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Whether we like it or not New Zealand is now known as the ‘leaky building capital of the world’.
A potent mix of exceptionally wet climatic conditions coupled with poor building design, inadequate product performance and sub-standard installation has resulted in this country now facing billions of dollars in repair work- and new cases continue to occur every week!
The exterior wall cladding system is the first line of defence for entry of water into a building. With good design and correct installation we can reduce the chance of moisture entering and lingering in the building envelope and hence help to eliminate ‘leaky building syndrome’. Here are some tips to optimise the performance of weatherboard cladding in wet and humid environments.
1. Consider the ‘4 D’s’ principle in cladding system design
Deflection – by eaves and cladding
Drying – by diffusion and ventilation
Drainage – of water from behind cladding
Durable – materials for the conditions
2. Use a cavity system
By installing weatherboards onto a vented cavity system, moisture that enters the wall is able to move freely out of the wall, hence reducing chance of entrapment. Abodo recommends 20mm thick cavity batten, though BRANZ research suggests that as little as 10mm cavity will do the job. A cavity closer with 1,000mm2 /lm opening area must be used at the base of the wall to stop vermin.
3. Choose the right cavity batten
For horizontal fixed cladding, regular H3 treated timber cavity battens can be used. For vertical fixed cladding, Abodo offers a proprietary castellated (notched and bevelled) H3.2 CB-H-20×45 horizontal cavity batten- this allows movement of moisture and air up and down the cavity. Cavibat fluted polypropylene battens may also be used vertically or horizontally.
4. Include eaves in the building design
Over 90% of leaky homes in New Zealand have eaves less than 100mm width.
5. Apply a flexible wall underlay (building wrap)
It is important to make sure that the underlay is fixed and restrained properly so that it doesn’t bulge out into the cavity and that the correct taping details are used around windows, doors and penetrations into the wall. Remember that building wrap is only a tertiary line of defence and should be designed to be ‘dry in service’.
6. Continuous air seals only
Continuous air seals are required around openings into the wall such as around windows and doors. This usually takes the form of sealant onto PEF back rod. It is important to make sure that the seal is continuous and not broken at any point.
7. Avoid capillary action
Capillary action generally refers to the forced movement of water due to high air pressure on outside of wall and low pressure on inside- often into places we don’t want it such as behind the cladding. It sometimes occurs due to inadequate gaps left between adjacent materials such as bottom of cladding to head flashing, between cavity battens and laps on weatherboards. The recommended gap does vary according to the application, but as a rule around 5-6mm will prevent capillary action.
8. Seal off the wall from roof space
Water vapour rises, therefore it is crucial to make sure that the wall cavity is completely sealed off from the roof space. If moist air mixes with the cool air in the roof, then condensation of water will lead to excessive moisture at the top of the wall- potentially leading to decay of building elements.
9. Top-vented cavities are a good option
While they are an alternative solution to E2/AS1, testing by BRANZ has confirmed that top-venting cavities allow greater drying potential than bottom-only vented cavities. Top- venting allows buoyant water vapour to escape outside rather than become trapped at the top of the cavity. It also allows equalisation of pressure in the cavity so that it performs in the correct manner.
10. Rigid wall underlays/ Rigid Air Barrier (RAB)
For the ultimate cladding system, apply a rigid air barrier to wall framing prior to installation of the cavity and weatherboards. There are a number of proprietary systems on the market including Magnesium oxide board (e.g.: Magnum Board), fibre cement (e.g. James Hardie HomeRab) or plywood (e.g.: ecoply Barrier). Rigid wall underlays are required in extra high wind zones. They also allow the building to be closed in a lot quicker at construction time, so interior finishing can begin earlier. Fibre cement and Magnesium oxide board can also offer increased fire performance (subject to design conditions).
11. Breatheable wall systems are the best
The Forever Breathe wall system offers the ultimate in healthy building design by allowing moisture to move through the interior lining, wall insulation and out through breathable wall cladding such as Abodo’s weatherboard eco cladding.