Dew point
The temperature at which air of a given humidity becomes saturated and its vapour begins to condense. It is a property of the air, not of the wall.
What is the dew point and why is it not a property of the wall?
The dew point is the temperature at which air of a given humidity becomes saturated: it can hold no more water vapour, and the excess separates out as liquid water. Warm air carries far more vapour than cold air, so the dew point rises with indoor humidity and falls again when the air is dried.
The consequence routinely lost in arguments about insulation is that the dew point is a property of the air, not of the construction. There is no fixed dew-point line in a build-up that can be drawn once on a detail and considered settled. The place where the temperature falls to the dew point moves with indoor humidity and outdoor temperature: in a January frost it sits somewhere quite different from a damp November, and in summer it is usually not inside the build-up at all. Assessment under the Slovak thermal standard therefore covers the whole year rather than one design state.
Why does the position of the insulation decide the whole argument?
With the insulation outside, the entire original wall stays on the warm side of the build-up. Through the winter its temperature remains above the dew point of the indoor air, the masonry keeps its thermal mass, and there is nowhere for condensation to occur. That is why external insulation, whether bonded as an external insulation system or ventilated, is the safer solution and the Slovak default.
Internal insulation inverts the geometry. The original masonry ends up on the cold side of the insulation, because heat passing out from the interior no longer warms it, and the dew point can land inside the insulation or at its junction with the wall. This is not forbidden, and with a protected facade it is often the only option available, but it is a calculation rather than a choice.
| Insulation position | Where the masonry sits | Consequence |
|---|---|---|
| External, bonded or ventilated | Warm side, above the dew point all winter | Safest; thermal mass retained; no condensation plane |
| Internal | Cold side, below the dew point in frost | Requires calculation, a vapour control plane and detailed junctions |
| Cavity, between two leaves | Outer leaf cold, inner leaf warm | Works if the cavity is drained and the inner leaf is tight |
| None | Whole wall cold on its inner face | Surface condensation and mould at corners and reveals |
A safe internally insulated build-up rests on a few rules, and each of them is routinely skipped on Slovak renovation sites. Prove it by calculation rather than by an estimate based on insulation thickness. Use a deliberate vapour control plane on the inside, whether a full barrier, a retarder or a humidity-adaptive membrane. Make that layer continuous, because every penetration, socket and junction with the floor slab is where the build-up loses. And resolve reveals, floor slabs and partitions at the same time, because those thermal bridges are where the surface will be coldest.
What is the difference between surface condensation and condensation inside the material?
These are two different problems with two different remedies. Surface condensation appears wherever the internal surface is colder than the dew point of the room air: a window reveal, a balcony slab junction, the corner of an external wall, the back of a wardrobe pushed hard against a wall. Mould arrives before any visible droplets, because sustained raised surface humidity is enough for it.
Condensation inside the material is hidden and shows up years later as wet insulation. Vapour passes into the build-up and separates out in a layer that is cold enough and unable to pass the vapour further outwards. The rule that governs it is simple to state: vapour resistance should fall from inside to outside, roughly by a factor of five or six across the build-up, tight inside and open outside. The consequences of getting it wrong are covered under interstitial condensation.
How does the dew point relate to the U-value of a wall?
Indirectly but decisively. The U-value describes how much heat passes through an element, while the dew point question asks how warm the inner surface stays. Better insulation raises the internal surface temperature, which moves the surface further above the dew point of the room air and removes the mould risk. That is why insulating a wall usually cures condensation in the middle of it.
The relationship breaks down at junctions, and this is the part that surprises owners after a renovation. Insulating the field of a wall while leaving reveals, lintels and the floor junction untreated raises the temperature everywhere except at those points, so the remaining cold spots collect all of the moisture that used to be spread over a larger area. Condensation after an insulation upgrade is almost always a junction problem, not a proof that the insulation was a mistake.
What does the household itself control?
| Activity | Effect on indoor humidity | Remedy |
|---|---|---|
| Cooking with the extract switched off | Sharp local rise in vapour | Run the extract during and after cooking |
| Drying laundry indoors | Sustained rise across the whole flat | Dry outside, in a vented room, or in a condensing dryer |
| Showering without ventilation | Very high short peak | Extract during the shower and after it |
| Window permanently on the latch | Chills the reveal without removing much moisture | Short bursts through a fully opened window instead |
| Furniture against an external wall | Cold pocket with no air movement | Leave a gap so air can reach the surface |
The construction determines how cold the surfaces become; the occupants determine the humidity of the air. Both sides of that sentence have to be addressed, and blaming either one alone is why the same argument between builder and owner repeats itself every winter.
What is the most reliable way to keep surfaces above the dew point?
Two measures, applied together. Raise the surface temperature by insulating continuously and by resolving the junctions rather than only the field. Then lower the vapour load by removing moisture at source and ventilating properly. Short bursts through fully opened windows remove moisture without chilling the structure, while a window left permanently on the latch does the opposite.
In an airtight house neither the owner's diligence nor the weather can be relied on, and heat recovery ventilation does the job continuously and without the heat loss. That is the real reason mechanical ventilation belongs in a well-insulated Slovak house: not comfort in the abstract, but keeping the indoor dew point low enough that the surfaces the design produced are actually safe.
Frequently asked questions
- Where exactly is the dew point in a wall?
- There is no fixed answer, and that is the point of the concept. The place where the temperature in a build-up falls to the dew point moves with indoor humidity and outdoor temperature, so it sits somewhere different in a January frost than in a damp November, and in summer it is usually not in the wall at all.
- Why does insulating from inside cause problems?
- Internal insulation leaves the original masonry on the cold side, because heat from the interior no longer warms it. The dew point can then land inside the insulation or at its junction with the wall. It is not forbidden, and on a protected facade it is often the only option, but it becomes a calculation rather than a choice.
- Does a warmer room raise the dew point?
- Only indirectly. The dew point depends on how much water the air actually holds, not on its temperature. Heating a room without adding moisture leaves the dew point unchanged and raises surface temperatures, which is why heating and ventilating together is effective while heating alone often is not.
- Why does mould appear before any visible water?
- Mould does not need liquid droplets. Sustained high relative humidity at a surface is enough, and that condition occurs at temperatures a few degrees above the dew point. By the time condensation is visible on a wall, the microbial problem has usually been developing for some time behind the furniture.
- Can drying laundry indoors really damage a wall?
- It contributes measurably. Drying laundry in a living room, cooking with the extract off and showering without ventilation raise the indoor dew point by several degrees, which is enough to wet a surface that had been fine. The construction decides how cold the surfaces get; the household decides how humid the air is.