Page 61 - Build 151

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Figure 1: Sorption curve for timber.
500 kg – that translates to 75 kg of water stored.
A cubic metre of air at 60% humidity and 20°C would only contain about 0.01 kg of water vapour. This means that a house lot of timber framing can safely absorb hundreds of litres of water. This is because there is roughly a 10% safe-working region of mois- ture content above normal in-service levels.
These points mean timber can act as a hygric bu er as moisture is stored in it and then released when conditions permit. WUFI, and programs like it, can take this bu ering into account because they utilise a much wider range of material properties, including the sorption curve, than the Glaser method.
Adding New Zealand materials to WUFI
The di culty lies in choosing a material from the database that adequately describes the material being used. This is an area for improvement BRANZ is looking to address by adding some speci c New Zealand mate- rials for use in WUFI (see WUFI develop- ment project on pages 52–53).
Variable climate conditions
The example in Build 147 used steady inte- rior and exterior conditions for a Glaser calculation. In reality, the indoor and outdoor climate varies.
In WUFI, you can apply any climate you like to the assembly you are looking at, including a selection of built-in New Zealand climate les and presets for indoor climates. As with the materials, the di culty lies in choosing what is appropriate.
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Relative humidity (%)
Users of WUFI often pick a sinusoidal indoor climate, which slowly varies over the year. This is often an oversimpli cation for New Zealand construction to the point where it leads to meaningless results.
Linking indoor and outdoor climate helps
A more realistic option is to link the indoor climate to the outdoor climate somehow or at least superimpose some daily variation onto the yearly sinusoid.
One of the selectable options in WUFI is to use the indoor climate from ASHRAE 160-2009 Criteria for moisture-control design analysis in buildings.
This option links the indoor climate to the outdoor conditions and takes account of moisture generation rates and heating in the building.
What’s typical for New Zealand?
Using ASHRAE 160-2009 would be a better choice for many New Zealand users than the basic sinusoids, but the question remains, ‘What is a typical indoor climate for New Zealand homes?’
Ascertaining this is likely to require a signi cant amount of eld measurements from around the country. A new BRANZ project is working on this (see WUFI develop- ment project on pages 52–53).
Findings in next article
In the next Build article on vapour control, we will look at the project’s ndings and what they might mean for designers and builders.
Build 151 — December 2015/January 2016 — 59
Equilibrium moisture content (%)