Cellar Tanking – The Condensation Problem and How to Avoid it Part 2 – What Causes Condensation?

The average cellar tanking system will cost many thousands of £’s. The primary aim is to keep out ground water, but how often do we think about the internal naturally occurring atmospheric humidity? We ignore this at our peril; cellar tanking and other forms of basement waterproofing can interact with humidity and cause condensation problems… My last article on cellar tanking and condensation looked at what condensation is. Condensation occurs wherever the humidity and temperature combine to create ‘dew point’. So we need to look at those specific factors in a tanked cellar or waterproofed basement that cause temperature and humidity to come together in this way.


Waterproof coatings or membranes are not usually very thermally efficient so they can get cold. And they are usually applied to the internal surface of the outside walls during a cellar conversion or basement waterproofing project this makes them even colder. If we put insulation in front of the waterproof membrane this insulation makes the colder still (insulation keeps heat in the room and therefore away from the membrane).

Heating patterns

If the basement or cellar is heated on an irregular basis this can give rise to periods where the temperature is too low to avoid condensation.


Moisture vapour in the air will move in all directions, so not only is moisture vapour coming into a basement or cellar from the ground outside, it is also moving from the internal air space of the cellar or basement to the outside. An effective waterproof barrier (tanking system), designed to be effective against the ingress of ground water will also prevent the escape of internal moisture vapour. Rooms below ground are generally more difficult to ventilate due to lack of windows and doors. General living generates moisture vapour, even breathing and perspiring, also cooking, drying clothes, boiling kettles using a shower or bathroom without adequate air extraction all contribute to internal moisture vapour. This in itself is not necessarily a problem. It only becomes a problem is any part of the cellar or basement as a temperature which is equal to or below the dew point temperature and the most likely place for this (cold) dew point temperature is the waterproof barrier itself. In my own professional experience I have seen many cases of otherwise good cellar conversion and basement waterproofing projects ruined due to condensation.

In my next article I will be looking at the damage that it can do, not only to the building fabric, but also to the health of the occupants.

Meanwhile to find out more about problems with cellar tanking and how to avoid them click cellar tanking.