Drying in wall cavities
This is a part of the
Weathertightness
feature
Weathertightness research completed by BRANZ over the last 6 years has shed light on evaporative drying in wall cavities, indicating that climate, vent sizes and cavity depth all play a part.
There have been regular updates in Build on BRANZ’s ongoing weathertightness project measuring the moisture properties of 24 specimen walls with different cavity types in BRANZ’s experimental building and of collaborative work with the Fraunhofer Institute in Germany putting the physics of moisture into a single computer program.
The project provided many scientific challenges along the way. Perhaps the most significant involved understanding ventilation inside wall cavities. Starting with very little knowledge, new light was shed on evaporative drying in wall cavities and how this depends on cavity dimensions, vent sizes and climate.
4Ds model
The research has supported the Canadian 4Ds weathertightness model that underpins the weathertight design solutions in New Zealand Building Code E2/AS1. The 4Ds identify key components in a roof or wall system that deflect incident rain from the cladding, drain and dry any water leakage from the back of the cladding and use appropriately durable materials (see pages 16–17). Here we look at deflection, drainage and drying.
Deflection – how weathertight are weatherboards?
The water leakage properties of 11 weatherboard walls were measured in the laboratory (brick veneer was used as a reference). The largest leaks in timber weatherboards were at knots and cracks around fixings and not as might be expected at lap joints. In metal and plastic weatherboards, the most significant leaks were at butt jointers.
Although weatherboard walls have been shown to leak, they have a successful track record in New Zealand. This is partly due to drainage and ventilation drying paths that were shown in the experimental building to help control water leaks.
Drainage
A non-absorbent drainage path is very important. Water drainage trials in all the walls applied water steadily to the back of the cladding at 100 g/hour for 8 hours. It was found that more than 90% drained out from non-absorbent claddings and as little as 10% from absorbent claddings.
There is no doubt that walls with absorbent claddings lose an important opportunity to control water leaks and will have to rely more on ventilation drying. However, ventilation drying is very effective.
Drying
Ventilation and drying rates were measured in cavities from water absorbed in wall claddings. The evaporation rates were surprisingly high (from 10 g/day/m of wall in bottom-only vented cavities up to hundreds of g/day/m of wall in top and bottom vented cavities). These are useful drying rates that add moisture tolerance to a wall.
It might be argued that all wall cavities should be top and bottom vented to capture the higher drying rates, but the risk of water entry through top vents, which can be difficult to rainscreen unless they are sheltered under eaves, needs to be balanced against the actual need for additional ventilation drying. Top vents certainly help when the cladding is both absorbent and prone to some leakage (for example, brick veneer), but the jury is still out on whether other claddings would benefit from the higher drying rates.
HOW BOTTOM-VENTED CAVITIES WORK
Looking at the physics of ventilation in a cavity vented at the bottom might suggest that ventilation will be very low. That is because all of the vent area is at a single pressure rather than at least two pressures to encourage cross-flow ventilation. Using tracer gases, much higher ventilation rates were found than expected.
It turns out that there are gaps other than vents (for example, gaps between battens and claddings and at the head of the cavity) that add alternative ventilation paths, delivering more ventilation than expected. In some walls, they are similar to cavities with top and bottom vents.
IMPORTANCE OF VENT SIZE AND CAVITY DEPTH
Cavity depths have to be quite small before they start to limit cavity ventilation rates. That is not surprising because cavity depth is primarily chosen to avoid moisture bridging paths to the building wrap. For example, the cavity depth in a brick veneer wall will need to cope with mortar irregularities, whereas in a fibre-cement clad wall, the deciding issue is likely to be insulation bulging the wall wrap towards the cladding. This leads to cavities that are wider than strictly necessary for drying.
That might suggest that enlarging the vents to make better use of existing cavity sizes is a good idea, but cavities already have a high capacity for ventilation drying. If you think ventilation is inadequate, it might be better to look at the competence of the cladding.
LIMITED DRYING OF WET FRAMING
Frame drying measurements in the experimental building showed cavity ventilation helped dry out wet framing in the summer months, but only marginally. This is because moisture diffuses through timber relatively slowly, and this turns out to be the rate-limiting step for getting rid of frame moisture. Warmer summer temperatures allow the wall to capitalise on the faster drying from the cavity.
LOCATION, SEASON AND ORIENTATION
There is a 2:1 difference between cavity drying on north and south walls and a similar difference between summer and winter. In comparison, differences due to the building location are comparatively small.
Research continues
The weathertightness research programme has a full agenda, and you can expect to see new information emerging on:
- drainage media walls – how drainage and drying performance compares with conventional cavities
- rigid air barriers – how and where they add to weathertight performance
- weather grooves – a clarification of their role and application
- direct-fixed claddings – taking better account of ventilation and drainage in the risk score associated with weatherboard walls
- education – creating better educational material that shows interactively how design choices influence moisture management.
For more
This research project was funded by the Building Research Levy and the Foundation for Research, Science and Technology.
Download the PDF
Articles are correct at the time of publication but may have since become outdated.
