Interstitial condensation
Water forming inside a build-up where the temperature falls to the dew point. The criterion is not zero condensation but an annual balance that dries out again.
How does condensation form inside a build-up?
Warm air holds more water vapour than cold air. In a heated house there is therefore substantially more moisture indoors than outdoors in winter, and that difference drives vapour outwards through the external construction. Inside the build-up the temperature falls step by step, and at the point where it reaches the dew point the vapour is deposited as liquid water, not on a surface where it would be visible, but within the material.
Moisture gets into a construction by two routes and they are not comparable in scale. Diffusion is the slow migration of vapour molecules through the substance of a layer, driven by a difference in partial pressure. Air flowing through a leak transports orders of magnitude more water: one untaped joint in the vapour-tight layer delivers more moisture into the construction than diffusion through the entire wall area. That is why airtightness matters more, in moisture terms, than the Sd value of the sheet itself.
How is interstitial condensation assessed?
The basic tool is the steady-state Glaser method standardised in STN EN ISO 13788. It calculates the temperature and vapour pressure profile of the build-up month by month, establishes where and how much water is deposited, and above all determines the annual balance: whether what condenses in winter evaporates again during the warm part of the year. The criterion is not zero condensation. The criterion is that the construction dries out.
The Glaser method has limits and they need to be known before the result is trusted. It does not calculate capillary transport in materials that conduct liquid water, and it does not capture build moisture from the construction period, driving rain, or moisture carried by air movement. Where one of these governs, and capillary-active internal insulation, a timber structure with a high build moisture content and an exposed weather-facing facade are the usual cases, a transient hygrothermal assessment is carried out with real climate data instead.
| Method | What it models | When it is the right tool |
|---|---|---|
| Glaser, steady state | Diffusion, month by month, annual balance | Ordinary walls and roofs with conventional layers |
| Transient hygrothermal | Diffusion plus capillary transport, rain, real climate | Internal insulation, heavy build moisture, exposed facades |
| Surface temperature check | Risk of mould on the internal face | Every design, especially at junctions |
| Blower door test | Air leakage, the largest moisture transporter | Every airtight construction, before linings close |
How does the failure show itself?
Condensation does not announce itself in the year it starts. It becomes visible once there is enough moisture for microbial growth or timber degradation, and by then the extent is usually larger than the appearance suggests, because water travels along timber and along membranes away from the point where it formed.
- Mould in cold corners, at window reveals and behind furniture pushed against an external wall.
- Damp bands on a ceiling or wall following the line of the rafters or studs.
- Staining and rot at the eaves and where a roof meets a wall.
- Blistering or peeling paint and discoloured plaster.
- A musty smell in a converted attic even without visible damp.
- Timber decay found only once a lining is opened.
What does the Slovak assessment actually require?
The requirements for internal surface temperature and for the moisture behaviour of constructions are set by STN 73 0540 Part 2. The first requirement protects against mould on the surface, the second against moisture accumulating inside the build-up, and a design has to satisfy both. They are separate checks because they fail in separate ways: a wall can be warm enough on its inner face and still be accumulating water four centimetres in.
A design that meets both must also resolve thermal bridges, above all reveals, ring beams and floor junctions, because those are where the surface temperature is lowest and where the isotherms bend deepest into the construction. A thermal bridge is therefore not only a heat loss item. It is the place where both failure modes are most likely to appear first.
How is the risk reduced in practice?
| Measure | What it prevents | How it is verified |
|---|---|---|
| Continuous airtight layer, taped at every detail | Bulk moisture transport by air movement | Blower door test before linings |
| Falling order of Sd values, inside to outside | Vapour trapped by a tight outer layer | Assessment to STN EN ISO 13788 |
| Correct choice of vapour barrier or retarder | Loss of the drying reserve, or too little resistance | The same assessment, per build-up |
| Ventilation sized for the household | Indoor humidity that overloads any build-up | Design airflow rates, humidity monitoring |
| Thermal bridge detailing | Cold spots where isotherms sink into the layer | Two-dimensional thermal calculation |
The practical measures are three, and none substitutes for another: a continuous airtight layer, a correct order of vapour resistances falling from inside to outside, and ventilation that removes the moisture the household actually produces. Without the third, even a faultless build-up will not save the construction, because the indoor vapour pressure it was designed against simply will not be the one it experiences.
How is this different from condensation on a surface?
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. It is visible, it is diagnosable in an afternoon with a thermal camera and a hygrometer, and it is usually fixable by warming the surface or lowering the humidity.
Interstitial condensation is the hidden version of the same physics, and its consequences are more serious precisely because nobody sees it. That is also why the response to it is a design response rather than an occupancy one. A vapour-open build-up forgives it, a closed one does not, and the decision between the two is made on a drawing years before anyone smells anything.
Frequently asked questions
- Is any condensation inside a wall automatically a defect?
- No. The criterion in the standard assessment is the annual balance: what condenses during the winter must evaporate again during the warm part of the year. A build-up that condenses a small amount in January and dries fully by August is compliant. One that accumulates year on year is not, however small the annual increment looks.
- What is the Glaser method?
- A steady-state calculation, standardised in STN EN ISO 13788, that works out the temperature and vapour pressure profile through a build-up month by month, identifies where condensation forms and how much, and establishes whether it evaporates again. It is the basic tool in Slovak practice and it is deliberately simple.
- When is a Glaser calculation not enough?
- When capillary transport, built-in construction moisture, driving rain or air movement governs the result, because the method models none of them. Capillary-active internal insulation, a timber structure with high build moisture and an exposed weather-facing facade are the usual cases, and they need a transient hygrothermal assessment with real climate data.
- Does airtightness matter more than the vapour barrier?
- In terms of the quantity of water moved, yes. Diffusion through a tight material is slow; air flowing through a gap carries orders of magnitude more moisture with it. One untaped joint can deliver more water into a construction than diffusion through the whole wall area, which is why the blower door test is not optional in a timber building.
- How do you know a wall has interstitial condensation?
- Usually not until the damage is done, because the water forms inside the material rather than on a visible surface. The early signs are indirect: mould in cold corners, damp bands following rafters or studs, staining at eaves, a musty smell in a converted attic, and timber decay found only once a lining is opened.