Page 60 - Build 151

Internal moisture
Condensation research
In this latest look at BRANZ’s vapour control in walls project, we outline the bene ts of WUFI, a sophisticated computer modelling software that calculates heat and moisture movement in building components.
BY GREG OVERTON, BRANZ BUILDING PERFORMANCE ENGINEER
BRANZ’S Vapour control in New Zealand walls project was introduced in Build 138, pages 63–64. It aims to de ne the condensation limit for typical New Zealand walls and should clear up some confusion in the industry about the role of vapour barriers and vapour retarders.
In Coming to terms with condensation (Build 144, pages 71–72), we introduced technical terms relating to water vapour. Water vapour in walls (Build 147, pages 83–84) looked at the Glaser method for assessing the risk of condensation in a structure and its limitations.
Calculation limitations
The limitations of the Glaser calculation are that:
● it does not account for convective (air) mois- ture transport, which can dominate over the
di usion process
● it does not account for moisture storage in
materials
● it does not account for variable moisture
properties, for example, many materials have a vapour resistance that varies with relative humidity
● it is a one-dimensional method, so, for example, it cannot easily take account of framing
● it does not account particularly well for variable indoor and outdoor conditions, including solar radiation.
Here, we look at two of these limitations in more detail – moisture storage and variable climate conditions.
Moisture storage
When using the Glaser method, we examine how the moisture content of air varies when water vapour di uses through a structure. The psychrometric chart can be used to iden- tify how much water is stored in the air for a given temperature and relative humidity.
However, other materials can also store moisture. For these, a di erent chart, known as the sorption curve, is used.
Equilibrium moisture content
The sorption curve tells us how much water is stored in a material in terms of an equilibrium moisture content (EMC)
and how this varies with relative humidity. The EMC is the weight of water expressed as a percentage of the dry weight of the material. Figure 1 shows a generic sorption curve for timber.
The sorption curve represents equilibrium moisture contents and it can take weeks, or months in the case of timber, to settle around equilibrium.
The EMC is not dependent on tempera- ture, only the surrounding relative humidity – this is a simpli cation but is good enough for our discussion.
There is a maximum amount of water that can be absorbed by a material from the air, and this corresponds to 100% relative humidity. For wood, this is known as bre saturation and is about 30%. If the moisture content is above this level, liquid water has to have been present.
Timber can store and release a lot of water Timber can store a lot of water vapour. At 60% humidity, the EMC is about 15% (see Figure 1). In a cubic metre of timber – dry weight of about
58 — December 2015/January 2016 — Build 151
FEATURE SECTION