Managing Damp in Old Buildings

It's inevitable that you will find dampness somewhere in an old house. Nothing lasts forever, but despite English weather, our old buildings stand the test of time better than any modern house. The worst enemy of old houses is we humans - it's what we do to them, or fail to understand about them, that causes problems and makes you think you have to 'Damp Proof'. Almost 100% of damp issues in buildings are caused by condensation, and lack of adequate ventilation.

What is an 'old building'?
The Official Definition of an Old Building used to be "Built before 1919".   That didn't really help anyone understand what made them different.  It's now defined as:

"A solid walled structure, built using breathable materials, which needs to breathe"

This definition very deliberately talks about breathability. It is the cornerstone of the whole concept of old buildings and architecture. It may sound a bit daft, but until the Last War, most materials used in buildings are fairly simple - timber, stone, brick, lime-based mortars and plasters, and stone or slate tiles. All of these materials had one thing in common - breathability. It is since the war that there has been an exponential explosion in building materials, there are now thousands of them, and most are artificial compounds - gypsum plaster, cement, plastic sheets and membranes, silicones, epoxy resins, polyurethane insulation sheets, fiberglass, plastic coatings, and paints.. the common thread to all of these is that they are NOT breathable. Use them on an old building, and it will get wet.

What is 'damp'?

Damp as most people know it, when applied to old buildings, is a collection of symptoms. Essentially: hollow plaster, stains on walls, flaky paint, rotting skirtings, salty fluff on walls, dark patches on walls, plaster falling off, damp smells, mould and mildew, and no.. damp doesn't Rise, and no, there is no such thing as Rising Damp. These are SYMPTOMS - they are just evidence of something happening which in 99% of cases, YOU can easily do something about. ~They are ALL to do with water in the air, and the temperature of both the air and the wall that shows the symptoms.

Common causes of damp...

• Insulation
• Heating on/off - must be constantly ON, but low temp = 15 degrees C
• Avoid your house getting warm, then cold
• Modern paints
• Cement render
• Gypsum plaster
• Ground levels outside higher than inside
• Broken guttering or missing downpipes
• Vegetation growing near the wall
• Trees creating shade and moist air near a wall
• Lack of ventilation - double glazing, no vents
• Blocked chimneys - fireplace blocked up, no vents
• Furniture against walls creating cold, damp areas

What actually is Water?

You think of water as something in a glass, pouring out of a tap, as rain, or a river.  But you are constantly immersed in water - as a gas. You just don't know it. You can't see it. And it's everywhere. Water as a gas is a tiny molecule - it's like a naughty child - small, fast-moving, high energy, gets everywhere, but doesn't actually do much damage! When you cool it down, it loses energy, slows down, and becomes a liquid - initially as minute droplets, and if there's enough of them, they clump together to form a liquid. The problem now is that the liquid molecules are very big, and they can't get out of non-breathable materials like cement, gypsum plaster, and plastic paints. This is where our problems stem from Gaseous water moves easily through stone, brick, concrete, gypsum, wood, many plastics - but if you cool those down, water as gas turns into a liquid and your wall gets wet - inside. THAT is what they call 'rising damp' and THAT is what causes all the flaky plaster and paint.

How does Temperature come into the picture?

There is a bit of science involved here. If you cool air that contains water as a gas (in air spaces within your walls for example) it condenses when it reaches the 'Dew Point'. The scientific bit is predicting the dew point temperature. This is governed by the Saturation of the air at a given temperature (Relative Humidity) but most importantly, the actual amount of water dissolved in the air at that temperature (The Absolute - measured in Grams per Cubic Metre of air). In short, the warmer the air, the more saturated it is, the higher the Dew Point, and the more likely your walls are going to be wet inside.

Armed with this knowledge, how do we end up with a warm dry house?

By controlling the Dew Point, you can prevent condensation from forming in the walls, and therefore create a dry house. To control the Dew Point, you have to monitor the total amount of water that's dissolved in the air - the Absolute. This is where it is vital to control moisture production in your home - cooking, showering, family life - all of these produce vast amounts of water - several gallons a day - and it is this water that ends up in your walls. All you have to do is control the sources of moisture, and introduce scientifically controlled ventilation to achieve a dry house - and dry walls don't transmit heat nearly as quickly as wet walls, so it's a warm house as well - win-win!

Don't stop walls breathing!

Moisture diffuses through all building fabric (walls, stone, mortar, plaster, paint, timber), regardless of temperature. This process of diffusion is known as 'Breathing' when applied to walls and other parts of the building fabric like the roof for example.  It is shallow when there is little temperature variation and deepest when the daily range is greatest.  Water always moves from where it is more concentrated to where it is less (humid to dry). If air within the wall is humid, and the wall cools below dew point then water vapour condenses as water droplets in the pores of the masonry (this is what Rising Damp actually is), though the wall may still appear ‘dry’. During warmer and drier times, some of this water will evaporate and leaves the wall.  Even walls that seem dry will contain water, the amount varying with changes in the season and climate. If there are salts or other hygroscopic (moisture-attracting) materials in the masonry, the amount of water drawn into (and retained in) the wall can be sufficient to make the wall visibly damp, even in dry weather. This is the same as happens when you leave a pot of salt on the kitchen table - it gets wet. Most of the time, the inside of a house will be 'wetter' than the air outside because we humans produce a lot of water from living.

Anything that prevents a masonry wall from breathing will reduce its life expectancy. Coatings designed to seal the surface of masonry walls (and so ‘protect’ them)  trap moisture behind the coating and cause a damp problem elsewhere, such as on the other side of the wall. If there are appreciable salts in the wall, damage caused by inappropriate use of coatings can be dramatic. Coatings themselves - cement renders, gypsum plasters, plastic emulsion paints - will eventually be forced off the wall.  This is why we do NOT recommend products like Stormdry, Sandtex, Weathershield - all of which we have found to cause major problems of water entrapment on walls.

Most problems of damp in pre-1920's buildings have been caused since the war when cement and gypsum plaster became widely available - these two materials are responsible for over 90% of the damage that we see. They are impervious, they trap moisture and cause rot - they are death to an old house. Note that they all allow water to pass through them AS A GAS - but not as a liquid. Add silicone sealants to a timber frame, and you have instant rot of oak which is probably 400 years old. I have seen new timber frames start to rot in 2 or 3 years when incompetent framers seal the panels using mastic instead of oakum, which can breathe.

Injection Damp Proofing old houses is a WASTE of time and money - it doesn't work, and it causes irreparable damage to the fabric of your house
Damp Courses are almost NEVER broken, bridged, or damaged, as damp proofing companies will always tell you. There is almost always a specific reason for damp, which is easily fixed
Do NOT be talked into hacking old plaster off and re-plastering with gypsum, or worse still, waterproofing compounds, or so-called 'waterproofing or renovating plaster'! Nearly every case of so-called rising damp is just condensation - the bottom of the wall is coldest, so moisture condenses near the floor - it doesn't rise!
If you have a rotten timber frame, it's because of all the modern materials used to plug the gaps and cover it - sill beams covered with cement, wattle and daub covered with cement, silicone sealant around the panels, and filling holes in the wood. NEVER use cement, tar, cement render, modern acrylic, or silicone caulking on a timber frame - it will rot in front of your eyes. Never use modern acrylic or emulsion paints - they are death to old walls - they are plastic - they trap moisture and cause damp.  Avoid Weathershield, Sandtex, Stormdry - any 'protective masonry coatings'.

Understanding your Built Environment

Restoring an old home is all about understanding the environment, and how it affects the materials it's built with. By understanding this, it becomes a relatively simple task to sort out and manage problems causing damp.

Remember - it is all about 'Breathability' - such a simple ethos - allows moisture to enter and leave the building fabric without hindrance and it will never build up.  Moisture is just water that has condensed - allow water to move around freely as a gas, and it will never cause a problem.  Put something in it's way, and you will regret it.  Just like putting a plastic sheet on the lawn - in the morning it is wet underneath.

Temperature is a vital component. To avoid condensation, you MUST keep your home constantly heated at a gentle, low temperature. Aim to keep it at 15 degrees C. Run heating all the time, but at a low temp - radiators just warm, not blazing hot. Heating must not come on for 2 hours and go off again - it must be constantly on, but set VERY low. If you want more heat in a room, just use additional heat in that room - a wood burner or fan heater for an hour or two. Constant heating ensures building fabric stays warm, and thus does not get cold enough for interstitial condensation to form within your walls.

Source: Heritage House