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Damp on your chimney?

Across the UK, millions of chimneys are now currently lying dormant. The reduction of solid fuel as a primary heat source means the majority of flues in homes aren’t used at all!

Sometimes the damp is near the ceiling. Other times it is near the floor.  On the odd occasion, it is in the middle.

damp chimney

Naturally, when people think of damp, they think of water… so they will call a roofer who will then look for leaks. And when they can’t find one (or do some minor works and it still doesn’t solve the matter) they call in a chimney sweep.

Although rainwater can get into a chimney, it rarely manifests itself as damp patches on the internal walls.

So if it’s not water from outside, what actually causes it? To understand this, you have to look at the history of your home.

If it is Georgian, Victorian, Edwardian or in fact, any sort of home built up until the 1970s, then your home was built to ‘breathe’. Most had very little insulation. Double glazing or cavity wall insulation was still decades away.

These older homes were draughty – as the open fires which used to warm each room (even bedrooms) relied on these draughts to keep the fires working properly.

Over the years though, we have sought to be more ‘energy efficient’ – in short cutting draughts and keeping the heat ‘in’. Hermetically sealing our homes which, remember, were built to breathe!

Along with other factors such as tumble dryers, central heating and so on, we are now expelling more moisture into the air than ever before.

And whereas previously the moisture would simply be absorbed by the natural draughts and materials of an older house, it now has nowhere to go, so it has to condense on cold surfaces – and something like a chimney breast is perfect!

The recent trend in wood burning stoves has allowed a lot of these properties to ‘breathe’ again, as the fireplaces are re-opened. When closing off a chimney, for example, when installing an electric fire, make sure that it is swept beforehand and kept well ventilated using a vented chimney cap on the chimney pot and ideally a vent in the breast.

A flue that is not swept can harbour moisture and damp. 

There is another major factor which stops a house breathing – the type of plaster used. Years ago, homes were built with lime mortar and plaster and, again, this would allow the property to breathe. Lime mortar works via a process called the lime cycle. Limestone – calcium carbonate – is burned and turns to calcium oxide. When water is added to calcium oxide it turns to calcium hydroxide. Calcium hydroxide can be added to an aggregate and used as a lime mortar.

At this point carbon dioxide forces out the water to turn the lime mortar back to calcium carbonate. This allows buildings to breathe and helps keep the building warm and dry.