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Hygric and Thermal Simulation

Flank diffusion – GLASER 2D examination of a roof structure

In the present example the combination of flanking diffusion and a disadvantageous roof structure are being examined. The term flank diffusion is used to describe a process when water vapor bypasses a vapor barrier through penetrating into components that disrupt the barrier (e.g. internal walls).The disadvantageous roof structure that has been examined consists of the following components (see also illustration at the bottom of the page):

roof tiles
30 mm air layer (ventilated) / sub beams
50 mm air layer (ventilated) / beams
4 mm bitumen sheeting (sd=200 m)
25 mm roof sheeting (spruce boards)
220 mm mineral wool / rafters
vapor barrier
50 mm mineral wool / wood frame
24 mm gypsum plaster board

Since we want to show the effect of flank diffusion we focus on a spot where a standard internal wall penetrates the vapor barrier layer. The internal wall consist of standard bricks with a 15mm lime-gypsum plaster on each side.

Although being widespread this setup is disadvantageous, as the bitumen layer prevents drying of the insulation layer towards the outside. Moisture that penetrates into the roof either by flank diffusion or by structural damages of the vapor barrier accumulates as the drying potential of this structure is poor.


In this examination we decided to use the monthly averages for Austrian climate defined by the relevant standards. Of course any reasonable set of data can be used for this purpose.


  • HTflux very clearly shows the effect of flank diffusion. Water vapor penetrates the plaster layer and the bricks to bypass the vapor barrier. The level of diffusion depends very much on the diffusion resistance of the materials used. In this case average values have been assumed for all materials.
  • HTflux also reveals that the thermal bridging through the rafters in combination with extra insulation layer behind the plaster boards causes further condensations spots on the internal side of the vapor barrier.

The condensate being formed inside roof sheeting is problematic as the drying capacity of this structure is poor. Most of the moisture can only leave the roof towards the inside (by flank diffusion again). In the present case this can only happen during four months of the year. During this period indoor humidity is elevated and the temperature difference between the roof sheeting and the internal air is low. Hence the rate of diffusion is low and the drying of the condensate accumulated during the cold period cannot be completely dried. Further accumulation of moisture with consequential damage is likely.

Flank Diffusion - Temperatures - Minimum, maximum values, total heat flux

Temperature view (January, condensation period)


Flank diffusion - heat flux view - January

Heatflux view / thermal bridging (January, condensation period)


Flank diffusion - Relative humidity in January

Relative humidity view (January, condensation period)


Flank diffusion - vapor stream density - January

Water-vapor stream density (January, condensation period)


Flank diffusion - water-vapor partial pressure view - January

Water-vapor partial pressure (January, condensation period)


Flank diffusion - vapor stream density - drying period

Water-vapor stream density (July, drying period)


Flanking diffusion - glaser 2d-condensation chart - residual moisture

GLASER 2D – accumulated condensate and residual moisture – monthly chart


Flank diffusion-glaser 2d-condensation table

GLASER 2D – temperatures, humidities and accumulated condensate – monthly table


Flank diffusion-materials and setup

Material view – roof structure and internal wall



Flank diffusion around a vapor barrier - vapor stream density

Flank diffusion – detail view


Notes on the GLASER 2D method

Our GLASER 2D algorithm represents an extension of the classical one-dimensional Glaser method on two dimensional configurations. Using the same model it solely describes vapour diffusion based on constant parameters and the so called “stable solution”. This means that the model does not include moisture storage or liquid transport processes. Consequently the GLASER 2D method shares the same advantages and disadvantages of the Glaser (1d) method:

  • The simulation basically only requires the knowledge of two constant parameters per material: thermal conductivity (known as λ) and water vapour diffusion resistance (expressed as µ or sd-value). These values are usually well known or can easily be found on test certificates or the relevant standards.
  • Forming the basis of the standards for hydrothermal assessment the Glaser method is well known and proven over decades.
  • It provides a conservative method to calculate if and how much condensation can be expected. The results of such an assessment are therefore on the safe side as long as vapour diffusion is the dominant process.
  • The method as well as its results are straight-forward and graphic.
  • The method calculates the so called stable solution of the underlying differential equation. Keeping this in mind it is still possible to process a series of simulations (e.g. with monthly temperature averages) once interpreted correctly.
  • On strong formation of condensation the method is less precise as moisture storage, liquid transport processes and moisture dependent change of material parameters are not covered by the vapour diffusion model. In such cases the amount of condensation is usually calculated too high, whereas the evaporation potential is underestimated by the model. Again, this represents a safe side assessment that can point out the need for a closer examination.
  • A more realistic simulation including liquidity transport and moisture storage would require a precise knowledge of the often complex temperature- and humidity-dependent parameter functions of all materials used. This information is often not available.

Therefore we believe that the GLASER 2D method offers an excellent and sometimes even the only practical way for hydrothermal assessment of two dimensional details – providing precious information with little effort.


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