Avoiding damp#
Whatever you do, you need to avoid your building being damp. Damp walls lose heat much faster, and damp makes the building deteriorate.
Buildings with traditional construction are designed to let water vapour freely through the fabric and they need breatheable materials throughout to avoid the vapour getting trapped. Modern buildings are more easy-going about these things.
Is my building traditional?
One rule of thumb is that pre-1919 buildings and materials are traditional and the later you get, the more likely it is to be designed in a modern way - but there’s a long changeover period and no guarantees. Stone is breathable, and timber-suspended floors are too. Most concrete is not, but there are lime-based forms of concrete that are.
Brick is a breatheable material like stone, but we don’t have a lot of experience of it and brick varies considerably in its composition and hardness, how it is treated, and we think, what you can get away with in practice.
If in doubt about what materials are appropriate for your building, our guidance is always to ask a professional, like an architect. Whatever you do, don’t go straight to a supplier whose interest will be in selling you whatever is easiest for them to do. Historic Environment Scotland puts out “Inform” guides that help for understanding best practice in traditional buildings and what kinds of things suppliers might try to sell you that will damage the building.
With that in mind, here are some of the main causes of damp:
Rainwater ingress. Make sure the roof is sound and that the rainwater clearance systems - gutters (called “rhones” in Scotland) and downpipes - are kept in good working order through regular maintenance to clear debris and spot problems. With the increased rainfall that comes with climate change, some buildings are already finding that they have to increase the capacity of their rainwater clearance to avoid wetting the walls of their building. Wet walls deteriorate and also make the inhabitants miserable because they lose 1/3 more heat than dry ones. You may also have water ingress at ground level into basement areas.
(c) Martinvl, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
Blocked vents. People and their activities release moisture into the building. If the building’s vents are blocked, then it’s hard for that moisture to escape. Some building operators deliberately block vents to avoid heat escaping and create damp that way. In many others, the walls have exterior air intake vents at ground level that get clogged with leaves and other debris.
Manual ventilation used incorrectly. Many buildings have ventilation features that are intended to be used only when needed to vent moist or stale air from the building - for instance, window trickle vents and extractor fans. If they are used too much, they waste energy, and if they are used too little, the building can be damp. The goal is to have the right amount of ventilation for the circumstances. Of course, this is hard to achieve in community buildings.
Plumbing leaks. Make sure they don’t recur. Older buildings often have pipework to areas that aren’t toilets any more. If you are having a problem with it, consider capping it off so there’s no water to leak.
In traditional buildings:
Non-breatheable finishes. Traditional buildings need breathable materials that will let water vapour through the building freely, but often workers will choose modern materials that trap moisture where it can damage the building - plastic membranes, for instance, or the wrong kind of insulation. Many buildings have had the original lime-based mortar between the stones replaced with hard, cement-based mortar over the years. If the mortar is softer than the stone, then it wears out faster but does its job of protecting the stones from damage - which is much harder to fix than just getting the mortar replaced by “re-pointing” the building. Non-breatheable paints, plasters, and renders are often the cause of mysterious damp problems.
Inappropriate or incorrectly installed insulation. Again, materials should be breatheable and the design also has to allow airflow in the correct places, for instance, with loft insulation laid on the floor of a loft, at the eaves to ensure the space and materials above the insulation stay dry.
Missing harling on rubble buildings. If you have a building where the outside surface is rubble - not just one with rubble infill between dressed stone (ashlar) - Historic Environment Scotland tells us it would have been harled originally, and re-harling it is the best step you can take for making it drier and warmer. Don’t forget you still need to use breatheable materials.
Churches and other stone buildings in occasional use
The Victorians were so worried about disease and had access to such cheap coal that their churches are sometimes over-ventilated. Churches also sometimes have ventilation features that were designed to be shut while the building was warming up for a service and opened afterwards to let the moisture people generate out, until the next time heating is required. Two examples are sliding vents under the windows and rope pulls for opening and closing the louvres on church towers. Although you might not have the people around to run them, it can still be useful to understand the design. It can be hard to find professionals with the right skills to assess how much ventilation you need, but it can save a lot of energy to get this right.
Also, it takes a lot of energy to heat up stone, so it’s often cold. Moisture in air can condense on cold surfaces, for instance, if you let lots of warm air into a very cold stone building on a fine spring day. In the winter, stained glass windows are often the coldest surfaces and so a good place to look for issues. Condensation on them can cause the colours the leach out of the glass.
What if the building is already damp?
If your building is already damp, you need to provide enough ventilation to dry it out. Trying to dry things out by keeping the heating on is expensive and doesn’t work as well. Dehumidifiers are OK in some buildings, but if you have a traditional building, current thinking appears to be that this will be slower than just increasing the airflow in the area inside the building. If there isn’t enough ventilation in the right area, one source advocates directing fans at the damp surfaces. We’ve seen a couple of competing explanations of why dehumidifiers are slower, so we’re still working on this one.
Your priorities are:
fix the leak or correct the wrong material
dry out the building
replace any components that are structurally damaged, retaining as much as possible
replaster and redecorate
Many firms sell damp-proof courses, impermeable membranes, and chemical treatments meant to prevent decay in timber. Sometimes they use damp meters incorrectly to demonstrate that an area needs treatment. Maybe they mean well, but our understanding is that these usually do more harm than good. For timber in particular, use your money to keep the area dry instead of applying something intended to keep rot from getting hold if the area gets wet again. Rot can’t survive if the area is dry in the first place.
More information