In this article Mike Parrett,an independent damp consultant and building pathologist with many years experience working in senior surveying and management positions within local authorities looks at the causes of dampness.

Condensation within occupied dwelling houses and flats occurs as a result of a number of factors that contribute to its formation. The principal factors could be any combination of the following:

• Use and occupation of a dwelling
• Design and layout
• Disrepair
• Construction

What is dry in building elements?

To best define dampness in buildings is to look first at what is acceptably dry in a range of building materials – the normal hygroscopic moisture content (HMC).

From extensive tests I have conducted both in Lewisham’s housing stock and all over the United Kingdom coupled with established research, we find that typical average moisture contents in a range of different materials can be tabled as follows:

For habitable rooms within dwellings the internal atmosphere should be in a range of 30 – 60% relative humidity for occupants to feel comfortable and for the avoidance of creating mould colonies to form and the proliferation of the house dust mite.

A typical construction brick, fletton, stock etc would be around 0.1 – 0. 5% total moisture content (TMC).

Typical mortar: up to 1-2%

Gypsum plasters (not Carlite): 0.2%

Softwood timber i.e. skirting board, in a range 8-12% air dry. Above 22% timber will decay if moisture above this level is sustained.

Solid concrete – cementitious floors - British Standard BS 8203: 1987 which covers the requirements of laying resilient floor coverings benchmarks 75% relative humidity or below as an acceptable moisture content upon which floor coverings may be laid.