Using some form of vertical barrier to hide damp has been around for a long time. Many walls in old schools and churches still retain the dado rails and match boarded linings at their base.
Recently, dimpled profile membranes, made by Newton, Delta, Oldroyd and Platon have really expanded into this market. The method involves fixing the membrane to the damp wall, usually with proprietary self-sealing fixings. The membranes are profiled, with the wall side looking rather like a well worn football boot, with studs, which hold the bulk of the membrane surface off the wall.
The facing side of the membrane may be naked, for subsequent fixing of laths and plasterboard, or have a mesh or ribbed surface suitable for wet plaster or ‘dot n dab application.
The plaster system may include insulation, either between the laths or bonded to the rear of the plasterboards.
There has been a trend for ventilating the rear of the linings, with a gap at the bottom, top or both. I am writing this article to highlight why this is unnecessary and could even cause problems later.
First, lets look at the temperature gradient in the room and through the insulation and external wall:
It’s cold outside and we have a solid wall; that’s why it is damp in the first place. Evaporation of water in the wall needs energy. So the wall is even colder – if it could, it would shiver, just like I do when I get out of the shower.
In the adjacent lounge, the family are watching telly and the heating is on – toasty.
Now then, what with a family of three with a combined lung surface area of two and a half tennis courts, there’s lots of water vapour being exhaled into the room. Remember; all that steam we see enveloping a bus queue in cold weather is just an indication of temperature; we don’t create more water vapour in the winter. Our imaginary family are blowing off a similar cloud, as they vegetate in front of their 50 inch plasma.
However, if we touch the wall, behind the sofa, it is slightly cooler; not much, thanks to the insulated plasterboard and our studded membrane. Dig a bit deeper though and we find cooler temperatures. Get through the insulation and touch the membrane and we feel a cold surface.
The air behind the plasterboard and insulation is sheltered and moves very slowly. In contact with the cold plastic and plaster, it loses all the energy our central heating put into it.
As I outlined in my article A word of Relative Humidity, air temperature is a mover of RH. The H in RH is Relative to the temperature of the air. There is no need to add any more water to get condensation – it is already there!
For example, let’s imagine the air in the room, is at 180C and has an RH of say 65%, using the psychrometric chart, we see that cooling of only 60C or so, will be enough to saturate our air as it reaches it’s dew point.
Now, as vapour pressure will always run down it’s pressure gradient, the movement of vapour will be from High pressure to Low pressure.
The vapour pressure in a property is almost invariably higher within the property than it is outside – or even within the cold outside walls. The humid air will be constantly trying to reach equilibrium, passing through the face of the decorations, plaster and insulation. Providing an easy route for this; via air gaps, will only hasten the situation.
In the example above, I would expect to see occasional condensation, happening in the space behind the membrane, and/or on the internal face of the membrane, behind the plasterboard or insulation. As mould spores are everywhere, growth conditions will be ideal. The face of the plaster, may still look great, but after some time, the house may become smelly and if enough condensate is forming, this may even drip onto the floor (if the membrane is not sealed into it or passed below a timber floor). This can cause problems. Bear in mind too, that because the condensate is forming in a small void and in a cold place, there will be no energy to engender evaporation; once it is wet, it will tend to stay wet. Though the parts of the wall which were not already saturated by the initial damp problem, will absorb some condensate by capillary action, eventually adding to the ‘wetness’ of the wall and chilling it further. Condensate forming on the internal face of the membrane will wet the insulation and the plasterboard or dab material. The paper on plasterboard is basically cellulose – moulds love it.
The situation will be worsened if the damp problem in the external wall has not been reduced. So the membrane is not a panacea for damp walls, care still needs to be applied, so that the external fabric is as dry as can be reasonably achieved. Research by Safeguard Europe of Horsham, has shown that merely reducing absorption of rainwater can save up to a useful 29% of heat loss, through a solid external wall. This applies to the base of the wall in particular, so if there is rising damp, installation of a DPC, such as DryZone is beneficial, even if a membrane is being applied to the inside face. Membranes are bloody useful, as part of a range of measures, which of course include sensible external ‘housekeeping’ gutters, pointing, render maintenance, managing ground levels and such. If the masonry is just porous and facing into driving wind and rain, consider a good quality breathable water repellent cream, such as Storm Dry.
Some simple tips on membrane use above ground:
- Always check to see if the property has potential issues with high humidity; fancy measurements are not needed, just check for extraction in the kitchen, bathroom and any utilities – do they work well?
- Reduce the damp problem before application of the membrane.
- Always hack off the plaster if this is damp and salt contaminated, though sound plaster or render can be left on, any paper on it will be home for mould, so strip it or seal all joints very well.
- Don’t ventilate the rear or face of the membrane
- If a humid environment is unavoidable – utility room, kitchen or bathroom, install an internal vapour membrane on the inner, warm face of the insulation. This will help stop vapour passing through to the cold side and condensing.
I hope this all helps.