We often get asked why our renders are lime and not cement based. People query why breathability matters if you’re rendering on to impervious materials and why you'd pay more for no clear benefit?
We supply Baumit renders because they are lime based, technically very good and high quality. Using lime as the main binder in a render instead of cement allows them very different properties which can be extremely beneficial to the life of the facade and the substrate beneath.
Lime based renders such as Baumit StarContact or MC55 are well known to be more flexible than cement renders, reducing the likelihood of cracking and also reducing the need for unsightly expansion joints. These additional details add cost to the cement systems, often allow moisture ingress over time and also restrict where they can be used. When rendering on to insulation the increased flexibility offered by lime renders reduces the risk (or completely removes the risk) of the render buckling when using darker finish colours on facades.
The assumption that ‘breathability’, or the correct term vapour permeability, is not beneficial when the substrate is not particularly vapour permeable, is false. Both new and refurbished buildings tend to contain lots of construction moisture that has to dry out. If this drying is restricted or completely prevented it can give rise to moisture related damage such as timber or metal decay along with surface staining from salts.
The ability to absorb and release small amounts of water from the surface of the render is also beneficial in the longer term. It reduces the amount of time during which the surface of the render is wet, through rain or condensation, thereby reducing the rate at which algae can begin to grow. Cement based renders tend not to absorb any moisture and acrylic finishes are prone to condensation, making them grow green/black moulds very quickly, particularly on Northerly facades or those near vegetation.
Baumit’s most active finish render, NanoporTop, relies on it’s ability to absorb and release small amounts of moisture to stay clean. During rain, the surface of this nonporous material absorbs water. As it dries, the water moves towards the surface of the render and pushes off dirt particles. Additionally it has a mineral additive which uses sunlight to break down staining, producing a self cleaning surface which will stay clean for many years.