Mould appearing in a consistent pattern on interior walls, at corners, along skirting boards, around window frames or in repeating lines that mirror the wall frame behind, is often not a ventilation problem or a hygiene problem. It’s a physics problem: thermal bridges are creating cold spots that cause localised condensation, and that condensation is sustaining mould growth in exactly those locations.
Understanding thermal bridging explains patterns of mould that otherwise seem random, and points to solutions that no amount of cleaning or ventilation alone will resolve.
What Is a Thermal Bridge?
A thermal bridge is a localised area of a building’s envelope, walls, roof or floor, where heat flows more readily through the structure than in surrounding areas. The result is a surface that is significantly cooler than surrounding surfaces when the outside temperature drops.
When warm, moist indoor air contacts that cool surface, it reaches its dew point and deposits condensation. Condensation on surfaces, even intermittent, invisible condensation, provides the sustained moisture that mould requires to grow.
Why Building Materials Create Thermal Bridges
All building materials conduct heat to some degree. The problem occurs when a highly conductive material (steel, concrete, timber) penetrates or links two faces of an insulated assembly:
Steel stud framing, in steel-framed Wollongong apartments and commercial buildings, each steel stud is a thermal bridge from the cold outer wall face to the inner plasterboard. Steel conducts heat 300-400 times better than insulation. Even with insulation between studs, the studs themselves conduct enough heat to create surface temperatures 3-5°C cooler than the insulated bays.
Concrete columns and beams, in concrete-frame construction (common in Wollongong high-rises and commercial buildings), concrete elements that penetrate the insulation layer bridge from outside to inside. The internal face of these elements runs cold.
Slab edges, the concrete floor slab typically projects to the outer face of the building. This slab edge loses heat to the outside and creates a cold band at floor level where the slab meets the external wall. In older Wollongong homes, this manifests as a consistent band of mould at the skirting board level around perimeter rooms.
Timber wall framing, less severe than steel but still a bridge. Timber plates, studs and noggings are more conductive than insulation batts, and in homes with no insulation, the framing creates a consistent pattern of moisture deposition that produces the repeating stripe mould patterns seen in some older homes.
Window frames, aluminium frames without thermal breaks are severe thermal bridges. Even double-glazed windows in aluminium frames can have condensation running off the frame itself in cold weather.
How to Recognise Thermal Bridge Mould Patterns
Thermal bridge mould has characteristic patterns that distinguish it from other mould types:
| Mould Location | Likely Thermal Bridge | Other Indicators |
|---|---|---|
| Regular vertical stripes on wall | Wall studs or steel framing | Pattern matches framing spacing (450/600 mm) |
| Horizontal band at skirting level | Slab edge | Present on external walls only |
| Corner mould, ceiling to floor | Corner framing, multiple studs | Always in corners, not on flat wall surfaces |
| Frame of every window | Aluminium window frame | Condensation visible on frame in winter |
| Along top of wall at ceiling | Top plate thermal bridge | Consistent horizontal line near ceiling |
| Around each structural column | Concrete columns or steel posts | Circular or column-shaped mould |
A thermal imaging camera during a mould inspection makes thermal bridges immediately visible, the cold zones show as dark blue against the warmer wall surface.
Thermal Bridging in Illawarra’s Climate
Wollongong has a temperate climate that is mild by Australian standards, but winter temperatures in the 8-16°C range are cool enough to create significant thermal bridging problems in:
- Poorly insulated 1970s-1990s brick veneer homes, common in Corrimal, Dapto, Albion Park and established western suburbs
- Pre-insulation era single-skin brick homes, the thermal mass of brick doesn’t protect cold corner areas
- Steel-framed apartments, Wollongong CBD and Fairy Meadow apartment buildings
- Uninsulated slab homes, the slab perimeter bridge is particularly effective in homes without perimeter slab edge insulation
The escarpment suburbs (Thirroul, Austinmer, Helensburgh) see colder winter temperatures than the coastal strip and have more severe thermal bridging problems as a result.
Solutions to Thermal Bridge Mould
Internal Insulation and Thermal Break Lining
The most effective retrofit solution is applying continuous insulated lining to the affected internal wall surfaces. This eliminates the thermal bridge effect by separating the cold outer wall from the inner surface with an insulating layer.
Options include:
- Insulated plasterboard (foil-backed or foam-backed) fixed directly to existing surfaces
- Battens and insulation batt system with new plasterboard face
- Spray foam insulation applied to the cavity behind existing plasterboard (specialist work)
The insulating layer must be continuous, any penetrations (power points, light switches) must be detailed properly to maintain the thermal break.
Improving Window Glazing and Frames
Replacing aluminium single-pane windows with double-glazed units in thermally broken frames (uPVC or aluminium with a plastic thermal break in the frame) eliminates the frame condensation problem. This is a significant cost but provides both mould prevention and meaningful heating cost reductions.
Slab Edge Insulation
For homes under renovation, perimeter slab edge insulation can be retrofitted externally during external works, rendering, cladding replacement or landscaping, by applying a layer of XPS (extruded polystyrene) or PIR (polyisocyanurate) board to the slab edge before the final finish. This is much easier to do during other external work than as a standalone project.
Thermal Bridge Detailing in New Construction
For new builds in the Illawarra, the best approach is designing thermal bridges out of the building structure, continuous insulation layers, thermal break pads under structural elements, and warm-side vapour control membranes. Engaging an architect or building designer familiar with thermal performance from the design stage is far more effective than retrofitting.
Treating Mould at Thermal Bridge Locations
If thermal bridge mould is already established, surface cleaning before addressing the thermal bridge is a short-term fix, the mould will return with the next cold snap. The correct sequence is:
- Remediate the existing mould, treat affected surfaces and allow to dry fully
- Confirm the thermal bridge location, thermal imaging during cold weather confirms exactly which areas are bridging
- Apply thermal break treatment, appropriate insulation or detailing for the specific bridge type
- Repaint with anti-fungal finish, quality anti-fungal paint as the final layer
- Monitor, if the bridge is properly addressed, mould does not return at the same location
Frequently Asked Questions
I have mould in a regular stripe pattern on my walls, is this thermal bridging? Vertical stripes matching stud spacing (typically 450 or 600 mm apart) are classic thermal bridge mould from wall framing. Confirm with a thermal imaging scan, which will show the cold studs clearly. The treatment is internal insulated lining to break the bridge.
My corner walls always have mould even though I ventilate well, why? Corners in external walls are high thermal bridge areas. Multiple studs meet at corners, increasing the conductive area, and corners have less airflow from room circulation. This is a thermal problem, not a ventilation problem. Corner-specific insulated lining or thermal mass adjustments are needed.
Does anti-mould paint fix thermal bridging? It slows the mould growth at the cold surface but doesn’t address the root cause. If the surface temperature remains below dew point for significant periods, condensation continues and the anti-fungal additives in the paint eventually exhaust. Anti-mould paint is part of the solution, not the complete solution.
Is thermal bridging covered by building codes? The National Construction Code (NCC) has been progressively strengthened on thermal performance (NCC 2022 in particular), but these requirements primarily apply to new construction. Existing homes have no mandatory retrofit requirements. Older Illawarra homes built before modern insulation standards often have significant thermal bridge problems that current codes would not allow in new construction.
Can I find thermal bridges without a thermal imaging camera? With difficulty. In cold weather, you can feel the temperature difference on a cold wall, run your palm slowly across the wall surface and feel for the colder spots. Dew or condensation appears first at the coldest points. However, thermal imaging is far more accurate and finds less obvious bridges that are still causing mould problems.
Mould in a regular pattern that keeps coming back? A thermal bridge is probably driving it. Contact Illawarra Mould Removal for a thermal imaging inspection and a treatment plan that actually addresses the root cause.