Water vapour diffusion

Water vapour molecules

A water vapour molecule has the size of only one ten-millionth of a millimetre (0.0000001 mm); therefore it is totally invisible to us. This also explains why substances that are absolutely impervious to water are relatively easily penetrated by water vapour. If a vessel were to be made of porous building materials instead of glass, then the pressure compensation would take place in the shortest possible time. The water vapour molecules would pass through the walls of the vessel. This process is what we call water vapour diffusion. Depending on the material of the walls, they offer more or less resistance to water vapour diffusion. The parameter for this is the diffusion resistance factor μ (pronounced as 'myu'). It indicates how high the diffusion resistance of the corresponding material is with respect to that of air of equal layer thickness.


Nice to know!

  • Air has a diffusion resistance factor of μ = 1
  • Mineral wool has a diffusion resistance factor of μ = 1.5
  • A vapour check, e.g. Ampatex DB 90 (thermoset endless fibres with polypropylene fill) has a vapour diffusion resistance factor of μ = 68,000
  • A vapour seal, e.g. Sisalex 514 (kraft paper + paraffin + glass fibre fabric + aluminium) has a vapour diffusion resistance factor μ = 6,900,000

Diffusion resistance

sD - value

From this point of view, our vapour check Ampatex DB 90, which previously reached the impressive μ value of 68,000, has only a relatively modest diffusion-equivalent air layer thickness. sD = μ value x thickness in m = 68,000 x 0.00033 = 23 m

Factors for water vapour flow

The quantity of diffusing vapour is dependent on:

1. Diffusion resistance factor (µ)
2. Layer thickness of the construction material (d)
3. Air temperature (ϑL) indoors and outdoors
4. Relative air humidities (φ) indoors and outdoors (vapour pressure gradient)

Diffusion resistance factor

The diffusion resistance factor μ is a constant, and is not dependent on layer thickness. It is not particularly relevant for the construction site, because each material has a different thickness. Therefore, we compare the diffusion resistance of a material usually used in construction (e.g. 20 cm concrete) with the resistance of an air layer of 1 meter thickness. We call this value diffusion equivalent air layer thickness sD, which we indicate in meters (m).


Nice to know!

Water vapour diffuses from the side with the higher absolute air humidity to the side with the lower absolute air humidity.


Water vapour diffusion

Water vapour condensation

We know that water vapour migrates. This is also the case with exactly the same temperatures inside and outside the building, because there is a difference in relative air humidity φ and thus a partial pressure difference Δp (pronounced as delta P) exists.

This vapour diffusion is harmless, so long as a significantly lower temperature does not exist on the side with less pressure. If this is the case, it may be critical. The temperature difference is called Δϑ (pronounced as delta theta).

First of all, it is of utmost importance to know that the maximum amount of water vapour that the air can absorb depends particularly on the air temperature ϑ (pronounced as theta).

  • At 30 °C, the air can absorb max. 30.40 g/m3 of water vapour. (ps = 4,241 Pa).
  • At 20 °C, the air can only absorb max. 17.31 g/m3 of water vapour. (ps = 2,337 Pa).
  • At 10 °C, the air can only absorb max. 9.41 g/m3 of water vapour. (ps = 1,227 Pa).
  • At 0 °C, the air can only absorb max. 4.85 g/m3 of water vapour. (ps = 611 Pa).
  • At -10 °C, the air can only absorb max. 2.14 g/m3 of water vapour. (ps = 260 Pa).
  • At -20 °C, the air can only absorb max. 0.88 g/m3 of water vapour. (ps = 103 Pa)

Nice to know!

The figure in brackets indicates the saturation pressure (ps), between the first example with +30 °C and the last one with -20 °C; there is a vapour pressure difference Δ p of over 4,000 Pa!


Heating air in a closed vessel

Starting conditions

The vessel has a normal ambient air temperature of 20 °C and a relative air humidity φ of 50%. Thus, effectively 8.65 g/m3 of water vapour is present, with a partial pressure p of 1,169 Pa.

Heating by 10 °C

The temperature is 30 °C, quantity of water vapour and partial pressure p remain unchanged. Since the air can now absorb max. 30.4 g/m 3 of water vapour, the relative air humidity φ falls to 27.6%.

Heating by 20 °C

Even at 40 °C, 8.65 g/m3 of water vapour is still always present. But the relative air humidity φ is still only 15.8%, and the partial pressure p still remains at 1,169 pa.

Heating by 40 °C

Even at 60 °C, 8.65 g/m3 of water vapour is still present. φ = 5.9%, p = 1,169 Pa.

This heating of the ambient air therefore leads to a reduction of the relative air humidity φ from 50% to 6%.

Cooling air in a closed vessel

Starting conditions

We will return to the starting conditions of the previous example of heating: Air at 20 °C and φ = 50%.

Cooling by 10 °C

Now the temperature is 10 °C, still 8.65 g/m3 of water vapour is present. The partial pressure p remains unchanged here. Since the air can now absorb only a maximum of 9.41 g/m3 of water vapour, the relative air humidity φ rises to 95.2%!

Water vapour saturation

At a temperature of 9.3 °C, the air is completely saturated with 8.65g/m3 of water vapour, φ is now 100%. p = 1,169 Pa is now the saturation pressure ps, this saturation limit of 9.3 °C is referred to as the "dew point temperature ϑT (pronounced as theta T).

Water vapour condensation

Now further cooling inevitably leads to condensation. At 5 °C the air can still have only a maximum of 6.8 g/m3, the remaining 1.85 g of water vapour condenses to water! The saturation pressure ps is now only 872 Pa!

This series can also be continued, but the result will be much more disastrous than in the case of continued heating. At -20 °C, already here 7.8 g of water per m3 of air would accumulate!

Water vapour condensation

Surface condensation during vapour production

If a lot of vapour is produced, for example, while taking a shower, φ rises typically to 80% at 22 °C. The dew point temperature ϑT for this climate is 18.4 °C. If the window pane has a surface temperature of less than 18.4 °C now, condensation water deposits on it!

Condensation zone in structural components

If the diffusion of vapour takes place at a large temperature gradient, then a pronounced condensation zone can occur in strongly vapour-permeable single-layer building materials. It is located in the part of the layer which is colder than the dew point temperature ϑT.

Condensation level in structural components

In multilayer structural components, local condensation levels occur in the case of an unfavourable interaction of various diffusion resistances.

Condensation on surfaces

At 22 °C and φ = 50%, the dew point temperature ϑT is at 11.1 °C. If a part of this surface is now cooled to below 11.1 °C, e.g. by thermal bridges, surface condensation occurs with mould formation.

Water vapour in buildings

Exactly the same happens when, on its way through the construction, the water vapour which is continually produced in the building (see example P. 11) passes over components or air layers whose temperature ϑ (theta) is below the dew point temperature ϑT (theta T) relevant for the indoor climate.


Nice to know!

Such condensation does not automatically lead to damage to the building. Often, the amounts are so small that they can easily dry out again without harming the building.


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