Sources of moisture

Introduction

The ultimate objective of any dampness survey is to identify the 'lead source' of moisture so that actions to terminate the source and remedy the dampness problem can be recommended.

The lead source is either the source responsible for the greatest amount of moisture present, or the source which, if remedied or controlled, would bring the moisture content down to an acceptable level. Identifying the lead source is usually a process of elimination, and this may involve monitoring. Once the lead is identified you may also be able to highlight secondary sources. Typical moisture sources are listed in the table below.

Moisture source Typical examples
air moisture condensation
  • cooking
  • breathing; overcrowding
  • washing; drying clothes internally over radiators on airers or in unvented tumble dryers
  • lack of adequate ventilation
  • type and level of heating
  • fuel poverty; over 10% of disposable income spent on heating the home
  • design, layout and type of construction
  • removal of internal doors particularly to the kitchen 
penetrating dampness
  • rainwater, snow, sleet, hail
  • leaking or overflowing external rainwater goods
  • external wall surcharge (flooding) cause by blocked gully, blocked stream or drainage ditch, etc.
  • raised external abutting ground levels
  • blocked cavity wall voids
  • insulated cavity wall voids
internal plumbing leaks
  • long-term breakdown of plumbing joints
  • accidental damage (e.g. fixings piercing through pipes)
  • overflow from bath/shower
  • water storage
  • cistern leaks/overflows
  • burst pipes (freeze)
  • leaks from washing machine/dishwasher
below-ground moisture
  • underground plumbing leaks
  • water entering a habitable space e.g. a basement ground level
  • leaking ponds
  • high water table
  • leaking potable mains water supply pipes
site/building specific sources
  • rising levels and water tables
  • building on flood plains
  • water from fire fighting
flooding
  • lack of adequate warning or protection to the building
  • lack of district or regional attenuation
  • blocked or defective drainage
  • leaking water mains supplies
other
  • entrapped moisture (typically under flat roof coverings)
  • wet trades - plasters/concretes/screeds
  • materials wetted due to poor site storage prior to construction
  • inadequate drying-out phase after/during construction
  • cleaning fluids/waste water
  • urine - pets/livestock/humans

It is often difficult to single out which moisture source is causing the greatest problem for a building. Figure 1 illustrates the various ways that moisture can enter a dwelling at ground level and shows how there may be several 'sources' operating in one small area. It can also be difficult to draw on mainstream sources for clear guidance, or even for examples of 'patterns of dampness', because most texts offer limited line diagrams that show one or two examples of damp problems, with patterns of dampness usually being restricted to visible symptoms on the inside faces of walls. Nevertheless, surveyors must be prepared to confidently express possible uncertainty in a report, making it clear that professionalism and not ignorance is the reason for appearing inconclusive. See What's going on here for more information.

Dampness investigations are often about eliminating what isn’t the cause until you are left with what is. In more complex cases it may be necessary to carryout invasive or destructive investigations and longer term monitoring, and the building surveyor should be confident to recommend such further work after they have operated tests and measurements to a level 4 and still cannot reach a safe professional opinion.

Figure 1: key moisture sources at the wall base of a traditionally built house

Various 'types' or 'sources' of moisture or dampness are described in surveyors' report forms, textbooks and research reports. These sources are commonly divided into 3: rising dampness, penetrating dampness and condensation. This analysis is challenged by Professor Malcolm Hollis in Surveying Buildings, where he includes 'pipe leaks' as a fourth source.

Traditionally much emphasis has been placed on distinguishing the symptoms of rising damp from those of condensation. Concentrating on these two sources sometimes results in misdiagnosis as penetrating dampness and moisture sourced from plumbing leaks have often been ignored.

So a more useful division of likely sources for surveyors would be:

  • air moisture condensation;
  • penetrating dampness;
  • internal plumbing leaks;
  • below-ground moisture; and
  • site/building specific sources.

This analysis reflects the fact that, strictly speaking, 'condensation' is a mechanism of moisture movement, not a source (the source is air moisture). Rising dampness is also included under the heading 'below-ground moisture', as well as plumbing leaks and other sources.

There will be occasions when the moisture source does not fall into one of these categories, so you must be aware of the wide range of moisture sources possible. You also need to appreciate the problems that can arise following natural flooding.

It is useful to remember that all of the causes of rising and penetrating dampness are also causes of condensation.