Hudson Lambert of Safeguard looks at the second of the two potential specifications: Dryzone Renovation Plasters.
In the UK, lime plaster – either applied directly to walls or onto timber laths – was used until the middle of the 19th century, until ceding a dominant position to gypsum plasters and then, since WW2, gypsum plasterboards.
These are all quality systems and quite durable when applied to dry walls. However, in the face of rising or penetrating damp, gypsum boards and lime or gypsum plasters can start to fail. This is especially the case where the dampness introduces salts into the plasterwork.
Consequently, there is a need for internal plastering systems that are impervious to this sort of damp damage. Previously, the solution would have been sand: cement plasters, based on strong mixes (typically 3:1), of sharp sand and cement and often with a waterproofing additive incorporated.
There is no doubt that these are very effective at holding back damp and salts, yet have from their own downsides such as condensation risk, poor insulation qualities and a cold feel that is unpopular with building occupants. Also slow and expensive to apply, they are not suitable for traditional buildings (because they can’t flex), don’t breathe and can damage underlying brickwork.
Alternatives were seen in the first generation of refurbishment plasters, typically composed of cement, lime, perlite, and calcium stearate; yet their resistance against damp and salts was relatively poor compared with sand: cement versions.
More recently, new ‘second generation’ plasters have been introduced that are more resilient to dampness and salts than traditional plastering methods. These are intended to replace existing plaster that has already deteriorated (having already also tackled the cause of the damp); or for use in new-build and conversion projects to limit potential damage caused by minor ingress of dampness during the lifetime of the building.
These modern replastering systems are capable of being applied to walls while they are still damp and can resist high levels of moisture and salt ingress. The suitability of these for a given project will depend on a number of factors, such as the age of the building, the required speed of application, and whether or not is possible to fully deal with the underlying cause of dampness.
The make-up of these plasters means they have large pores and a high pore volume. This allows salts to form within the plaster rather than on the surface; and the high pore volume results in higher thermal resistance, reducing the risk of condensation.
Other properties include a low compressive strength, ensuring that the plaster can easily be removed at a later date without damaging underlying brickwork; high water vapour diffusion (breathable) and conformity for the EN998-1 CE Standard as ‘Renovation Mortars’.
Dryzone Damp-Resistant Plaster is a modern formulation of breathable damp and salt resistant plaster that can be applied with traditional plastering techniques. For older buildings, Dryzone Hi-Lime Renovation Plaster is also available.
Unlike sand and cement renders, Dryzone Renovation Plasters are non-destructive to the underlying masonry, do not require gauging and provide a warmer surface. Simply mix and apply two coats.
These highly effective renovation plasters can be applied to walls while they are still damp and has a porous structure which controls salt migration, allows walls to dry out by evaporation and results in higher thermal resistance, reducing the risk of condensation.
The plaster is applied in two layers as a 5mm anchoring rough coat and a 15mm to 20mm top coat. The working time of the plaster is 30 – 60 minutes depending on temperature and humidity; and a 2mm skim coat can be applied after 24 hours.
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