With winter well on the way, anyone who has recently purchased an older property made of cob or stone or both, may want to keep a close eye on the existing exterior render if they suspect that it is not lime mortar, but instead has a cementious base that may also have been sealed with a plastic paint.
The reasons for this are straightforward- cement is a strong mortar and because it's waterproof, actually creates a sealed surface which is rigid and non-permable, resulting in any moisture underneath getting trapped. Cement is susceptible to frost and so in very cold conditions will crack. Not only does it attract moisture, it also retains it within the walls and other materials around it such as lintels and windows.
Once frosted, cement render will often fall off in clumps whilst inside, there will probably be patches of damp appearing on walls and condensation problems, especially in bathrooms and kitchens, as cement is not able to regulate the moisture content.
Lime mortar render on the other hand is permeable which which means that vapour can pass through it at all times, providing a much healthier option. It's weatherproof, not waterproof, meaning that your building is protected without sealing it. Lime mortar does not attract moisture so it's not wet material and therefore not susceptible to frost; nor does it require the foundations to be below the frost line.
Soft and flexible, lime mortar allows stone or brick to move without cracking. As the seasons change so the mortar will flex without causing instability to the structure, and unlike cement, there is no need to provide expansion joints. Any small cracks that appear tend to do so in a much reduced number of tiny micro cracks. Subsequent movement of rainwater through the surface of the mortar joints dissolves the free lime which then gets deposited in the micro cracks as the water evaporates. The lime consequently reacts with the carbon dioxide in the air and is converted to calcium carbonate and in a short perid of time the cracks are healed- this is known as autogenous healing. Whatever the weather, lime mortar is a mortar for all seasons.
Heritage Cob and Lime are manufacturers and suppliers of lime mortars and traditional building materials. Based in North Devon they export across the UK. The company can be contacted on 01237 477431 or through the website: www.heritagecobandlime.com