Why a Drainage Cavity Behind Cladding Matters So Much

This is the least visible and most important detail in external cladding, and it is the one most often compromised to save a few hundred dollars.

Water always gets in

No cladding system is watertight. Wind-driven rain finds its way behind boards through joints, around fixings, past flashings and through capillary action in tight gaps.

This is not a defect. Every cladding manufacturer assumes it. The system is designed around it.

The question is not whether water gets behind the cladding. It is what happens next.

How a rainscreen works

A properly built wall has four layers working together:

1. The cladding. Sheds most of the water and takes the UV and impact.
2. The cavity. A gap, usually 18 to 20mm, formed by vertical battens. Water that gets past the cladding runs down this gap.
3. The sarking. A breathable membrane over the frame. It stops water reaching the timber while allowing vapour to pass outward.
4. The frame. Stays dry.

The cavity must be open at the bottom so water can drain out, and vented so air can move through and carry away moisture as vapour.

That airflow is what dries the assembly. A cavity that is sealed top and bottom is just a gap.

What happens without one

Cladding fixed directly to the frame over sarking, with no cavity, gives water nowhere to go. It sits against the membrane and the framing.

Over time:

• Timber moisture content rises and stays high
• Above roughly 20% moisture content, decay fungi become active
• The bottom plate and the base of studs go first, because water runs down
• Fixings corrode, which accelerates local damage
• Mould develops in the wall cavity, which becomes an indoor air quality problem

None of this is visible from outside until it is advanced. By the time paint bubbles or a floor feels soft, the frame has been wet for years.

Where cavities get compromised

Blocked at the base. Mortar, render or a paved path built up over the cavity opening. The drainage path is sealed and the wall cannot dry.

No vermin mesh, so it gets blocked. The base opening needs mesh to keep insects out while letting water and air through.

Battens installed horizontally. Horizontal battens dam the water. They must run vertically, or be notched to allow drainage.

Insulation stuffed into the cavity. The cavity is not for insulation. Insulation belongs between the studs.

Flashings that direct water into the cavity rather than out of it. Window head flashings in particular must turn out over the cladding, not behind it.

Where it matters most

Every wall benefits, but the ones that suffer most without a cavity are:

• South-facing walls in Newcastle, which get less drying sun
• Walls exposed to prevailing weather
• Coastal walls, where salt-laden moisture is relentless
• Two-storey walls, where more water runs down the lower portion
• Any wall with a lot of penetrations

What to ask your builder

• Is the cladding installed over a drained and vented cavity?
• What batten spacing and depth?
• Is there vermin mesh at the base?
• Is the cavity open at the top for airflow?
• What sarking is being used, and is it breathable?

Any competent builder will answer these immediately. Hesitation is informative.

The cost

Adding a proper cavity to a recladding job costs roughly $6,000 to $12,000 on an average home, mostly in battens and labour.

Replacing a rotted wall frame costs several times that, and it means stripping the cladding you just paid for.

We build every cladding job over a drained and vented cavity, because it is the difference between cladding that lasts forty years and cladding that quietly destroys the wall behind it.