Cider House Conservation
Dampness in older buildings
Two common problems with older buildings are dampness and poor energy efficiency. These are often related since a damp house is a cold house, requiring greater heat input to achieve comfort. So reducing damp is an important first step to improving energy efficiency.
Most buildings were not damp when first built so any subsequent appearance of dampness is usually due to structural alterations, changes in use and/or changes in surface finishes.
Old buildings, in particular, need to breathe (allow water vapour to escape) since many materials used in their original construction are relatively "soft" and tend to absorb excess moisture. This is not generally a problem provided the moisture can readily escape. Breathable finishes (e.g. lime mortars, lime plasters/renders, limewash) enable the easy escape of this moisture as soon as conditions allow - a gentle breeze across a wall will quickly remove dampness following rain. Old brick walls without moisture-trapping finishes are usually very dry.
Unfortunately many widely used "modern" materials such as cements renders and most petrochemical-based paints and mastics are not breathable in any significant way. So moisture, often entering the structure by capillary action through microscopic cracks and pores (almost impossible to prevent), becomes trapped and gradually builds up. This leaves walls increasingly damp and cold whilst causing associated timbers to rot, plaster to crumble and paint to bubble and peel.
Removal of these materials, where possible, and replacement with breathable alternatives is recommended.
The external ground levels against buildings often become raised over time, (e.g. paths being built on top of earlier paths) so that the internal floor level is now at or even below the external ground level. This often leads to excessive dampness in the lower walls. Older buildings rarely had a damp course although a slate course became more common from the Georgian period and later a bitumen course may be found. Membrane damp-proof courses only started to be used from about the mid 20th century. A damp course, if installed, would normally have been set 2-3 brick courses (or equivalent) above the original ground level at time of construction. Any less than this suggests ground levels have been raised and would be best lowered back if possible. Older building without a damp course depend on high breathability in the lower walls to prevent internal dampness, so any raised ground level is likely to cause problems. External ground levels should ideally be at least 150mm (6") below internal floor levels although any digging out needs to take care not to go below the footings of the wall as these may be very shallow. If ground levels cannot be reduced then installation of a French drain may help.
Internally, replacement of old, somewhat breathable floors (e.g. flagstone or brick pavers set on sand and lime, or suspended wood floors) with a concrete slab laid over a plastic membrane often drives naturally rising ground moisture sideways where it escapes into the softer lower walls, potentially making them much damper. Installation of a breathable limecrete floor is a much better option than a concrete floor in older buildings and is now widely specified in churches, cathedrals and other historic structures.
Concrete or other impervious materials laid tight up against external walls can have a similar effect, again driving moisture into the lower walls. A breather strip of open soil or gravel against the building will minimise this problem.
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