Page 57 - Build 152

Ventilation
Replacement air needs to be fresh outside air.
Roof styles are changing as well. Skillion- type roofs are very popular, with a much reduced cavity volume.
An easy pathway for moist air to migrate into the roof cavity has also been provided through open downlight fittings in the ceiling. These factors have created a situation where moisture can cause a problem in the roof space.
Condensation in cold roof construction
Harry Trethownen found that, in New Zealand conditions, a cold roof construction will sometimes experience condensation. Cold roof construction is where the insulation layer is at the ceiling level and the roof cladding is cold on both the outside and the inside.
In a recent study, BRANZ looked at a completely ventilated roof cavity – the air in the roof space is always the same make-up as the ambient air.
Simulating this baseline case gives an idea of how often condensation occurs simply by looking at the temperature and humidity conditions over an average year at di erent locations in New Zealand.
A roof can generally handle such events. The underlay and timber elements can absorb some moisture, and as long as this is removed again later, no lasting damage is done.
Ventilation channels one strategy
BRANZ modelling has also shown that ventilation is bene cial in removing excess moisture that nds its way into the roof space. Does this mean every roof needs additional ventilation channels?
There are roofs in New Zealand that perform well without ventilation elements. Well managed indoor humidity along with
an airtight ceiling is still a primary means of reducing the risk of condensation problems in roof cavities.
Therefore, additional ventilation channels should be viewed as just one element in a range of moisture control strategies.
Passive and mechanical measures
When installing additional openings, consider a few basics. Passive ventilation of roof cavities is driven by two mechanisms:
● Temperature di erences – stack or chimney
e ect.
● Wind action.
These driving forces generally result in a nega- tive pressure within the cavity. Air enters the cavity from below and is expelled at the top.
Bearing this in mind, avoid situations where exhaust air, for instance, exiting through a ridge vent, is sucking up moist air from the living quarters below. The replacement air needs to be fresh – outside air coming through vents around the eaves for instance.
Testing for weathertightness
Weathertightness is another important consid- eration. In the past, BRANZ has tested roo ng elements to AS 4046.9-2002 Methods of testing roof tiles. However, this standard does not speci cally address the issue of vent elements as part of the roof design. Work is required to establish its suitability in this context.
The same is true for cutting the roof underlay beneath the ridge. This is necessary for the vent to function properly, but conden- sation developing underneath the ridge cap must not be allowed to drip into the roof cavity.
These items will be addressed as part of the BRANZ research programme over the coming months.
Build 152 — February/March 2016 — 55
FEATURE SECTION