How is this possible? Damp on an internal wall? – So no water ingress from blown render or poor pointing, no leaking gutters, no cracked tiles, no raised pavements above damp proof course, so what is going on?
Internal walls in older buildings were not built with very good foundations or very good damp proof courses. Some had slates installed, others bitumen, but many have nothing at all. The basic idea for these old houses was that they either had suspended wooden floors, or solid packed earth floors with quarry tiles. These coped with damp is different ways.
The suspended wooden floors were well ventilated so that the moisture from the soil would be ‘wicked’ away by the draughts. The solid floors were covered with a breathable covering thus allowing moisture to slowly evaporate into the house itself.
Both of these systems was helped by having breathable walls – the bricks were covered with lime plasters and breathable lime paints, so again any moisture in the walls could be dispersed into the internal atmosphere with little fuss.
So why are we seeing problems now?
There are a few reasons:
1. We have replaced many suspended floors with solid concrete floors. This process involves laying down a damp proof membrane under the floor and wrapping this up against the internal walls and filling the floor area with insulation and concrete. This means that any moisture in the ground can no longer come up through the floor. Sounds good. Unfortunately, this only puts extra pressure onto the internal wall. Any moisture is effectively concentrated into the internal wall foundations and hence the walls become wet through capillary action.
2. We have blocked up the vents under existing suspended wooden floors. This means that the moist air above the soil is not wicked away. This allows moisture to build up under the house and hence the walls can get wet.
3. The internal walls in our homes have often been replastered over time. Old lime mortar has been removed and more modern gypsum plasters used. These gypsum based products are not designed to survive in a moisture rich environment. So any sustained moisture in a wall will cause the plaster to fail. This in turn means that the plaster starts to become hydroscopic – attract more moisture to it. A self fulfilling failure system is hence in place.
So are there any solutions?
1. For replacement solid walls it is a bit late really as the pressure on the walls is so great. However, you can inject damp proofing silicone creams (Eco Home Centre sells DryZone) or install a new physical damp proof course. This will help to reduce the amount of water that is able to reach the living area. Care also needs to be taken to ensure that the plaster used is breathable and also that there is a gap between the floor and the plaster (normally hidden behind the skirting boards). I would also ensure that the skirting boards are treated against moisture so that they do not rot from the back.
2. Unblocking the vents and ensuring that there is a good draught under the floor should allow the walls to dry out.
3. Using breathable renders and plasters internally (predominately lime based) will allow moisture to travel through it without harming the finish (note that paint finishes need to be breathable as well – lime wash, claypaint and natural emulsions are all possible).
Choosing the right solutions can be difficult, so please contact the Eco Home Centre for more guidance if required.