How it works
What the dew point is and why walls get wet
Indoor air always carries water vapour, and warmer air holds more of it. When warm humid air touches a cold surface, part of that vapour condenses. The dew point is the temperature at which air of a given humidity becomes saturated.
At 20 °C and 50 % humidity the dew point is about 9.3 °C — so any surface colder than that will be wet. In winter those surfaces are the corners of external walls, window reveals, the wall behind a wardrobe and the strip along the floor slab.
Vapour does not stop at the surface: it diffuses through the wall. If a layer inside the assembly is already colder than the dew point while vapour still reaches it, condensation forms inside the wall — the insulation gets wet, loses performance, and the render fails. The wall looks dry while it slowly degrades.
How to read the result
- U-value — heat lost per m². For an external wall in Tbilisi a working target is no more than 0.4 W/(m²·K).
- Surface temperature θsi — below the dew point, water forms directly on the wall. This is what the check point changes: plain wall, corner or behind furniture.
- Humidity at the surface — the key number for mould, which grows from 80 % with no visible condensation.
- fRsi — the temperature factor. Below the required value you have a thermal bridge, and drying the air will not help: the construction has to be fixed.
- Condensation zone — the layer where vapour pressure exceeded saturation, plus the calculated accumulation in grams per m² per day.
Why mould appears without condensation
Mould does not need a puddle — sustained humidity above 80 % at the surface and a temperature between 5 and 30 °C are enough. That is why it grows in the corner behind a sofa, where the wall is only 2–3 °C colder: the air does not mix, moisture is not carried away, and relative humidity right at the surface climbs to 85–90 %.
Two conclusions follow. First, you fight the surface temperature and the air humidity, not the stain. Second, painting over mould, hiding it behind plasterboard or washing it with bleach without fixing the cause is wasted money — it comes back within 2–4 weeks.
Georgian climate specifics
Tbilisi. Winters are mild, yet uninsulated brick and panel buildings give surface temperatures of 11–13 °C with 20 °C indoors. Combined with 60 % humidity or more, that is the classic cause of mould in corners and on reveals. Heating is often run in one room only, so the rest cools down — and that is where mould appears.
Batumi and Kutaisi. Humidity stays at 75–80 % all year. The heating season is not the only risk here: in summer air conditioning cools the wall from inside, condensation forms on the internal surface, and humid sea air feeds the structure from outside. Vapour-tight outer layers are especially harmful in this climate — the wall cannot dry.
Gudauri, Bakuriani. Design temperatures reach −6 °C and below. Without 100–150 mm of external insulation and correct vapour control in timber assemblies, any mistake soaks the insulation within a single season.
When proper ventilation is not an option
Air exchange is the only measure that removes excess vapour without touching the walls. But in older Tbilisi buildings the duct is often blocked or absent, and a full supply-and-extract system runs into ceiling height, ductwork and the renovation budget.
The answer then is a single-room device — a supply unit with heat recovery: a breather or a Prana-type recuperator. A Ø100–160 mm core through the external wall, 30–120 m³/h of fresh air, 70–90 % of the outgoing heat returned. It does not chill the room in winter, needs no ducts through the flat and is installed in a day. We size it by room volume and occupancy and install it turnkey.
Check the effect in the calculator itself: the “How to fix it” block has a heat-recovery option that recalculates the humidity in your room with the extra supply air and shows whether the mould risk disappears.
Where the limit is. The unit lowers air humidity but does not raise the surface temperature. If you have a thermal bridge and fRsi below the limit, ventilation alone will not close the case — the detail needs insulating too. The calculator shows this honestly: apply the option and see whether the risk remains.
What to do with the result
This is a steady-state Glaser calculation (EN ISO 13788). It honestly shows whether an assembly works in principle and catches the gross mistakes: internal insulation without a vapour barrier, vapour-tight render on aerated concrete, insufficient insulation thickness. It does not model seasonal moisture accumulation, rain, solar gain or rising damp — disputed details need a transient simulation and an on-site survey.
Save the link to your calculation or send us the report — we will review the detail, check the assembly and propose a solution that actually dries out.
What indoor humidity should I keep in winter?
Between 40 and 55 %. Below 35 % the air feels dry and furniture shrinks; above 60 % mould risk starts on cold parts of the walls. A hygrometer costs less than any repair.
Can a wall be insulated from the inside?
Yes, but only by calculation. Internal insulation leaves the structural wall in the cold zone, so you need a vapour barrier on the room side (sd from 10 m, ideally a smart membrane) with every joint sealed, or a vapour-open mineral system. XPS fitted by guesswork without vapour control is the most common cause of soaked masonry.
Why is there mould only in the corner while the wall is dry?
The corner is a geometric thermal bridge: it loses heat in two directions, its surface is 2–4 °C colder and the air stagnates. Select the “Internal corner” check point in the calculator to see how far the surface temperature drops and the surface humidity rises.
Does a dehumidifier help?
As a temporary measure yes, especially while things dry after building work. But if the cause is a thermal bridge or missing ventilation, the dehumidifier will run forever. Calculate the construction first, then decide whether you need equipment.
Is airing the room more often enough?
Often it is. Opening windows wide for 5–10 minutes 3–4 times a day removes excess vapour without over-cooling the walls. Check it in the “Room moisture balance” block: it shows the humidity your air exchange settles at and what is needed for 55 %.
What if the façade cannot be touched?
Then we work from inside: vapour-open insulation systems, local insulation of corners and reveals, restored vapour seals at window joints, trickle vents and extract fans. Every such detail is calculated separately — with this same tool, checked in the corner and behind furniture.
Will a breather or heat-recovery unit stop mould?
It helps where the cause is over-humid air: the unit supplies 30–120 m³/h and carries away the vapour from people, cooking and showers, while the heat exchanger returns 70–90 % of the heat, so winter stays comfortable. If the cause is a thermal bridge — low fRsi, a cold corner or reveal — ventilation alone is not enough and the detail needs insulating. Try the heat-recovery option in the calculator: it recalculates your room and shows whether the risk goes away.
Send the report or call us — an engineer will review the construction, measure the surface temperature and propose a solution with a warranty.
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