Understanding dampness within chimneys
We all have a bias towards the skills and knowledge we hold, however, when looking at the repair or restoration of an old building, those bias beliefs can lead us away from the actual building defects that need to be addressed. Therefore, it’s very important to seek support from either an independent surveyor, or, perhaps a company similar to us who cover multiple trades. There are many great companies such as this in Cornwall and always please request at least three quotations to not just compare price, but also the findings and repairs that are suggested by each induvial surveyor.
Understanding all aspects of a chimney construction and the methodology that can be used to diagnose dampness within a chimney is vitally important. Analytical diagnosis prior to any work being undertaken can save you anywhere in the region of £1500-5000!
listed below are common building defects found within chimneys, taking a birds eye overview of chimney repairs, no pun intended, is by far the best way to evaluate a complex part of your building structure and fabric that changes immensely from its base to its adorning cowl. Due to the wide verity of issues that can found within a single chimney, you may wish to further research the use of limes, types of damp and hygroscopic salts, all of which are detailed within our Knowledge Hub.
Vegetative growth within chimney masonry provides strong evidence of water ingress. Where possible aerials should be removed and the stack should be repointed with lime mortar.
The lead cap that has been bossed over the chimney pot will be reducing air flow within the chimney and potentially causing damp.
3 visual checks for defects in your chimney..
Chimney construction
Lime Pointing & Rendering
The majority of traditional Georgian and Victorian cottage chimney stacks were built using clay brick that suffer greatly when pointed with modern cement mortars. As moisture is unable to move through the cement it is forced through the clay bricks and in cold spells frost shattering reactions can occur.
Cement pointing should be removed, the joints raked out 25mm and an NHL 3.5 lime mortar should be used which will allow for vapour permeability whilst also being tough enough to withstand our 70mph Cornish sideways rain!
Cement renders should also be removed with care and be replaced with an NHL 3.5 lime based render system.
Chimney Crowns
The purpose of the chimney crown is two fold, firstly it holds the chimney pots in place and secondly it is shaped to shed water from the top of the stack.
For these reasons we opt to use an NHL 3.5 or NHL 5 lime, dependant on location and size of the chimney pots due to having to withstand the harshest of our Atlantic ocean driven weather.
Image note: The cement crown that was installed on this chimney in the 90’s had broken down severely, allowing water to ingress down the stack, causing high moisture levels and mould on the bedroom wall below. It was replaced with an NHL 3.5 lime mortar.
Chimney Cowls
Chimney cowls play three incredibly important roles in protecting your home. Firstly they restrict the ingress of rain water from above, secondly they keep out wildlife such as birds that might nest within the stack and lastly but most importantly, they ensure that air flow is maintained through the chimney breast. For this reason, non vented cowls should never be fitted to a heritage building.
A lack of airflow causes condensation to form within the stack, they can mobilise hygroscopic salts within the masonry that appear as wet patches on chimney breasts and internal walls. Fireplaces should also never be blocked off, and air flow should be encouraged wherever possible.
Flue lining and condensation issues
The fitment of a flue liner can restrict airflow through a chimney and reduce or fully eliminate the naturally occurring updraught that reduces the potential for condensation forming within the stack.
Condensation forming within a chimney can present as dripping (black) water within a fireplace or can also mobilise hygroscopic salts within masonry, that present on internal chimney breasts as damp or wet patches.
At least one chimney pot should be left open, and where this is not possible, vented steel spacers can be fitted beneath the vented cowl to allow airflow through the chimney pot.
Vented cowls are both fine to use and recommend. The issue being highlighted only occurs when a flue liner is connected to the cowl base, effectively sealing the rest of the chimney and preventing updraught.
Hygroscopic Salts
Hygroscopic salts are water attracting in nature, they commonly present as either powder like salt deposits on masonry walls or damp patches on internal wall papered or painted surfaces.
The types of hygroscopic salts and how they find their way into our homes:
Chloride and nitrate – typically found in ground water and drawn up into masonry via capillary action (rising damp) due to some form of building defect, typically high ground levels, a broken drain or water pipe.
Sodium carbonate and ammonium nitrate – which are released by the combustion of wood, coal and other fossil fuels and therefore are the most common form of hygroscopic salts found in chimney breasts.
Calcium sulphated dihydrate (calcium salt) – the primary ingredient in modern gypsum plaster.
Calcium chloride – found in cement and ‘add mixtures’ which can manipulate the characteristics of cement mortar.
Chimney Construction
The adjacent image shows the route the chimney and its flue take through this solid masonry wall. In this image you can see swelling, cracking and algae growth that was caused by a defect to the render on the chimney stack allowing moisture to penetrate for many years.
The cause of the defect? The four and a half metre high arial that was attached to the chimney was buffeted by the wind, causing the chimney to move and the render to crack.
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