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Assessing the internal environment and condensation


  • Surface temperature thermometer (infra-red or direct contact)
  • Thermo-hygrometer
  • Psychrometric chart (included below)

Important note!

When using electronic instruments such as an electronic thermo-hygrometer and infra-red thermometer it is essential to allow them to come to equilibrium with ambient temperatures whether indoors or outdoors before making any readings.  Leave the instrument in the appropriate environment for a suitable time to this occur - if you do not then erroneous results will be obtained!


2. Condensation assessment:

 Worked examples are given below

  1. Using your thermo-hygrometer measure the internal temperature and humidity.
  1. Using the psychrometric chart mark at that temperature on the temperature scale on the bottom and draw a vertical line, then follow down the appropriate relative humidity (rh) line to where it intersects your drawn vertical temperature line.
  1. Now draw a horizontal line from the intersection across to where it cuts the 100% relative humidity line and from this point drop down a vertical line to the temperature scale at the base of the chart – this gives us the DEW POINT temperature, ie, the temperature at which condensation begins to occur. (Note: some thermo-hygrometers will calculate the DEW POINT temperature for you)
  1. Using your surface temperature thermometer take surface temperatures in places of interest such as base of external walls, corners, etc. If the recorded temperatures are at or below the DEW POINT temperature then surface condensation will be occurring. If, say, the surface temperature is 1 – 2ºC above the DEW POINT then a distinct risk should be considered.

In the example attached (marked in red), we have found that the internal conditions are 22ºC and 60% rh – these are marked as described above. Draw the horizontal from the intersect and then where it cuts the 100% rh line drop the vertical to the temperature scale at the base – this gives us a dew point temperature of 14ºC. Any surface at or below this temperature will be subject to surface condensation at the time of your visit.


3. internal environmental assessment:

  1. Using your psychrometric chart, mark the internal temperature and relative humidity as described above; where they intersect draw a horizontal line right across to the left hand scale – the vapour pressure scale. This gives us the INTERNAL vapour pressure.
  1. Now go OUTSIDE and take the temperature and relative humidity, mark these on the appropriate temperature and humidity scales as before. Now extend the point of intersection horizontally to the left vapour pressure scale as we did above – this gives us the EXTERNAL vapour pressure
  1. Subtract the EXTERNAL vapour pressure from the INTERNAL vapour pressure (the internal pressure is almost always greater than the external pressure) – this gives us the DIFFERENTIAL vapour pressure.

In the example attached, extending horizontal line (red) to the vapour pressure scale from the intersection of the 22ºC and 60% relative humidity gives us the INTERNAL vapour pressure of 1.6kPa. Outside we find a temperature of 6ºC and 90% relative humidity (blue) – marking these on our chart as above and extending the intersect to the left vapour pressure scale we get 0.9kPa for the EXTERNAL vapour pressure. INTERNAL minus EXTERNAL vapour pressures gives us 0.7kPa – the equates to the internal conditions being a ‘wet occupancy’ (see below)


Interpretation of results:

The following is described in BS5250:2002 ‘Condensation’ in relation to the differential vapour pressure:


0 – 0.3kPa   =   ‘DRY OCCUPANCY’ - Where ventilation balances moisture production


0.3 – 0.6kPa   =   ‘MOIST OCCUPANCY’ - Where ventilation does not quite balance moisture production/large water  production


Greater than  0.6kPa   =   ‘WET OCCUPANCY’ - Where Ventilation does not balance water production/very high water production/large occupancy.



When undertaken the results will only give a ‘snapshot’ in time, ie at the time of your inspection – conditions will frequently change. Do bear in mind that open windows, doors, etc, can strongly influence results as can someone who has recently used shower, bath, etc



You may occasionally need to know what effects a temperature change will have on the environment. For example, imagine that we have a property where the RH is always around 85% in the winter because the property is cold, say at 16.5ºC – the tenant does not want to use heat but complains of mould growth on clothes, leather, etc.


We need to drop the RH down to around 70% and to do this we would need to increase the temperature. To do this we need first to mark the 16.5º mark on the baseline (green) and take a vertical line up to the 85% RH position, and mark it. Now draw a line horizontally to the right until it touches the 70% RH line – now drop a vertical to the base line and we get 20º. So in order to drop the RH from 85% to 70% we have to increase the temperature of the room from 16.5º to 20º



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