Mould Taxonomy

The Fungal Kingdom

Mould is not a plant and it does not photosynthesise. It belongs to the fungi, a kingdom of its own, and fungi feed the way animals do: by breaking down something that is already there. A fungus secretes enzymes onto its food, dissolves it externally, and absorbs the result. Everything a mould touches is, potentially, a meal.

The black growth on window frames, silicone beads and uPVC sills is most often Aureobasidium pullulans, with Cladosporium close behind. Both are everywhere in the outdoor air. Both are dark because they produce melanin, the same pigment that darkens human skin, and for the same reason: protection from ultraviolet light.

Digesting Your Home

Plastic is not food. Plasticiser is. Flexible PVC and most silicone sealants contain plasticising compounds, oily esters blended into the polymer to keep it soft and workable, and those compounds are a carbon source that certain fungi have learned to use. Aureobasidium in particular is well documented growing on plasticised PVC and drawing the plasticiser out of it.

So the mould is not eating your window. It is eating the ingredient that keeps your window's seals flexible, which in practice amounts to the same thing.

Seal Embrittlement

As plasticiser is lost, the sealant hardens, shrinks and loses its grip on the glass and frame. It cracks along its length, pulls away at the corners, and stops doing the one job it was installed for. Water finds the gap. Draughts follow. The thermal performance of the whole unit degrades, and a sealed double-glazed unit that loses its perimeter seal will eventually mist between the panes.

The frame itself suffers more slowly but in the same direction. Surface plasticiser loss on uPVC shows first as chalking, then as a brittle, easily marked surface that stains more readily and cleans less well each year.

The Bleach Illusion

Household bleach makes black mould disappear in seconds, and that is the whole problem with it. Bleach destroys the melanin pigment on the surface. It does not reach the hyphae, the fine feeding threads that have grown into the textured surface of the sealant, and on a porous or textured material bleach is largely spent before it gets there. The colony goes colourless, carries on feeding, and is visibly back within weeks.

Repeated bleaching also attacks the sealant and the frame directly. Chlorine is not kind to silicone or to PVC, and the net effect of a bleach habit is a seal that is both still colonised and now chemically degraded as well.

Health and Air Quality

Mould on the outside of a window is a moisture indicator before it is anything else. It tells you that the frame, the sill or the drainage slots are holding water. It also sits immediately beside the trickle vents and openings through which a house breathes, and mould spores are a recognised respiratory trigger, particularly for people with asthma and for young children.

Exterior mould rarely causes an indoor problem on its own. But a frame that grows it outside is a frame whose drainage and seals deserve a look, because the same conditions are often producing condensation on the inside face as well.

Polymer-Safe Decoupling

The treatment has to reach the hyphae without stripping the plasticiser that the mould has not yet taken. We use a fungicidal cleaner formulated for plastics and sealants, held at a pH that PVC and silicone tolerate, applied with gentle agitation so it works into the textured surface, and rinsed clean. No solvents, no abrasives, no chlorine.

Then the moisture source is addressed, because a treated frame that still holds water will be recolonised from the air within a season. Clearing blocked drainage slots, checking the sill fall and identifying where condensation is forming are part of the job, not an optional extra.

How to Identify It

Black or dark grey speckles, spots or smudges on window seals, the corners of frames, sills and the rubber gaskets around the glass. It usually starts in the bottom corners and along the lower edge, where water sits longest. Wipe it with a damp cloth: surface mould smears away, while mould that has grown into the sealant stays put.

What It Is Not

Traffic dirt is an even grey and wipes off completely. Perished rubber gaskets shed black crumbs or streaks when touched, which is the rubber itself breaking down. Chalking uPVC leaves a white powder on your finger. Misting or marks between the two panes of a double-glazed unit mean the seal has failed, which is a glazing fault rather than surface mould.

Where to Look on Your Property

North-facing windows, frames behind shrubs or climbers, ground-floor windows close to soil, and bathroom and kitchen windows where moist air escapes through vents. Check the drainage slots along the bottom of uPVC frames, the silicone around external doors, conservatory roof bars and the underside of sills.

What You Can Safely Do Yourself

Wash with warm water and a little mild washing-up liquid on a soft cloth or brush, then rinse with clean water. Clear the drainage slots with a pipe cleaner or a thin brush so water can escape. Avoid bleach, solvents, cream cleaners and scouring pads: they damage uPVC and sealant, and bleach only hides the growth.

When to Call a Professional

When the mould has grown into the sealant and returns within weeks of cleaning, when the sealant is cracked or pulling away, or when a whole house of windows is affected. Deeply colonised sealant sometimes cannot be saved and needs replacing, which is a glazier's job. We will tell you honestly if that is where it has got to.

How to Keep It Away

Keep trickle vents open and use extractor fans, so less moist indoor air reaches the frames. Cut back shrubs that touch the windows. Keep the drainage slots clear. And clean frames at least once or twice a year, before growth has a chance to establish in the sealant.