Some of the most expensive mould problems we see were built into the home before anyone moved in.
Every new home goes through a stage that rarely makes the brochure. The frame is up and the roof may be on, but the walls are still open. There’s no wrap, no insulation and no plasterboard yet. Then it rains for a few days, as it does.
If water is running down the inside of the walls or pooling on a slab under a finished roof, something isn’t right. The timber and OSB (oriented strand board) bracing the walls are sitting bare and soaking up water.
It’s easy to assume it will all dry out. Sometimes it does. But if those materials are still wet when the walls are insulated and lined, the moisture gets sealed in, out of sight, for good.
What a soaking does to the structure
OSB is made from wood strands bonded with resin and wax. Kept dry, it performs well. Left wet for long periods, it swells, especially along the cut edges, and the strands can start to separate. Much of that swelling doesn’t reverse when it dries, so the board can end up thicker and weaker than it was designed to be.
Structural timber has a limit too. Once its moisture content rises above about 20%, decay fungi and mould have the conditions they need. Long periods of saturation can also lead to warping, cupping and loss of strength.
Timber that gets wet and then dries out in the open air is usually fine. The trouble starts when it’s closed up inside an insulated, lined wall before it’s had the chance to dry.
The mould you’ll never see
Mould needs only two things: moisture and something organic to feed on. Wet timber and OSB give it both. Given steady moisture, mould can start growing within 24 to 48 hours.
Once the walls are wrapped, insulated and lined, that growth is hidden inside the cavity. The home looks perfect at handover. Months or years later, the owners may notice:
- a musty smell they can’t trace
- stains or bubbling paint on walls and ceilings
- damp, cold patches or mould around skirting boards and corners
- allergy-type symptoms, coughing or headaches that ease when they’re away from home
By then the moisture has had a long time to do its damage, and the fix often means opening up walls.
Building or renovating? Ask these questions
This can happen on a traditional site build and on a prefab or panelised build. The construction method matters less than how well the structure is protected while it’s open to the weather. Before work starts, ask your builder:
- How long will the frame sit exposed before the building is weathertight?
- How will the timber and OSB be protected from rain during that time?
- Will moisture levels in the framing be checked before insulation and linings go in?
- What happens if the frame gets soaked? Who decides it’s dry enough to close up?
- How will the finished home be ventilated so everyday moisture has somewhere to go?
A good builder will have clear answers. If they can’t tell you, that’s worth knowing before the walls are closed.
Already moved in? Here’s how we can help
A newer home with a musty smell, recurring mould or damp corners isn’t something to live with. At Clean Air Solutions Australia, our two brands tackle both sides of the problem.
MouldPlus Environmental: find it and remove it. We inspect for hidden moisture and mould, including inside wall cavities and roof spaces, then safely remediate affected areas and recommend a lasting fix.
Ventit: stop it coming back. Even a perfectly built home needs somewhere for moisture to go. As the Official Australian Partner of Blauberg Ventilation, Ventit designs and supplies continuous, energy-efficient ventilation that removes damp air and keeps fresh air moving.
Whether you’re planning a build or worried about the home you’re in, we’re happy to talk it through.
Call 1300 100 113 or visit mouldplus.com.au and ventit.com.au.
