There are certain rituals that define a British winter, and not all of them involve mulled wine or the orange glow of a wood burner. Some involve pulling back the curtains on a grey November morning to find the windows streaming with condensation, the sill puddled with water, and – if the season has really been making its point – a dark fringe of mould already establishing itself along the frame. It is the kind of sight that activates a very specific category of domestic dread: the knowledge that this is not a one-off event to be dabbed at with a piece of kitchen roll and forgotten, but a seasonal problem that, left unmanaged, has the potential to become a genuinely serious one.
For those living in poorly insulated flats – which, in London, covers a considerable proportion of the housing stock, from Victorian conversions with single-glazed sash windows to 1970s purpose-built blocks whose thermal performance has aged rather less gracefully than everything else – condensation is not a winter inconvenience. It is a winter fixture. Understanding why it happens, what it does over time, and how to manage it properly is the difference between a damp windowsill and a mould problem that has migrated into the plaster, the curtains, and, in the most dispiriting cases, the ceiling.
Why Poorly Insulated Flats Are Condensation Traps
The Physics Nobody Explains
Condensation is not a mystery, though it is rarely explained in terms that make the practical implications clear. Warm air holds moisture – the warmer the air, the more of it it can carry. When that warm, moisture-laden air comes into contact with a cold surface, it cools rapidly and can no longer hold the same amount of water. That water has to go somewhere, and it goes onto the surface. In winter, the coldest surfaces in any flat are invariably the windows and exterior walls.
In a well-insulated flat with double or triple glazing, the interior surface of the glass stays relatively warm even on a cold day, because the insulating layer of air or gas between the panes buffers it from the outdoor temperature. In a poorly insulated flat – or, more commonly in London, one with single-glazed sash windows that were considered perfectly adequate when installed in 1887 – that buffer does not exist. The interior glass surface is essentially at outdoor temperature, and the warm air inside the flat deposits its moisture there with reliable, damaging efficiency.
Add to this the particular dynamics of flat living: multiple people in a small space, cooking, bathing, drying laundry indoors, simply breathing – all of which add significant moisture to the air – and the cold-surface problem becomes considerably more acute. The flat is not doing anything wrong. The physics is simply doing exactly what it always does.
Why Condensation and Mould Are Not the Same Problem
The Timeline From Damp Window to Black Spores
Condensation and mould are related, but conflating them leads to the kind of reactive cleaning that treats visible symptoms while the underlying situation quietly continues.
Condensation is water. On its own, and managed promptly, it causes limited damage: wet sills, timber frames that swell and stick over successive winters, and the general unpleasantness of a perpetually damp-feeling room. The problem escalates when condensation is left to sit – on sills, in the corners of frames, in the folds of curtains that touch the glass – providing the persistent moisture that mould spores require in order to germinate and establish a colony.
Mould is a living organism. It needs three things to thrive: moisture, a surface to colonise, and warmth. A window that is consistently wet through a winter provides the moisture. The paint, timber, sealant, and plaster of the surrounding frame and wall provide the surface. The heated interior of the flat provides the warmth. Once mould takes hold in window frames and sills, it can spread to soft furnishings, curtains, plaster, and – in serious cases – into the structural fabric of the wall itself.
This is why daily condensation management matters. It is not fastidiousness for its own sake. It is the removal of moisture before it becomes the foundation for something considerably harder, and more expensive, to address.
Cleaning Condensation Properly – The Daily Ritual
The Right Tools, in the Right Order
The goal of a morning condensation wipe-down is not to redistribute moisture across the glass – which is precisely what happens when the wrong tools are used – but to remove it from the window, the sill, and the frame entirely.
A small rubber-bladed squeegee is the most effective single tool for this job. Worked from top to bottom in overlapping strokes, it moves the bulk of water off the glass surface quickly and cleanly, directing it downward where a cloth can collect it. Squeegees designed for shower screens work perfectly well on windows and are cheap enough that keeping one per affected room is an entirely reasonable investment.
Follow the squeegee with a dry microfibre cloth to collect the water pooled at the base of the glass and along the bottom seal. Then – and this is the step most consistently skipped – wipe the sill and frame thoroughly. The frame holds moisture in its joints and corners long after the glass has been cleared, and this is precisely where mould first gains its footing. A damp cloth is not sufficient here. Use a dry one, or one barely dampened, pressed carefully into the corners and along the joins where timber or uPVC meets the surrounding wall.
The whole process takes roughly ninety seconds per window. Performed every morning through the colder months, it produces a result over a full winter that is very difficult to achieve any other way.
Treating Mould That Has Already Appeared
Surface Mould vs. Embedded Mould
When mould has established itself on window frames, sills, or the adjacent plaster, the approach depends entirely on how deeply it has penetrated the surface.
Surface mould – present for a relatively short time, sitting visibly on paint, sealant, or timber without having discoloured the material beneath – responds well to undiluted white vinegar applied and left to work for at least an hour before being scrubbed away with an old toothbrush or firm cloth. White vinegar kills a broad spectrum of mould species without the harsh fumes of bleach-based products and is safe to use around most painted surfaces and sealants. Rinse thoroughly with clean water afterwards and dry the area completely. Leaving any moisture behind after treatment simply recreates the conditions that allowed the mould to establish itself in the first place.
For sealant that has developed mould within it rather than on it – visible as dark discolouration that refuses to shift regardless of cleaning – the sealant needs to be cut out and replaced. This is a maintenance task rather than a cleaning one. Treating the surface of compromised sealant yields temporary improvement at best, because the mould source remains intact beneath.
Where mould has penetrated painted plaster or timber, a solution of one part bleach to four parts water, applied carefully in a well-ventilated room, addresses most surface-penetrating cases on paintable surfaces. Wear gloves, protect the surrounding area, and air the room thoroughly during and after treatment. Where mould has reached bare plaster or clearly extends beyond the decorative surface, a professional assessment is the appropriate next move.
Reducing Condensation at the Source
Ventilation, Heating, and the Habits That Matter
The daily wipe-down manages condensation once it has formed. Reducing the moisture load in the air to begin with lessens the scale of the problem significantly – and, in a poorly insulated flat, represents the most effective long-term intervention available without structural work to the building.
Ventilation is the most powerful lever here, and also the one most resisted during winter for entirely understandable reasons. Trickle vents – the small adjustable slots fitted along the top of many window frames – should be left open year-round, including in January. Their effect on room temperature is negligible, but their contribution to moisture management is substantial. Where trickle vents are absent, opening a window briefly after cooking, bathing, or a drying session achieves considerably more than most people expect.
Consistent, moderate heating makes a greater difference to condensation than most domestic heating habits allow for. Cold surfaces condense moisture; warmer surfaces condense far less of it. A flat kept at a steady 18 or 19 degrees will generate less condensation than one that drops to 12 overnight and climbs to 22 in the evening, because those temperature swings create precisely the cold-surface conditions that moisture is waiting to exploit.
Drying laundry indoors in a London flat in January is, in most cases, unavoidable. Where possible, confine it to one room with the door closed and a window open – directing the moisture outward rather than allowing it to distribute itself freely through the rest of the flat.
When the Problem Is the Building
There are flats in London where condensation and mould cannot be resolved through cleaning and daily management alone, regardless of how consistent or thorough the effort. A flat with single glazing throughout, no cavity wall insulation, and no mechanical extraction in the kitchen or bathroom will generate moisture faster than any individual can realistically remove it. In these cases, the cleaning work still matters – it prevents moisture from becoming established mould on surfaces where it would otherwise go unchecked – but it is working against a thermal failure that sits well beyond the occupant’s ability to fix.
Condensation-related mould is generally the tenant’s responsibility to manage on a day-to-day basis. Structural damp – rising damp from the ground, penetrating damp from a failing roof or defective exterior wall – is the landlord’s. The two can look superficially similar, particularly at window level where they sometimes coincide, and distinguishing between them matters both practically and legally. Getting a professional assessment to establish which problem is actually present is worth doing before either side of a landlord-tenant relationship draws firm conclusions and begins assigning blame.