Glazing keeps the rain out, but it also creates a cold, closed box in which moisture has nowhere to go.
An unglazed balcony practically never has a mould problem. It is an outdoor structure: the wind dries it, the rain runs off, and moisture does not accumulate anywhere. The problem appears precisely when the balcony is glazed.
Glazing creates a closed volume. That volume is not heated, so in winter it is only a few degrees warmer than outside. At the same time humid air enters it — from the flat every time the balcony door is opened, from laundry hung there, from rain and snow that have come in through gaps, and from the things stored there.
The result is a space where air humidity is high, surface temperatures are low and there is almost no air exchange. Condensation settles on the cold surfaces — the glass, the frames, the lower part of the external wall, the ceiling and the floor edges — and stays there. On a balcony in winter you can regularly see the glass misted or frosted on the inside; that is direct visible proof that humidity in the space is too high.
Nobody heats a balcony and nobody spends time in it day to day — so a situation that would be noticed within days in a room can persist on a balcony for a whole season.
A misted window or a patch in a corner is noticed quickly in a living room. On a balcony, where in winter you go once a fortnight for potatoes or winter tyres, the problem is only noticed in spring, when the spring clean starts.
A glazed balcony can work in three ways, and only two of them are trouble-free.
The middle way is what people choose most often, because it is the cheapest option. In practice it means the balcony works as a large condensation surface all winter.
Partial insulation deserves a separate mention. If the balcony walls and ceiling have insulation fitted without a properly built vapour barrier on the room side, humid air gets inside the insulation and settles on the cold concrete surface behind it. The visible finish can then look perfectly normal while exactly what would happen on an exposed wall is happening behind it. Cases like this only come to light when the finish is removed — or when a smell appears.
| Type of balcony | Moisture risk | What to do |
|---|---|---|
| Unglazed | Low | Nothing needs doing — the wind dries it |
| Glazed, uninsulated, with airing | Low | Keep airing it in winter too |
| Glazed, airtight, unheated | High | Air it regularly, do not dry laundry, remove cardboard |
| Partly insulated, no vapour barrier | Very high | Inspection — the moisture may be inside the insulation |
| Fully insulated and heated | Low | Maintain air exchange as in a room |
A common additional mistake is drying laundry on a glazed balcony in winter. One washing machine load gives several litres of water back into the space. In summer with the sashes open that is not a problem. In winter on a closed balcony it is a direct route to condensation.
A balcony usually serves as storage, and the stored items are most often the first things mould appears on. The reason is simple: they are organic, porous and nobody moves them.
A practical principle: if a thing is organic and porous, then on an uninsulated balcony it belongs in a sealed plastic box, or somewhere else. And whatever the material — leave a gap between the items and the external wall so air can move behind them.
Another frequent factor is the floor covering. Balconies often get a rug, artificial grass or a timber deck, and moisture builds up beneath it and does not dry out. A rug on a cold balcony is the same problem as mould under floor coverings in a flat, only without a leak — condensation and rainwater coming in through gaps are enough.
If the balcony really is used as storage and you are not going to give that up, the minimum programme is this:
A balcony slab is a reinforced concrete structure passing straight through the building's external wall. It cools from the outside, and that cold is conducted into the room through the concrete. So even when the balcony itself is fine, a patch often appears in the adjoining room — typically at the floor edge along the balcony wall, in the balcony door reveal or in the room corner by the balcony.
Recognising this situation is fairly simple. The patch is not in a random place but precisely where the slab crosses the wall — often in a narrow, almost straight strip. It appears in the cold season and seems to disappear in summer. And it comes back in exactly the same place, however many times it is painted over.
This is a structural matter and we do not call it anything else. Treatment cleans the surface, but it does not remove the cold bridge. The real fix is insulating the external wall in that zone, which is usually dealt with at building level. More on causes of this kind — on the page about the causes of mould.
We measure air and material moisture on the balcony and in the adjoining room, and look for the coldest surfaces.
We recommend removing cardboard boxes, textiles and affected timber items — without that the treatment is incomplete.
If the fungus has got into the finish, the insulation or timber cladding, we remove the affected section.
We treat the walls, ceiling, floor and a band in the adjoining room if it is affected.
Airing routine, storage solutions and, if the cause is a cold bridge, a direct statement that insulation is needed.
Inspection from €40. Treatment from €1.50/m², minimum order from €80. A balcony is usually a small area, so the work often falls within the minimum order. If the adjoining room wall is affected too, or ozone treatment is needed because of a smell, we include that in the quote after the inspection. For the order of work, see the section on mould removal.
Yes, and winter is when it matters most. Cold outdoor air is dry, so brief airing takes moisture out. If the glazing is airtight and the sashes stay shut all winter, the moisture stays inside.
In summer with the sashes open, yes. In winter on a closed balcony it is not advisable: one load gives several litres of water back into the space, and it settles on the cold surfaces.
Often yes, indirectly. The balcony slab acts as a cold bridge that carries cold into the room, and condensation forms on that cooler zone. Treatment cleans the surface, but the lasting fix is insulating that zone.
Describe the situation — we will tell you whether treatment is needed at all, and which one.
Fill in the form — we will call you on the number you provide, ask a few clarifying questions and propose a solution.
Thank you! We will contact you on the phone number you provided shortly.
