Old vs. Modern Buildings
Historic buildings work in very different ways to more modern ones. Probably the most crucial distinction for retrofitters is the way water is dealt with.
A 21st-Century house is built to be as impermeable as possible. Low-absorption bricks are used with rigid cement mortar, damp-proof courses are included in the brickwork, and ground floors will usually be solid concrete slabs. By contrast, older houses were typically designed to be 'breathable'. Semi-porous materials such as lime mortar were used so that any dampness that got into the brickwork could harmlessly evaporate during dry periods. Similarly, suspended timber floors allowed moisture from the ground below to dissipate without damaging the fabric of the building.
A problem can occur when modern insulation materials are added to an old house. If impermeable insulation (such as foil-coated foam boards) is added to one face of a historic stone or brick wall, that wall's behaviour can change dramatically. The risk is that any water that gets into the wall can't escape again, leading to a build-up of condensation which causes dampness over time. This dampness may be hidden behind the insulation and could cause mould growth or rotting of timber elements which might only become apparent years later.
Moisture balance in a brick wall
Image credit - www.greenspec.co.uk
There are two philosophies for dealing with moisture in insulation. One approach is to make the insulation as airtight as possible to minimise the amount of water vapour that can get in and cause problems. This would typically done with foam (PIR) insulation boards, with the joints fully sealed with special tape, finished with plasterboard and a coat of plaster.
The second approach - the one which we’d recommend whenever dealing with pre-WW2 homes especially – is to stick with the historic principle of breathability. By using “vapour-open” insulation materials such as mineral wool, wood fibre and lime-based plaster you can retain the characteristics of absorption and breathability. This is generally considered to be a safer method, but it has some drawbacks. A thicker layer of insulation will typically be required (losing more floor space if you’re insulating internally) and you may require a specialist supplier for skills such as lime plastering. Specialised paints are also recommended for use on lime plaster.
Both approaches are discussed in more detail in our section on Internal Wall Insulation. If you have any further questions please feel free to contact us.