We are very aware of a current bad practice, which is being encouraged by some suppliers & builders, of using NHL limes (Natural Hydraulic Lime) on Cob/Earth buildings. We think one of the main reasons for this is both builders and customers are seeing it as a quicker/cheaper option than using Non-Hydraulic Lime (fatty lime mortar). The results are fairly damning, with signs of NHL renders delaminating quite quickly from the cob underneath.
There is a simple explanation as to why NHL should not be used on Cob.
Pure NHL mortars & plasters end up setting as hard, if not harder than sand & cement. Historic England & Historic Scotland is now banning the use of NHL's in the restoration of listed properties & there is a huge movement away from NHL's as a render system. Unfortunately NHL's are being abused & incorrectly used by many & this movement will take time to filter down through to general builders.
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Ignorance is not an answer when there are many case studies occurring & many results already available. We are currently carrying out three case studies on Cob properties & we are busy collating the information. This helps us produce the correct materials required for properties in our local area. We encourage maximum breathability for any property built with lime or earth mortars & this includes having good drainage.
Non- Hydraulic mortar (fatty lime mortar which we make here in our warehouse) has the best breathability. The porosity is good, as the aggregate we use are varied (limestone, sandstone, quartz & flint) ranging from 0-4mm, which gives the best breathable binder to the mortar & the mortar won't shrink or crack. (All Non- Hydraulic Limes should contain a large element of limestone as this creates the capillaries, which gives the breathability.)
We use NHL 3.5 only as a pozzalan to help tickle along the curation period at a ratio of 6% NHL to Non Hydraulic Lime mortar. This is an effective way of helping the set & not detracting from the breathability & porosity. The use of NHL 5 should be restricted to use under water, i.e. footings that stand in water and occasionally for flaunchings on high chimney pots which are exposed to severe elements.
If in any doubt, please ring and ask us, it could save you a small fortune by ensuring you have the correct lime mortar on your property.
Office, 01237 477 431
www.heritagecobandlime.com
