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

With the UK experiencing higher monthly rainfall figures – whether from climate change, global warming or typical British weather – the remedial measures needed to dry out wet masonry and protect external walling from wind-driven rain and flooding have become an increasingly important part of the specifier’s armoury.

That nice man from the Met Office never seems to tire of telling us how this month’s rainfall has beaten all records. In fact, over the last ten years, May to September rainfall has regularly exceeded 30-year averages. Increasingly, there also is no let-up during summer. Flooding, once a relatively rare threat associated with winter, has now also become a summer hazard. In the past, those responsible for surveying or maintaining property had some respite from moisture-related issues during the summer months, with walls drying out between April–September, making it safe to redecorate peeling, detached and moisture-affected internal finishes. However, that’s no longer the case.

What causes moisture

Moisture is a primary cause of building materials deterioration and, combined with diurnal temperature variation, has the most significant influence on a material’s overall performance. Moisture can influence external walling in all its states – as a solid (ice, snow), liquid (wind-driven rain) and gas (water vapour). Moisture aggravates most constructional defects, including:

  • Movement
  • Cracking
  • Fungal attacks
  • Chemical reactions

The primary sources of moisture include:

Penetrating moisture. Rain, especially when driven by strong winds, can penetrate the thickest masonry walling through stones and defective or dirty wall ties, usually at mortar joints. The relative exposure of external walling 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. For instance, walling on the periphery of an urban area may have a significantly higher exposure than those positioned more centrally.

Damaged or blocked gutters and downpipes also serve as a source of penetrating moisture. Damaged rainwater goods typically concentrate large quantities of water in one area, resulting in high moisture contents and localised damage externally.

Water vapour. Water vapour often results from typical activities like washing, bathing, laundry, and food preparation, producing as much as 7–10 litres of moisture/ water vapour per day. With an adequate vapour pressure difference, the moisture will transfer freely to the outside through ventilators or dissipate through vapour-permeable building fabric. However, it can condense as liquid moisture on cold surfaces, which increases surface moisture content when absorbed by surface finishes and linings. This increases the risk of mould, fungi or disrupted finishes. 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.

Ground moisture. There is some debate within the industry as to the extent moisture can transfer vertically within masonry; however, there are clear instances where low-level moisture can readily move through external walling. (One example is where adjoining soil or paving is in direct contact with the exterior wall above moisture barriers (DPC).) The higher moisture content at these lower positions increases the risk of thermal bridging and localised condensation, resulting in staining and disruption to lower-level internal finishes.

Flooding. Flooding can be from inundation of water due to leaking internal services (heating system, water supply, etc.) or extreme weather conditions causing increased river or tidal levels. In both cases, water impacts the building violently and immediately, causing physical damage and wetting. Fortunately, the period of exposure to flood waters is usually relatively short –a matter of days rather than months. However, the building fabric may be exposed to water-delivered contaminants in addition to high moisture content. Sometimes, these contaminants can be more complicated than the moisture to resolve effectively.

Measuring moisture

There is a wealth of guidance and research material explaining how moisture impacts walling and how it can transfer (as liquid or vapour) through the constructional build-up, supported by software that models moisture transfer through building materials and components under steady state or dynamic parameters. While terrific tools for academics, 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?

How do we determine to what extent moisture has penetrated the wall, and how long will it take to dry to an acceptable level?

Assessing moisture in building materials. Part 2: Measuring moisture content’ guides a simple, incremental drilling procedure for obtaining samples to determine walling material moisture content and establish the extent of moisture transfer through the depth and height of the wall. The process uses readily available hand tools, and the book also guides 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. When repeated over time, the process gives you an idea of how the moisture content profile changes.

Also, as part of a coordinated site condition survey and analysis, surface moisture content readings indicate the extent of surface condensation and even extreme penetrating moisture. Knowing this helps us better determine the time needed for the wall to dry.

Moisture conditions within external masonry

References providing information on moisture content data that offer guidance and point of reference for surveyors or estate managers in the field include CIBSE Guide A, Environmental Design (2015, 2021 update), which provides an indication of the standard moisture content of ‘exposed’ and ‘protected’ masonry for thermal calculations. Standard moisture content is quoted as:

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

While the BRE Good Repair Guide doesn’t quote standard values, it indicates ‘typical action points’ for a range of masonry materials. For instance, brick moisture contents (facing and commons) above 20%, lightweight concrete above 12% and high-density concrete above 15% are suggested to be within the ‘damage likely’ category. Although not definitive, they give those at the ‘sharp end’ some indication of the moisture content levels we can expect to find and at what level to consider remedial action.

Based on experience gained over 30 years of measuring and monitoring moisture content – including some over extended periods – I can suggest that masonry moisture content is typically below the values indicated by CIBSE. However, new masonry, particularly within the depth of the wall or where plaster or render has been applied, can take several years to reach that level of standard moisture content. In addition, the area where masonry is exposed can experience moisture content close to the ‘damage likely’ level (for instance, at a level to splash back or low-level ground wetting). Moisture contents over winter can also increase by 2–3 %, and rendered or externally clad walls, which typically have a lower average moisture content, can take longer to dry after flooding or severe wetting.

In highly exposed locations, wind-driven rain can penetrate the mortar joints of unrendered/ unclad solid walling (one and two bricks 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 the mortar joints. This moisture typically dries out over the following weeks.

Protecting external masonry walls

Depending on the moisture source, the following suggests remedial measures to protect external walling from wetting:

Penetrating moisture. Protection is primarily by rainscreen or overcladding systems, which reduce the risk of moisture impacting the external wall face. Traditionally, tile hanging and ship-lapped weatherboarding (timber siding) are used to protect the upper storeys of exposed elevations. However, with additional requirements for reducing heat loss, thermally insulated, proprietary, boarded and render systems have become more readily available.

Water vapour. Ventilate internally-generated moisture or condensation to the outside, preferably at the source. 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.

Groundwater. Ensure that any damp-proof courses are not bridged externally by soil, paving, or ramp systems. Adjoining paving or ramps may require extensive work to install an impermeable layer or granular fill (field drain). An impermeable layer laid against the face of the wall protects from wetting by materials in contact 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 can also improve the evaporation of trapped moisture. In addition to these externally applied interventions, several proprietary horizontal barrier systems are available. These typically require cutting, drilling or removal of walling materials, and their success as a barrier can depend on the continuity of the walling; e.g., it may be impractical to provide an effective horizontal barrier in solid walling with a rubble core. (Check with manufacturers on the suitability of their systems.)

Flood waters. While a complicated moisture issue to mediate against, BRE, Historic England, and others have produced detailed guidance based on a building’s age, construction and location. Some protection measures, like physical barriers, can be introduced to reduce the impact of flood waters, while others must be introduced at the refurbishment stage. These include the introduction of water-resistant insulation and finishes at ground floor level, raising electrical sockets and providing ventilation to enclosed voids.

With changing climate conditions resulting in increased rainfall, flooding, and wet masonry issues, those involved in building maintenance and refurbishment must get used to regularly managing. There is a vast wealth of experience and knowledge in measuring and monitoring moisture conditions, refurbishing buildings affected by moisture and reducing future risks. We also need to become more adept at accessing and sharing this information as weather conditions continue to change, increasingly testing existing or traditional building techniques. Alternatively, 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 Pimm’s!

Additional reading

  • Summer condensation on vapour checks; tests with battened, internally insulated, solid walls. BRE Information Paper IP 12/88, 1988.
  • Assessing moisture in building materials, Part 2: Measuring moisture content. BRE Good Repair Guide 33, 2002.
  • Assessing moisture in building materials, Part 3: Interpreting moisture data. BRE Good Repair Guide 33, 2002.
  • Environmental Design. CIBSE Guide A, 2015 (with 2021 update).
  • Flooding and historic buildings. Historic England, 2015.
  • Repairing flood damaged buildings. An insurance industry guide to investigation and repair. BRE EP 69, 2006.
  • Design guidance on flood damage to dwellings. The Scottish Office, 1996.
  • Applying flood resilience technologies. BRE Good Building Guide 84, 2014.


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