Each building component can be examined for the occurrence, location and rate of build-up of condensation.
Tas calculates the condensation rate through the construction based on BS EN ISO 13788 which contains three criteria for assessing structures.
To avoid mould growth the thermal design of the structure should be sufficient to keep relative humidity at internal surfaces below 80% in the most severe month of the year, given internal conditions appropriate to the use of the building.
Any interstitial condensation that occurs within the structure in the winter should all evaporate during the next summer to prevent an accumulation from year to year.
If interstitial condensation occurs over the winter and evaporates in the summer, the risk of degradation to the materials present should be considered in terms of the maximum accumulated condensation.
At present it is not possible to provide hard information as to the amount of condensate that would cause a problem in most situations. However, the following guidelines can be followed.
— Extensive work on flat roofs with continuously supported membranes has shown that they will perform satisfactorily if the winter peak in condensate retained within the roof does not exceed 350 g/m2 provided that there is no accumulation from one year to the next.
— The prediction of any amount of condensate on wood or wood based materials, especially structural components, should be treated with great caution and steps should be taken to eliminate it.
— An exterior leaf of masonry, which will be wetted by the rain can withstand substantial amounts of interstitial condensate without adverse consequences.
— Condensate on impermeable surfaces such as a metal roof or plastic will not always cause any damage where it occurs but can run or drip onto more vulnerable areas. The effect of various amounts of condensate is summarized in Table 1.
Amount of condensate (g/m2) |
Effect |
0 - 30 |
A fine mist that does not run even on vertical surfaces |
30 - 50 |
Droplets forming that will start to run on vertical surfaces |
50 - 250 |
Large drops forming that will run on sloping surfaces
70g/m2 will run at a 45 degree slope
150g/m2 will run at a 23 degree slope |
>250 |
Drops forming that are large enough to drip from horizontal surfaces |
The user must enter the following parameters:
Inside Temperature
The dry bulb temperature in the zone. This value is in degrees centigrade.
Inside Relative Humidity
The relative humidity inside the zone. This value is entered as a percentage.
Internal Surface Resistance
This is the surface resistance for the surface within the zone. It is measured in m2K/W. This value is usually 0.13 for glazing and frames and 0.25 for all other internal surfaces.
Outside Temperature
The external dry bulb temperature. This value is in degrees centigrade.
Outside Relative Humidity
The external relative humidity. This value is entered as a percentage.
Internal Surface Resistance
This is the surface resistance for the surface exposed to the outside. It is measured in m2K/W. This value is usually 0.04 for all external surfaces.
There is also a table displaying the condensation rate for each node in the construction. The table has the following parameters:
Surface or Node
The name and number of the node. The nodes are placed on the surfaces and where two layers of the constructions meet. There are also rows for inside and outside conditions.
Position of Node
This shows where the node is. It gives the material name of the two layers the node is touching or the name of the surface it is on.
Temperature
In the case of the 'Internal' and 'External' rows this is the dry bulb temperature. For the others it is the temperature at the node. In all cases it is measured in the degrees centigrade.
Vapour Pressure
The vapour pressure at the node or surface. This is measured in N/m2.
Saturated Vapour Pressure
The saturated vapour pressure at the node or surface. This is measured in N/m2.
Condensation Rate
The condensation rate at the node or surface. It is measure in kg/m2/s. If this value is positive then condensation has occurred and the row will become highlighted.
Two graphs are also displayed:
Vapour Pressure
This displays the vapour pressure and saturated vapour pressure in N/m2 for each node. The x-axis goes from 'External' through all the nodes, finishing with 'Internal'.
Condensation Rate
This displays the condensation rate in kg/m2/s for each node or surface.