Page 52 - Build 151

Internal moisture
Roof space moisture
Recent BRANZ research has con rmed how moisture gets into roof spaces and highlighted the critical role ventilation has in removing it.
BY MANFRED PLAGMANN, BRANZ SENIOR PHYSICIST, AND STEPHAN RUPP, BRANZ PHYSICIST
BEFORE THE PUBLICATION of BRANZ Bulletin 302 in 1992, conden- sation problems in roofs were thought to be caused by di usion driven by water vapour pressure di erences.
Air movement key, not di usion
Recent BRANZ research has con rmed that air movement from internal spaces into roof spaces is the predominant means of mois- ture transport. It’s greater than moisture movement from di usion by at least two orders of magnitude. So, why is air movement the predominant moisture transport agent?
Typically, air pressures within a roof space at night are lower than those within the building. This means that there will be a ow of air from inside spaces to the roof spaces through gaps or cracks in or around the ceiling lining or open downlights. If moisture is present in the space below, it will be carried into the colder roof space and potentially condense on colder surfaces.
Stopping internal air ow into roof space
The transport of air through the ceiling and other surfaces connected to a roof can be dealt with by:
● avoidingopeningsintheceiling,particularlyopendownlights
● sealingpenetrations,suchasforwiringandlights
● installinganairtightlining,suchas ush-stoppedplasterboard
● installinganairbarrierbehindair-leakyliningssuchasT&Gboarding ● ensuring spaces that have high moisture levels, such as kitchens
and bathrooms, are well ventilated so that the moisture is removed
before it can be transported into the roof spaces.
BRANZ research shows that controlling the relative humidity in the living quarters reduces the probability of roof moisture reaching high humidity levels by about half without the use of air barriers and vapour retarders.
Dealing with di usion
As with air pressure, night-time vapour pressure is usually higher within building spaces than within a roof space.
Specifying vapour retarders, such as an oilborne coating, on the warm side of the building envelope for spaces with a high moisture load, such as kitchens and bathrooms, can be useful. This will prevent the moisture vapour transmission through lining materials and lower the risk of roof space moisture.
Ventilation important but ...
Ventilation of a roof space was discussed in Ventilation dries attic space (Build 148, pages 78–79). Calculations at BRANZ show ventilation of the roof space will prevent some condensation but not all. One reason is that cold outside air has very little moisture capacity so is of limited use in controlling condensation e ectively, particularly during the night.
Some moist air will be removed by air ow through the roof, but because New Zealand’s night-time outdoor air typically has a very high relative humidity, very little drying will occur. The cold night-time air introduced by ventilation can also be a source of condensation if, on clear nights, radiative cooling brings the temperature of the roof cladding below the dew point temperature. These conditions are reasonably frequent in parts of New Zealand.
50 — December 2015/January 2016 — Build 151
FEATURE SECTION