NBS Technical Author, Charles Stirling, reminds us of the hazards of moisture penetration in external walls and potential ways to avoid or correct problems...

Be it due to climate change, global warming or just typical British weather, we are becoming increasingly conditioned to those record breaking monthly rainfall figures. That nice man from the Met Office never seems to tire of telling us how this month's rainfall has beaten all records. And, increasingly, there is no let up during the summer; over the last 10 years, rainfall from May to September has routinely exceeded 30-year averages. As a result, flooding, once a (relatively rare) hazard normally associated with the winter months, has become an increasing hazard during the summer months.

In the past, those of us who had a responsibility for surveying or maintaining property typically had some respite from moisture-related issues during the summer months. The period April through to September was a time when we could confidently explain to clients that external walls would begin to dry out; the risk of condensation would abate and they could redecorate peeling, detached and moisture affected internal finishes.

If the natural drying agents are failing, the remedial measures used to dry out wet masonry and protect external walling from wind-driven rain (or flooding) have become an increasingly important part of the specifier's and constructor's armoury.

Sources of moisture

Moisture is the primary agent of deterioration and, combined with diurnal temperature variation, has the greatest influence upon the overall performance of building materials. Moisture can influence external walling in all its states, i.e. as a solid (ice, snow); liquid (wind-driven rain) and gas (water vapour). Most constructional defects, e.g. movement, cracking, fungal attack, chemical reaction, are initiated and aggravated by the presence of moisture.

The primary sources of moisture include the following:

  • Penetrating moisture: Rain, particularly when driven by strong winds, can penetrate the thickest masonry walling, typically at mortar joints, through stones and defective or dirty wall ties. The relative exposure of external walling, see photograph 1, will determine the potential risk to wind driven rain. In addition, the height and proximity of neighbouring buildings or the local topography can considerably influence this exposure, e.g. walling on the periphery of an urban area may have a considerably higher local exposure than those positioned more centrally, see photograph 2. Damaged or blocked gutters and downpipes, see photograph 3, provide another source of penetrating moisture. Damaged rainwater goods typically concentrate large quantities of moisture in one area, resulting in high moisture contents and localized damage externally, see photograph 4, and potential for damage internally.


    1. A beautiful view, but very little protection from the elements


    2. Dense urban environments may provide some local protection


    3. Overflowing rainwater goods resulting in localized moisture and staining


    4. Internal disruption due to overflowing gutter and damaged downpipe
  • Water vapour: This arises from many of the activities we undertake within buildings, e.g. washing and bathing, laundry, and preparation of food. These activities can typically produce 7 to 10 litres of moisture/ water vapour per day. With an effective vapour pressure difference, this water vapour will freely transfer to the outside through ventilators or dissipate through vapour permeable building fabric. However, it may also condense as liquid moisture on cold surfaces. Where this condensation is absorbed by surface finishes and linings, increasing surface moisture contents, there may be an increased risk of mould, fungi or disrupted finishes, see photograph 5. In addition, water vapour from external sources, driven by higher external temperatures, can diffuse through the wall from outside to inside, condensing on the back of high (vapour) resistance internal linings or vapour control layers (see further reading 1).


    5. Mould and disrupted internal finishes as the result of an extreme condensation issue
  • Ground moisture: Over the past few years there has been some debate within the industry as to the extent moisture can transfer vertically within masonry. However, there are clear instances when low level moisture will readily transfer through external walling, e.g. where adjoining soil or paving is in direct contact with the external wall above dpc moisture barriers. Higher moisture contents at these lower positions can also result in an increased risk of thermal bridging and localized condensation, resulting in staining and disruption to low level internal finishes.

  • Flood waters: A typically infrequent event where the building is inundated as a result of leaking internal services, e.g. heating system, water supply, drainage; or due to extreme weather conditions resulting in increased river or tidal levels. In both instances water impacts upon the building in a violent and immediate fashion, typically resulting in physical damage and wetting. Fortunately, the period of exposure to flood waters is typically relatively short, usually restricted to days rather than months. However, in addition to high moisture contents, the building fabric may also be exposed to contaminants carried into the building by the flood waters. These contaminants can sometimes be more difficult than the moisture to effectively resolve.

Measuring moisture

There is a plethora of excellent guidance, reference and research related material dedicated to explaining how moisture impacts on walling and how it can transfer, either as liquid or as vapour, through the constructional build-up. This is supported by a wide range of software which will model moisture transfer through building materials and components under steady state or dynamic parameters. These are terrific tools for the academic but what about those poor surveyors and maintenance managers who need to analyse and interpret moisture-related issues on the ground.

How do we differentiate between a minor condensation issue and rising ground moisture on site? To what extend has moisture penetrated the wall and how long will it take to dry out to an acceptable level?

The BRE provides guidance (further reading 2 and 3) on a simple incremental, drilling procedure for obtaining samples used to determine the moisture content of walling materials. In particular, it can establish the extent of moisture transfer through the depth and up the height of the wall. It involves the use of readily available hand tools to obtain samples and provides guidance on measuring and analysing the moisture content results.

The primary advantage of this procedure is the ability to assess the moisture content profile through the wall. It allows us to establish whether the moisture content is higher towards the external face, suggesting wetting by rain penetration; or higher towards the inner face, suggesting condensation. The procedure becomes particularly powerful when repeated over a period of time to assess changes in the moisture content.

Undertaking surface moisture content readings is sufficient for the competent surveyor or architect to assess any impacting moisture issue and propose appropriate, remedial actions. However, when undertaken with incremental readings as part of a coordinated survey and analysis of site conditions, they can give us an indication of the extent of surface condensation and even extreme penetrating moisture. The coordinated survey is able to provide an indication of the volume of moisture which may be entrapped within the building fabric. Knowledge of the extent of this moisture assists in providing us with more accurate information on the time necessary for the wall to dry.

Moisture conditions within external masonry

There are very few reference documents which provide information on the moisture content data which could be used as a guide or point of reference by surveyors or maintenance managers in the field. CIBSE (further reading 4, Table 3.2) provides an indication of the standard moisture contents of 'exposed' and 'protected' masonry for the purposes of thermal calculations. Standard moisture contents are quoted as follows:

  • Exposed (external leaf) brickwork and concrete 5%
  • Protected (internal leaf) brickwork 1%
  • Protected (internal leaf) concrete 3%.

Although not quoting standard values, BRE Good Repair Guide 33 (further reading 3, Table 2) provides an indication of 'typical action points' for a range of masonry materials. It is suggested that brick moisture contents (facing and commons) above 20%, lightweight concrete above 12% and high density concrete above 15% are within the 'Damage likely' category. Although not definitive, they do give those of us at the 'sharp end' some indication of the levels of moisture content we may expect to find and at what level remedial actions should have to be considered.

From experience gained over 30 years of measuring and monitoring moisture contents, some over extended periods of time, it can be confirmed that:

  • Averaged masonry moisture contents are usually below the standard values as suggested by CIBSE
  • Over the winter period, moisture contents can increase by 2 to 3 %
  • New masonry, particularly within the depth of the wall or where plaster or render has been applied, can take many years to reach these standard moisture contents
  • Where masonry is exposed, e.g. to low level splash back or ground wetting, then moisture contents close to the 'Damage likely' level may be experienced
  • Rendered or externally clad walls, which may typically have lower averaged moisture contents, can take longer to dry out following flooding or severe wetting.

In highly exposed locations, wind-driven rain can readily penetrate the mortar joints of unrendered/ unclad solid walling (one and two brick thick). This penetrating moisture may cause severe disruption to internal finishes.

However, there may be little impact on the masonry other than a slight increase in the moisture content of the outer brickwork and of the mortar joints. This moisture typically dries out over the following weeks.

Protection of external masonry walls

Depending upon the source of the moisture, the following provides some guidance as to some of the remedial measures which can be utilized to protect external walling from wetting:

  • Penetrating moisture: Protection is primarily by rainscreen or overcladding systems which reduce the risk of moisture impacting upon the external face of the wall. Tile hanging and ship lapped weather boarding (timber siding) were traditionally used to protect the upper storeys of exposed elevations. However, with increased requirements for reduced heat loss, thermally insulated, proprietary, panel and render systems are increasingly available, see photograph 6.


    6. Rendered external insulation system providing additional thermal and weatherproofing performance

  • Water vapour: Moisture or vapour generated internally should be ventilated to the outside, preferably at source, see photograph 7. In addition, thermal insulation applied either externally or internally to the face of the wall will increase internal surface temperatures, thereby reducing the risk of condensation on cold surfaces.


    7. Mechanical extract fans in kitchens and bathrooms can reduce the risk of condensation

  • Ground water: Ensure that any damp-proof courses are not bridged externally by soil or by paving or ramp systems. Adjoining paving or ramps may require extensive working to install either an impermeable layer or granular fill (field drain) against the face of the wall. An impermeable layer provides protection from wetting and should be continuous with any horizontal damp proof course. Granular fill assists with drainage at the wall face, reducing the water pressure on the outer face of the wall, and improving evaporation of entrapped moisture. In addition to these externally applied interventions, a number of proprietary horizontal barrier systems are available. These typically require cutting, drilling or removal of walling materials and their success as a barrier may be dependent upon the continuity of the walling, e.g. it may be impractical to provide an effective horizontal barrier in solid walling which has a rubble core. Check with manufacturers on the suitability of their systems.

  • Flood waters: This is a particularly difficult moisture issue to mediate against and a number of agencies (see further reading 5, 6 and 7) produce detailed guidance depending upon the age, construction and location of buildings. Some protection measures, e.g. physical barriers, can be introduced in an effort to reduce the impact of flood waters; others, typically, have to be introduced at the refurbishment stage and can include the introduction of water-resistant insulation and finishes at ground floor level, raising electrical sockets and introducing ventilation to enclosed voids.

Changing climatic conditions that result in higher rainfall are something that those of us involved in building maintenance and refurbishment will increasingly have to get used to and manage. This will include making clients aware of the full extent of moisture damage and the considerable time that may be required for materials to effectively dry out. There is a vast wealth of experience and knowledge on measuring and monitoring moisture conditions, in refurbishing buildings affected by moisture and on reducing future risks. We need to become more adept at accessing and sharing this information as weather conditions continue to change and existing or traditional building techniques are increasingly tested.

Or, should those six numbers come up next Saturday, we could consider moving to warmer climes and continue to watch that nice man at the Met Office, with a knowing smile and celebratory Pimms!

Related reading on theNBS.com

Building pathology: moisture conditions within traditionally insulated pitched roofs

Further reading

1. Summer condensation on vapour checks; tests with battened, internally insulated, solid walls. BRE Information Paper IP 12/88, 1988
2. Assessing moisture in building materials, Part 2: Measuring moisture content. BRE Good Repair Guide 33, 2002
3. Assessing moisture in building materials, Part 3: Interpreting moisture data. BRE Good Repair Guide 33, 2002
4. Environmental Design. CIBSE Guide A, 2006
5. Flooding and historic buildings. English Heritage, 2004
6. Repairing flood damaged buildings. An insurance industry guide to investigation and repair. BRE EP 69, 2006
7. Design guidance on flood damage to dwellings. The Scottish Office, 1996.