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Internal condensation

12 May 2019

What is internal condensation?

Air can contain a maximum amount of moisture (the saturation vapour pressure) at any temperature.  Cold air can contain less water vapour than warm air, and therefore becomes saturated with water vapour when cooled sufficiently.  The relative humidity will then be 100%.  This is the dew point.  When the temperature drops or humidity increases, the excess water vapour turns into water droplets as condensation.

Internal condensation occurs when the dew point is reached inside a wall and part of the water vapour turns into water droplets as condensation.  The excessive concentration of water vapour can be the result of residual moisture convection or diffusion in the building.

Residual construction moisture

There is no chance of residual building moisture in a steel frame construction since no moisture is required for its construction.  This is the dry building principle.  In traditional buildings, moisture is always required for parts of the construction or the finish.  For example, we first need poured concrete and screed for the foundations, and a plaster layer often comes later.

Convection

Convection is a form of heat flow due to the displacement of air, in this case, caused by a temperature or pressure difference.  With a non-airtight outer wall, (warm) moist air can be “blown” through the construction, which causes large heat losses and can cause internal condensation.  The solution to this is correct detailing and the use of airtight materials and/or foils.

Diffusion

Diffusion is the principle of water vapour travelling through materials.  It occurs when there is a different relative humidity on both sides of a material.  Water vapour then diffuses from the highest to the lowest relative humidity.  The extent to which this is possible depends on the vapour diffusion resistance of a material.  In winter, the warm indoor air is more humid than the cold outdoor air, so water vapour will diffuse from the inside to the outside.  In summer, this phenomenon is reversed.  To prevent this, a vapour barrier material must be provided on the warm side (in practice, the inside) of the insulation and a vapour-permeable material on the cold side.

Internal condensation and steel frame construction

The better a building is insulated, the greater the potential temperature difference between the hot and cold sides of a wall.  In other words, this means a higher risk of internal condensation.  Since it is easy to build a high-performance wall with steel frame construction, we will look in detail at some variants of construction from the point of view of possible internal condensation for the following conditions:

Ambient temperature Relative humidity
internal 21 °C 80 %
external -10 °C 50 %
façade masonry
material thickness layer (mm) lambda (W/mK)
Thermal insulation (PUR) 100 0.023
OSB/3 18 0.170
beSteel frames + acoustic insulation (glass wool) 89 0.454(1)
(damp protection)
OSB/3 12 0.170
plasterboard 12.5 0.240

(1) Since this building layer is heterogeneous, this value is an approximation based on weighted heat resistances.  The actual thermal conductivity coefficient (lambda) depends on the precise frame/insulation ratio.  It can be calculated with numerical calculations looking at the two- or three-dimensional heat flows.

  • Temperature through the wall
  • The saturation vapour pressure can be determined on the basis of the temperature (psat) – this is the point where the relative humidity reaches 100%.

In this first arrangement, a vapour barrier is applied on the warm side of the acoustic insulation.  This is where the vapour pressure p goes down.

The vapour pressure always remains well below the saturation vapour pressure, so there will be no internal condensation.

A vapour barrier is not applied in the second arrangement. Note that here too, the vapour pressure always remains below the saturation vapour pressure, and no internal condensation occurs here either.

 

façade plaster
material thickness layer (mm) lambda (W/mK)
façade plaster 5 0.700
reinforcement layer and underlayer 10 0.930
Thermal insulation (PUR) 120 0.026
OSB/3 18 0.170
beSteel frames + acoustic insulation (glass wool) 89 0.454(1)
(damp protection)
OSB/3 12 0.170
plasterboard 12.5 0.240

1) Since this building layer is heterogeneous, this value is an approximation based on weighted heat resistances.  The actual thermal conductivity coefficient (lambda) depends on the precise frame/insulation ratio.  It can be calculated with numerical calculations looking at the two- or three-dimensional heat flows.

  • Temperature through the wall
  • The saturation vapour pressure can be determined on the basis of the temperature (psat) – this is the point where the relative humidity reaches 100%.

In this first arrangement, a vapour barrier is applied on the warm side of the acoustic insulation.  This is where the vapour pressure p goes down.

The vapour pressure always remains well below the saturation vapour pressure, so there will be no internal condensation

A vapour barrier is not applied in the second arrangement. Note that here too, the vapour pressure always remains below the saturation vapour pressure, and no internal condensation occurs here either.