Why is damp so misunderstood?

Almost every household and business premise has been affected by damp at some point in time. It might manifest as a musty smell, black mould dots in the corners of the room, moist carpets, wet looking patches on your walls; you might even see beads of moisture running down your window frames, or dripping off pipes.

In this post I want to talk about damp specifically. Not leaks, not holes in the roof; but damp. The mystery kind. The kind that you can’t resolve, or have no idea how to deal with. In fact I want to go further and discuss how your previous attempts to resolve the damp may have either made matters worse or has moved the problem elsewhere.

The reason why I wanted to write this post, is I have become increasingly aware that damp is misunderstood and people don’t appreciate that traditional and modern buildings work in opposite ways when it comes to keeping them dry. This lack of understanding means that many builders are giving clients bad advice and wasting them a lot of money. Let me explain why:

Traditional buildings (pre-WW2 to make an arbitrary mark in the sand) were more often than not built with natural materials; stone, brick, lime, timber, that sort of thing, and were designed in a way that would accommodate a degree of moisture both from the outside world and from internal processes like cooking and breathing. Walls were often solid and porous and insulation mostly absent. Surely buildings of this age were originally very damp and we have improved them over time? Perhaps not. Traditional buildings were designed to breathe. They were well ventilated due to gaps in windows and doors, drafts; they had fireplaces and chimneys with updrafts; they often had stoves or fires burning all though the winter driving air circulation; but most importantly, the walls were permeable. When the exterior walls got wet with rain, once the rain stopped the wind would wick the moisture away, or the sun would heat and dry the surface, drawing moisture out of the wall. Any moisture produced inside the building would also escape with the drafts and what did not, would be absorbed into the porous surfaces as a ‘moisture sink’, later to be drawn back out in a natural cycle. ‘Air movement’ and ‘permeability’ are the fundamental principles keeping traditional buildings dry.

Modern buildings on the other hand are designed to exclude moisture. The cavity walls prevent moisture penetrating inside. In fact, cement renders can even prevent moisture from penetrating the surface. Gypsum plasters and sealed glazed windows inhibit vapour escape from the inside and instead we rely on extractor fans in our kitchens and bathrooms to vent the vapour we produce. Modern buildings work on ‘exclude’ and ‘extract’ principles.

Now I am not going to get into the added complexities of insulation, humidity, and dew points in this post, because this is where it starts to get complicated. But, you will probably have noticed by now that modern day damp solutions are not very compatible with the traditional building’s strategy for dealing with moisture and can make matters worse.

Let me give an example; you own a traditional solid brick victorian mid terrace building, which used to have single glazed sliding sash windows, but in recent years the windows have been upgraded for sealed double grazed units with draft excluders. The fire places have also been blocked up and central heating has been installed. You have a gas hob in the kitchen and tech walls are now skimmed with gypsum and decorated with plastic based paints. Now you are noticing black mould spots in the corners of the room and salt like crystals in patches above the skirtings. Damn, you have a damp problem. You call in a builder who tells you you have penetrating damp causing the mould spots and that he will render your exterior walls to keep the rain out. He also suggests you need to inject a damp proof course to stop rising damp above the skirtings. You agree to both works and they get carried out at great expense. These interventions seem to work at first but then the smell of damp gets worse and suddenly larger patches of damp start appearing on walls in other places. Your builder then comes around and suggests that your exterior walls are very wet now and you need to tank them on the inside to keep them dry, which you agree to at huge expenses. You’re also now no longer able to fix anything to the tanked walls. Fast forward a number of years and you start to notice that your floor is becoming springy. You lift the floor boards and discover that the joist ends are sodden at the point in which they bear onto walls have rotten through. The builder returns and advises that you need expensive repairs to replace the rotten timber.

Now this is quite an extreme and theoretical example; however, I have seen elements of these issues occur time and time again and it has drawn me to the conclusion that most people don’t understand how modern building techniques and materials can damage the way a traditional building interacts with moisture. The keen reader may have realised that in the example given above, modern building interventions have increased the amount of moisture that gets trapped inside the traditional building. Trapped behind render, trapped behind gypsum plasters. Moisture can find its way into walls through cracks in the exterior, humidity from cooking etc, but if it is not able to breathe like it was originally intended then that moisture will either build up in the wall or condense on the interior surface or both. The knock on effects of persistently high moisture is a a discussion for another day, as are potential solutions.

I have written this post with the intention of introducing this issue to the reader only. It is not meant to be used to diagnose damp problems. Damp is very complicated and there are always many factors at play. However, if you feel like you need some extra advice, I recommend that you speak to a building surveyor who can demonstrate a good understanding of these issues.