doors
– Access door to garage
– Chimneys & flues
– Skirtings

All of these paths of air travel can potentially carry unhealthy air, (sucking fumes in from the garage is not preferable). This intrusion of unhealthy air is compounded when we chose kitchen range hoods, clothes dryers, bathroom exhaust fans etc. with a high flow rate without considering a strategy for a controlled and safe source of make-up air.

To manage this problem we need to:
– Select closed combustion appliances and have them serviced regularly.
– Provide a properly sealed building envelope
– Provide controlled make-up air when large exhaust fans are installed.



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