Page 56 - Build 152

Ventilation
Passive roof ventilation
Twenty years ago, roofs didn’t need to be speci cally ventilated. Today, things are di erent. That’s because 21st century homes are more airtight, and if they aren’t aired, condensation may form.
BY STEPHAN RUPP, BRANZ BUILDING PHYSICIST
Physics same but houses di erent
Did BRANZ get it wrong back in the 80s and 90s? I would like to argue no, we didn’t. The physics hasn’t changed, but the way we build and use our houses has.
Moisture is constantly being added to the inside of houses from cooking, taking showers and unflued gas heaters (these unflued gas heaters should be avoided wherever possible).
In the past, this excess water was often removed from the building by natural venti- lation. Draughty windows, doors and oors provided a channel to constantly replace moist inside air with fresh outside air. The inside air that did nd its way through the ceiling into the roof cavity – most commonly large volume gable-type roof spaces – didn’t generally cause any trouble.
More airtight, skillion roofs and downlights
These days, buildings are signi cantly more airtight, which is good to keep the warm air in. However, building occupants now need to provide the necessary air exchange by regularly opening windows.
Custom-made passive ridge vent elements.
QUESTIONS AROUND THE NEED for passive ventilation elements to give New Zealand roof cavities a breath of fresh air have been investi- gated by BRANZ researchers since at least 1988.
Extra ventilation not favoured in past
Back then, a scienti c article concluded that ‘additional roof ventilation has been found to be of no assistance’. BRANZ scientist Harry Trethownen argued that drawing in air from the outside into the roof cavity on a cold clear
night would even aggravate the problem. The humidity of the outside air will be close to saturation and given that the roof cladding can be at temperatures below ambient due to a process called radiative cooling, condensation on the metal might be the consequence.
How does this align with increasing calls to add ventilation to roof cavities? Some building authorities elsewhere recommend or even demand passive vents to be part of the roof design.
54 — February/March 2016 — Build 152
FEATURE SECTION