Home Condensation Explained Condensation 13 - "The Relative Humidity level is high so it must be condensation"
Condensation 13 - "The Relative Humidity level is high so it must be condensation" PDF Print E-mail

It might be but 'it depends'. It is very common for people to quote high relative humidity levels when trying to explain condensation but relative humidity levels when quoted on their own are meaningless.

Imagine the lounge and bedroom in your property. If the relative humidity in the lounge is 60% and the relative humidity in the bedroom is 70% which room do you think is 'wetter'? Most people automatically think it is the bedroom but the real answer in our example is that they are both equally 'wet'. How is this possible? The answer is down to temperature.

If the lounge air temperature is 20°C then the vapour pressure in that room @ 60% relative humidity is 1.5kPa (kilopascals), equivalent to a moisture content of 8.7 g/kg (dry air). If the air temperature of the bedroom is 16.5ºC and the relative humidity is 70% then the vapour pressure and moisture content of the atmosphere is the same as the lounge. Because of the differing temperatures both rooms will therefore have different relative humidity levels but the amount of water vapour present in each room will be very similar.

Explained another way; if you now imagine two identical buckets each half full of water and that this represents 50% relative humidity. If one of the buckets is heated it will expand and the level of water will reduce to say 40% of the available volume. If the other bucket is cooled it will contract and the level of the water will therefore increase to say 60% of the available volume. In both cases the volume of water is identical but the space available to contain it has varied because of temperature differences. This same principle applies to warm air and cool air.

Contact Peter: Email - info@dampdecay.co.uk - Telephone: 015242 71794