High humidity is often found in old cob and stone cottages which have had their traditional clay and lime mortars and plasters replaced with modern cement.
It is advisable to seek advice from a local lime mason skilled in old houses rather than from a general builder used to dealing with modern properties.
Maintaining a low humidity within your property is important for a number of reasons. The most commonly recognised one is that high humidity promotes the growth of mould, fungi, and other organisms potentially damaging to health. A less commonly recognised problem is the effect high humidity can have on the efficiency of heating a property.
To understand this effect, we need to consider the concept of relative humidity. This concept refers to the fact that air can hold substantially more water vapour at higher temperatures. When warm air with a high relative humidity cools, some of its moisture vapour settles out in the form of condensation. This occurs on the coolest surface with which the air is in contact, commonly window reveals and north facing external walls.
Interestingly, this explains why the areas where damp is exhibiting in old houses is not necessarily the point of moisture ingress. Lime may be used to manage this in old properties, but we’ll come to that in a minute.
When a room is heated, the air warms up first. The walls warm up much more slowly (this is particularly true of cob and stone buildings where the walls have a very high thermal mass).
As the air warms, any residual condensation on the walls evaporates. This evaporation takes energy, so rather than heating the room, heat is expended on drying the walls out.
Moisture will naturally move from a cold area to a warm area, so where the structure of the wall is damp, moisture will move towards the interior of the room and evaporate, further wasting heat energy. This commonly exhibits as plaster blowing away from the wall and also as salt crystals forming on the surface of the plaster, blowing off any non breathable paint.
Why are Lime Finishes Good at Managing Humidity?
Breathable finishes such as lime are particularly good at managing humidity within a building.
Used externally for pointing or rendering, lime allows walls to shed excess moisture to the outside world through means of evaporation. This prevents the structure of the wall becoming overly damp, and thereby reduces the amount of water that is subject to being drawn out of the walls internally.
When lime is used internally, it gives the walls what is referred to as ‘moisture mass’. Because lime is hygroscopic (able to readily absorb and diffuse moisture vapour), it is able to regulate internal humidity to a degree.
It is more difficult for condensation to form on lime, as it is able to absorb excess moisture and then disperse it into the structure of the wall.
Lime finishes are not a panacea – they must be combined with good external drainage and intelligent use of heating to be at their most effective. I will cover these topics in a separate post.
If you feel that your house would benefit from the use of some traditional techniques and approaches coordinated by an experienced lime mason, please call or email us and we will be happy to discuss options with you.