Each winter Dynamic Composite Technologies hears stories about leaking roofs or “rain in the roof” even when there has been no rain for several days. Often known as “ghost leaks”, these leaks and instances of rotting or corrosion of bottom plates are often a result of the building getting wet from the inside. Buildings are increasingly being designed as air tight and well insulated and problems with leaks and corrosion are becoming more widespread.
Methods for keeping out rain and wind involve covering the building with vapour tight materials, preventing the water from getting in. In the winter, these materials do not let vapour escape safely, resulting in the cold roof space and walls getting wet due to condensation.
If building insulation is not installed correctly in relation to vapour barriers and breather membranes, it can lead to structural damage and health problems for occupants due to mould spreading unnoticed within the building fabric. If moisture gets trapped in the insulation it prevents it from doing the job it is meant to do.
Being aware of the potential problems, expensive and embarrassing rectification can easily be avoided at the design stage.
Foil, perforated foil or reflective bubble foil type sarkings continue to be installed behind the external wall claddings, brick veneer or under roof tiles when vapour barriers should be used instead on the warm (interior) side of the insulation. A breathable membrane should be used on the cold (exterior) side of the building insulation. This carries out the same job as foil sarking but also lets moist air escape.
Condensation should be allowed to drain safely out of the wall to help wall assemblies dry out, common practice overseas is for a vented cavity between the external cladding and breather membrane. This was a key measure introduced in New Zealand in response to the leaky building syndrome and is increasing being adopted by good builders in Australia.
Typically, breather membranes are textile products that work much in the same was as Gortex fabric and have a very low vapour resistance of less than 0.3MNs/g. These should not be confused with some perforated foil type products labelled as “breathable.” Although well intentioned, these perforated products do not really breath enough and their vapour resistance is 10 to 70 times the standard for a breather membrane set by the National House Building Council in the UK.
High humidity projects such as swimming pools, museums and libraries are particularly at risk and a condensation risk analysis should be carried out.