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Equilibrium Relative Humidity (ERH) - the pitfalls.

With the advent of better moisture meters, more knowledge on dampness and affordable hygrometers, measuring moisture in buildings can become quite sophisticated.

Take condensation for example. Measurement of relative humidity, air temperature to determine the dew point temperature, and then determined the surface temperature in relation to the dew point will tell us, or at least give us an indication, as to whether surface condensation is occurring or the likely risk of it developing at a particular time. So the measurement of relative humidity is a very important - in some cases.

However, relative humidity (RH) alone can be very misleading. For example, two rooms in a property may record significantly different comparative rhrelative humidities (see figure). At first glance this can (and usually does) give the impression that one room is damper than the other. This may be so but it is more likely that there is a temperature differential, the room with the highest relative humidity being the colder room (See 'Condensation - the basics'). In practice, they could have the the same water vapour content as indicated by vapour pressure.


It is common practice for those laying floor tiles on a newly constructed solid floor to measure the relative humidity of the air in or just above the floor (this is sometimes called the equilibrium relative humidity - ERH). If the relative humidity is greater than 75% then the floor is considered still to be too damp for laying tiles.

But wait. Relative humidity on its own can be misleading as described above. Take the particular room I am in a moment. It is midsummer, all doors and windows are open (very good ventilation) and, like many summer days, quite humid. The internal relative humidity is 71% at 21°C; this leads to a vapour pressure of 1.77 kPa (note that vapour pressure is actually a measurement of moisture in the air and is independent of temperature). However, the solid floor (35 years old and perfectly dry) is 19°C, thus cooler than the air as might be expected. With the above vapour pressure this results in a relative humidity (i.e., an ERH) of around 80% immediately above the floor! So a surface hygrometer or relative humidity probe used on the floor now would declare that it was damp! Clearly this is not true - it is just that the floor is colder than the surrounding air.

Some are now suggesting that 'equilibrium relative humidity' should be a new approach to check for dampness in walls. This can clearly be severely flawed and lead to misdiagnosis for the same reason as the floor above except this time the error could be more serious and significant!

Imagine a 215mm solid external north-facing wall during the winter - likely to be quite cold. Conditions inside could be quite warm, pleasant and moderately humid depending on internal moisture production. Someone then decides the wall may be damp so they drill a hole and use a hygrometric probe to measure the 'equilibrium relative humidity'. Note that the movement of water vapour is almost always from within a building to the outside thus the moisture laden air will pass through the wall. As it does so it is cooled by the wall and the relative humidity will increase. So if one simply compares the relative humidity in the wall with that in the room clearly it will be higher in the wall but not as a result of dampness, just the lowering of temperature. So in the wrong hands this could be interpreted as a damp wall when obviously it might not be.

It becomes clear that one must be careful when using the concept of it 'equilibrium relative humidity' for looking at moisture in walls because as described above it can lead to a serious misdiagnosis of a problem or even find a problem where one doesn't exist!


One final point, in order to insert a humidity probe into wall a hole needs to be drilled - this is 'destructive'. Where this is the case then the process of drilling can also obtain a sample of the material which can be used for a proper and direct analysis of moisture (and salt) content rather than the more dubious practice of measuring ERH!


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