“[Lime mortars] represent the primary mortars of craft practice and construction over thousands of years.”
Nigel Copsey - Hot Mixed Lime and Traditional Mortars. A practical guide to their use in conservation and repair (2019)
Traditionally buildings were built from porous materials, including the lime mortars. This allowed the buildings to ‘breathe’.
‘Breathable’ is a term that has largely been hijacked by the building products industry to sell ‘breathable’ lines of paint, plasters, mortars and much more. Whilst not technically accurate, it is quite often used when describing the properties of lime.
Issues arise in old properties when modern, non-porous materials are used when renovating, the cardinal sin. This leads to issues of rising damp, rotten plaster and efflorescence. This can be easily remedied by removing the non-porous materials and replacing with lime plaster.
Why hot lime?
Hot lime has a much higher free lime content than Natural Hydraulic Lime (NHL). NHL, over time, will harden almost to the levels of cement, causing problems for the building.
I mix the hot lime plaster myself, with my recipe depending on the requirements of the job. I can include things such as hemp, pumice and perlite to provide natural insulation and I always use horse hair as a binder for my mix.