Revealed: Why DAMP On Your Chimney Is Usually NOT What You Think…

Right across the UK, millions of chimneys are 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.

Just another legacy from a different era.

But that doesn’t mean they don’t have the capacity to cause issues.

I regularly get calls asking for help with ‘damp’ on chimney breasts. Sometimes it is near the ceiling. Other times it is near the floor. And on the odd occasion it is slap bang in the middle of both.

Naturally, when people think of damp, they think of water… so they will call a roofer who will then spend 15-20 mins looking 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 obviously rainwater can get into a chimney, it rarely mainfests itself on damp patches on 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 Victorian, Edwardian or infact 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 – which is just as well 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 heat ‘in’.

Hermetically sealing our homes which, remember, were built to breathe…

Coupled with things like tumble dryers, central heating and so on, we are now expelling more moisture into the air from ever before.

And whereas before that 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 for that.

There is also 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, would allow property to breathe.

For more about this, I had a good chat with Steve Murray from Lime Mortar Restorations in Yorkshire. I met Steve on Twitter and was impressed with his crusade to educate people, especially those who own older properties, about how they really shouldn’t be allowing builders and plasterers to use cement-based products.

“The problem is that most plasterers now were trained using cement and gypsum plaster,” says Steve.

“So that’s what they use, no matter what type of building they’re working on.

“When you have a wall in a Victorian property, they’ll put plasterboard over it and then skim it. It’s all about straight edges, they’re obsessed with how it looks. And to be fair so are customers.

“But what they don’t realise is that they’re spending money on something which will cause a problem, namely trapping moisture. I’ve been to properties, like the one pictured below, where I’ve had to pull off five layers of plasterboard – all soaking wet with damp – just to get it back to the original wall.

“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 breath and helps keep the building warm and dry.

“Lime is burned at temperatures much lower than the high temperatures used for burning portland cement and draws in carbon from the air. So as well as allowing the building to breath lime is also kinder to the environment.

“The best way to sort a chimney breast, to be honest, is strip it, clean it and point it in a soft lime with a fine aggregate. No moisture gets trapped and it looks amazing.”

So as you can see, there are plenty things at play when you see ‘damp’ on a chimney breast. Lifestyle and building materials are key.

Of course, if you are closing off a chimney now make sure it is swept beforehand and well ventilated.

An unswept flue will harbour moisture and damp. Not only will it give off unpleasant smells, it can cause aesthetic issues.

It would, as stated earlier, also be wise to take Steve’s advice and ensure any plasterwork is appropriate for the age of the building.

Otherwise a simple job could turn into quite an unpleasant and expensive one…

Mr Soot Chimney Sweep is a HETAS Approved Sweep, and member of the Guild of Master Chimney Sweeps, offering a professional and friendly chimney sweep and stove service. Book Mr Soot online or call 0800 0541154.

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