Humidity and Dampness

What is Humidity?

Humidity is a measure of the amount of water vapour in the air. There are two ways to measure humidity.

Absolute humidity – This is a measure of the amount of water are held in a given volume of air. Usually in the UK, this is measured in grams per m3

Relative Humidity – This is a measure of the amount of water in the air relative to the maximum amount of water that can be held in the air without condensing out as dew or fog, and is usually expressed as a percentage.

The amount of water which can be held in the air increases with temperature in a non-linear fashion – roughly doubling for every 11 centigrade rise in temperature.

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Understanding Rain

The Earth's weather systems are largely driven by convection currents and the hydrological cycle. As the air warms under the influence of the sun, it is able to absorb more and more water from the soil and bodies of water. The warm air then wants to rise, and as it rises, it expands as pressure falls so causing its temperature to drop. When damp air rises, expands, and cools, water vapour begins to condense out as liquid in the form of small droplets of water. By condensing out, the water droplets release latent heat which causes air to keep rising carrying the small water droplets up further forming clouds. The droplets then coalesse into larger drops which eventually fall out of the cloud as rain. In some cases, the vertical convection currents are very strong – sufficient to more than balance out the gravitational forces on the drop of water so that it continues to rise, freezes, and continues to accumulate further droplets of super cooled water which have not yet frozen leading to the formation of ever bigger hail stones – which in extreme cases can exceed the size of cricket balls.

Weather Phenomena in Very Large Buildings

In a very few extremely large buildings such as the NASA Vehicle Assembly Building, it is in fact possible for “weather” to occur, for clouds to form, and even for a light rain to fall.

A building with an open space 250 meters tall with 50% relative humidity at ground level would form clouds and experience internal rain without humidity control. If ground level humidity were 80% or 90% such weather effects could be experienced in a significantly smaller building.

What of Smaller Buildings?

In any inadequately ventilated building where evaporation takes place, you can get a humidity build-up and condensation. This is most easily seen in a bathroom when you have a hot bath with the window closed. Water evaporates from the bath, and you will often see windows steaming up. This is a result of the water which evaporated from the bath condensing out on the coldest available surface. The reason water condenses on the coldest available surface is that a cold surface results in locally cooler air than elsewhere. Cool air holds less moisture, and relative humidity locally reaches 100% on the windows, and possibly on any cold spots on the wall.

Why might you get cold spots on the wall? Due to cold bridging where there is a break in insulation and in that particular spot, the wall consequently is more influenced by the outside temperature.

In almost all buildings, activities take place which give off moisture – anything from having a bath or shower, to cooking, sweating, or even evaporation from toilets. For this reason, absolute humidity in grams of water vapour per m3 of air is almost always higher inside than outside, and in the absence of ventilation, or moisture transport through the fabric of the building, absolute humidity will continue to rise until somewhere in the building, the air can hold no more moisture. When this happens, condensation will take place on a cool surface such as a poorly insulated spot on a wall which as a result can become damp, grow black mould, and otherwise be put at risk of rot or structural issues.




How Does Moisture Get out of Buildings?

There are two ways for moisture to get out of buildings, ventilation and vapour permeation with both applying in most older buildings, and for the most part, only ventilation applying in modern buildings. It is important to the continued good repair of buildings that a suitable level of ventilation is maintained and where applicable that the vapour permeable nature of old buildings is kept.


Suppose it is 20 centigrade in a building and 9 centigrade outside. Both indoors and outside, relative humidity is 50% HOWEVER the air inside is able to hold roughly twice the amount of moisture as it is 11 centigrade warmer. Suppose you take that outside air and warm it to 20 centigrade, its relative humidity will only be around 25% so any ventilation exchange between indoor and outdoor air carries moisture out of the building.

If ventilation takes place using an open window or an extractor fan, the heat in the inside air is lost along with its excess moisture, however with modern mechanical heat recovery units MHRUs, a heat exchanger uses the heat in the warm indoor air to heat the cold outdoor air on its way in. Some MHRUs exceed 90% heat recovery so that the 9 centigrade outdoor air would enter the building at close to 19 centigrade with relative humidity only a shade over 25%.

Extract fan

Vapour Permeability

In older buildings, there is no damp proof course, and no vapour barrier, so there tends to be more moisture in the walls as they will absorb a certain amount of water during rain, or rising from the ground.

The materials used in old buildings tend to be natural materials which are mostly but not totally waterproof, and which as a result are vapour permeable (like breathable cloth used in sports clothing). This allows a steady flow of moisture through the walls and floor etc. meaning that even when a building is sealed up, some moisture is able to escape by traveling through the structure. (In practice, no building is totally sealed and old buildings rely on a combination of ventilation, and moisture permeating through their structure in order to get rid of excess moisture).

Important Note: - When working with old buildings such as Churches or almost any building pre 1920's with a vapour permeable structure it is absolutely critical to continue to use only vapour permeable materials for any decoration, refurbishment, or reconstruction. Use of modern materials like vapour barriers, portland cement, modern plaster / mortar, plastic based paints, or even linoleum floors or bitumen backed carpet tiled could trap moisture in the structure leading to conditions like black mould, wet rot, and dry rot.

With any building, remember that not all moisture in the building is necessarily a result of normal daily activities. Look out for signs of leakage from water or drain pipes, blocked or broken gutters and downpipes, missing or damaged roof tiles etc.

1st Associated offers and recommends a Quinquennial inspection service (every 5 years) for historic buildings such as Churches, and also recommends trustees setting up a regular contract to get gutters and downpipes cleaned, and roof and gutters inspected twice annually. By following this advice any issues are likely to be identified and remedied more quickly before serious damage can occur.

We hope you have found this article useful

We hope you found the article of use and if you have any experiences that you feel should be added to this article that would benefit others, or you feel that some of the information that we have put is wrong, then please do not hesitate to contact us (we are only human). For more information contact us on Free Phone 0800 298 5424.

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