For most skillion roofs the amount of air movement in the roof cavity, although limited, is sufficient to remove small amounts of vapour. The trick is to keep those amounts of water vapour small.
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BRANZ was called into a Dunedin school where too much ventilation caused excessive condensation in a large roof space. Understanding why this happened points to warm roofs being better in some locations.
Twenty years ago, roofs didn’t need to be specifically ventilated. Today, things are different. That’s because 21st century homes are more airtight, and if they aren’t aired, condensation may form.
Ventilation of cold roof designs in New Zealand doesn’t always go smoothly. Here, BRANZ physicists hand over to a couple of professional builders who have long been at the coalface dealing with roof moisture issues on their projects.
BRANZ research has provided reassuring results for those wanting to add passive ventilation in roof cavities to counter condensation. Corrosion of metal fixings inside the roof space remains low, even in extreme marine environments.
BRANZ physicists have developed a useful calculation for designers to work out the minimum number and area of vents required in roof spaces.
Some roof designs, such as skillion roofs, must include ventilation to manage roof moisture. Recent BRANZ research provides advice on the best place for effective vent openings in low wind zones.
In response to industry questions, BRANZ recently launched a research project into corrosion rates in vented roof spaces. Two roof spaces have been built in an unforgiving coastal environment and will be monitored.
As we push for higher performance from our houses, expectations for airtightness and ventilation are changing rapidly. Some changes will be needed in design to improve airtightness, and mechanical ventilation is likely to soon be the norm.
There has been a big growth recently in the installation of mechanical home ventilation supply systems. We are told they can solve many of our indoor environmental problems, but sometimes basic issues need tackling first.