5.1 Moisture in Buildings – Biological Decay

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Main contributors to Module 5

Andy Simmonds, Tim Martel (modelling), Dr Eric Rirsch (Safeguard Europe), Joseph Little, Tina Holt (editing). Many others have offered expert academic and experimental insight or shared valuable feedback from live retrofit projects.

Note that dark blue text in this module denotes material used with permission from Joseph Little’s Tech 15 paper, a full version is available below if you wish to read it: hs-technical-paper-15_UseApproved_credited

The key objectives of Module 5 are:

  • To introduce the key issue of moisture in buildings.
  • We look at the bugs, rots and moulds that are some of the consequences when moisture is not well-managed in a building (old or new).
  • We then look in more detail at the way water in its different states behaves under the influence of changing environmental conditions, and how these different states of water interact with different kinds of building material.
  • After that we will look at these processes and conditions in the complete building, to understand what might make a building or part of it too damp (before and/or after retrofit) – and suggest how we might avoid this happening.

Module 5 (Moisture in Buildings) consists of the following lessons:

Module 5 Part A

Lesson 5.1 Biological Decay

Lesson 5.2 The Moisture Performance Gap

Lesson 5.3 Relative and Absolute Humidity

Lesson 5.4 Diffusion and Bulk Air Movement

Lesson 5.5 Evaporation and Condensation in Building Materials

Lesson 5.6 Liquid Water and Building Materials

Lesson 5.7 Wood Moisture Equivalent (WME)

Lesson 5.8 Moisture Movement – Magnitude and Direction

Module 5 Part B

Lesson 5.9 Problems and Solutions

Lesson 5.10 Heat Sources and Microclimates

Lesson 5.11 Moisture Reservoirs, Bulk Air Flow and Diffusion Flow

Lesson 5.12 Moisture Movement – Liquid Water Flows

Lesson 5.13 Rising Damp and Hygroscopic Moisture

Lesson 5.14 Suspended Floors

Lesson 5.15 Hygrothermal modelling, surveying, monitoring and analysis

Lesson 5.16 Introduction to the CLR case studies

Lesson 5.17 How to look at your retrofit

By the end of this lesson you will have learned about:

  1. An introduction to Moisture in Buildings
  2. Biological decay (Rots, Moulds, Bugs)
  3. Understanding rot
  4. Understanding mould growth on substrates – critical thresholds
  5. Examples of rots, moulds and bugs in action

1. An introduction to Moisture in Buildings

One of the most important challenges to any building, and one of the major potential risks arising from retrofit is the damaging effect of excess moisture on:

  • Indoor air quality
  • Thermal performance of insulating materials
  • The durability of building materials and components
  • The comfort, health and wellbeing of the occupants.

Sadly, hastily executed and partial retrofits too often pay little or no attention to existing moisture issues in the house before retrofit, and to the likely impacts of the retrofit.

A ‘moisture robust’ retrofit strategy should include:

  • Identification and rectification of existing problems – some rectification work may be required well in advance of the main retrofit works e.g. the ‘drying down’ of the building through rising / penetrating damp, rain protection or external drainage measures
  • Identification of any related repair and renovation work resulting from the above – e.g. relating to the treatment or removal of vulnerable or damaged materials
  • Contingency planning for potential ‘hidden’ problems – i.e. in hard to survey areas where issues cannot be accurately identified until retrofit works are underway
  • Assessing how new work intended to be integrated with the retrofit – such as extensions or internal remodelling – will impact the moisture balance of the existing building
  • Energy efficiency measures appropriate to the building’s materials and construction type

Issues caused by moisture

Too much moisture in the wrong place in a building – on or in the structure or in the living spaces – has a number of undesirable and in some cases, potentially dangerous effects:

  • Visible moisture (e.g. condensation) looks bad in itself, and can attract dirt and cause stains. This is often annoying for occupants, and can also become disruptive and costly e.g. because of the need to redecorate.
  • Moisture can worsen the thermal performance of materials. For example, the rate of heat loss through saturated masonry or insulation will rise significantly. This in turn will decrease comfort and increase energy use.
  • Visible and unseen moisture can damage building fabric by chemical and mechanical means. For example, salt crystals can migrate into and damage – even destroy – parging layers, plasters and renders, insulation materials and decorative finishes. Ice crystals can, through freeze thaw action, damage materials e.g. causing spalling in mortar or bricks, making them flake and erode away.
  • Moisture can promote the growth of surface moulds, wet and dry fungal rot, wood boring beetle populations and also support high populations of ‘pests’, including house dust mites and silverfish.
  • Airtightness measures should be designed for long term performance. Excessive moisture may lead to early failure of airtight junctions, which in turn can lead to worsened conditions and even more moisture in the wrong place.

All of these conditions, when visible, are likely to cause occupant distress, and whether visible or hidden they can cause damage, including structural damage, to the building fabric. These conditions also endanger the health and wellbeing of occupants, in particular via house dust mite faeces and airborne spores from fungal growth.

2. Biological decay

  • Building materials are decayed by the effects of adverse environmental conditions
  • Vulnerable materials are subject to microbiological deterioration as a result of the activity of a broad range of micro-organisms
  • Principal factors favouring bio-deterioration are: availability of oxygen and nutrients, temperature, moisture content/humidity and lack of ventilation
  • Moisture sources may be related to: penetrating or rising damp; condensation; building defects, historic or works-related rain ingress and / or other leaks; construction moisture introduced during building works e.g. replacement floors e.g. in situ concrete slabs, parging or plastering layers
  • Among the most vulnerable materials are timber, paint, textiles and paper – this course tends to concentrate on risks to timber components
  • Timber provides specialised ecological niches and many organisms have evolved to use it as a food
  • The most common and destructive to timber are dry rot, wet rot, common furniture beetle, and death watch beetle.

Further reading (1): see article by Dr Jagjit Singh: http://www.buildingconservation.com/articles/envmon/timber_decay.htm

Bugs, Moulds and Rots

Bugs – the wood boring larvae of beetles (and other pests)

“Beetles display some of the most diverse morphological forms in the animal kingdom. Wood boring beetles are no exception and demonstrate quite variable forms as both larvae and adults. Some adults range from the very large, i.e. >50mm (some Buprestidae and Cerambycidae), while others are minute, i.e. <5mm (most Ptinine Anobiidae and some Bostrichidae).”

From a useful online beetle identification tool: http://idtools.org/id/wbb/families/index.htm accessed 05.11.15

Beetles (or more accurately their larvae) that are responsible for timber decay include:

  • Woodworm (Anobium punctatum)
  • Death watch beetle (Xestobium rufovillosum)
  • Powder post beetle (Lyctus spp)
  • House longhorn beetle (Hylotrapes bajulus).

See required reading (1) and (2)

Fungi – including Moulds and Rots

  • ‘Fungi’ are one of the sub-kingdoms in the Eucarya Kingdom and includes all fungi, moulds & mushrooms
  • Fungi are very diverse
  • Fungi are differentiated from algae on the basis they lack chlorophyll
  • Fungi are subdivided into groups (or subsets) within the sub-kingdom on the basis of their life cycles, the presence or structure of their fruiting body and the arrangement of and type of spores they produce
  • Most fungi are microscopic in size, some are extremely large (the largest known organism on earth is a fungus)

The three major groups of fungi are:

  1. Multicellular filamentous moulds (referred to as ‘moulds’ in this course)
  2. Macroscopic filamentous fungi that form large fruiting bodies. Sometimes the group is referred to as ‘mushrooms’, but the mushroom is just the part of the fungus we see above ground which is also known as the fruiting body. (included in ‘rots’ in this course)
  3. Single celled microscopic yeasts” (not considered in this course)

Decay fungi – through the action of enzymes – degrade complex cellulosic materials, such as timber, into simpler digestible products.

Moulds‘multicellular filamentous moulds’ are one of three major subgroups of Fungi

Moulds grow as hyphae or slime, and are able to form fruiting bodies that allow them to generate spores. These fruiting bodies are the stalk like structures that you see on the surface of the mould (the mould is present for a period before it starts to form fruiting bodies).

Mould growth is considered to act as a ‘precursor’ to other forms of decay as a result of enzyme action and its ability to attract and concentrate further moisture from the environment thus increasing local humidity.

Moulds potentially present in buildings include:

  • Cladosporium spp
  • Penicillium spp
  • Aspergillus spp
  • Trichoderma spp
  • Alternaria spp
  • Aureobasidium spp

Slime moulds may be present in some specific areas such as waste traps e.g. Myxomycetes

Photo of slime mould
Photo of slime mould

Above: a slime mould growing in (and blocking) the pipe used to drain condensate from an MVHR. The MVHR condensate backed up and leaked from the unit wetting the floating timber floor construction – a population of silverfish appeared to benefit from the increased moisture contents in this area. Similar was seen in a ‘bladder type’ trap used for the same purpose – leading to a failure of the trap to block external air. Again in a similar situation of a bladder type trap serving a wash hand basin and connected to a foul drainage system, foul air could be smelt entering the room. In the former case described above, remedial action included changing the condensate pipework to larger diameter and using a U bend type trap, and in the latter case, cleaning the bladder trap with a view to replacing at some point with a U bend type. Following remedial action the silverfish population diminished markedly.

See Required reading (3)

See Further reading (3)

Rots – ‘macroscopic filamentous fungi that form large fruiting bodies’ are another of the three major subgroups of Fungi

The main types of wood rotting fungi from this group are:

  • White rots cause wood to become lighter in colour and fibrous in texture, without cross-cracking
  • Brown rots cause the wood to become darker in colour, and to crack along and across the grain. When dry, very decayed wood will crumble to dust.
  • White and Brown rots are referred to as ‘wet rots’ except for one brown rot, Serpula lacrymans, which is commonly known as ‘dry rot’
Wet rots include:

Cellar rot fungus (Coniophora puteana); Poria fungi, (eg Amyloporia Xantha; Fibroporia vaillantii and Poria placenta); Phellinus continguus; Donkioporia expansa; Oyster fungus (Pleurotus ostreatus); Asterostroma spp; Paxillus panuoides; Lentinus lepideus; Dacrymyces stillatus; Ptychogaster rubescens

Soft rots:

Chaetomium globosum

Plaster fungi:

Coprinus spp; Peziza spp; Pyronema spp

Stain fungi:

Cladosporium spp; Aureobasidium spp

Further reading:

The fungal decay of timber – featuring ‘dry rot’

Used with permission from: http://static.safeguardeurope.com/pdfs/dry_rot_book.pdf


Wood is a natural material being the end product of a complex chemical process – photosynthesis – which occurs in green plants:

Diagram of photosynthesis
Diagram to illustrate photosynthesis

Wood basically consists of boxes and tubes made of sugars which are linked together to form cellulose, the basic building material of plants. Chains of cellulose are laid down in different orientations and bonded by another material, hemicellulose. A further material, lignin, adds rigidity and strength. It is the arrangement of cellulose with the two other materials which give wood its characteristic properties and its ‘cellular’ structure.

The wood forming the outer part of the tree is known as the sapwood and transports sap and stores food:

Cross section through a tree trunk
Cross section through a tree trunk

This is the most vulnerable part of wood to fungal decay and attack by wood-boring insects. The inner wood is the heartwood and forms the older wood in the centre of the tree; it does not conduct sap or store food but it does contain some excretory products and is more resistant to decay than the sapwood. It is also more resistant to the movement of water and preservatives in general. The heartwood of different timbers varies in its resistance to fungal decay and it is this heartwood resistance to decay by which timbers can be classified, i.e., non-durable, durable, etc.


Wood decay is basically the reverse of wood formation. Dry rot attacks the cellulose and hemicellulose of the wood to break it back down into its sugar components (Fig. 3). The sugars are respired with air to produce carbon dioxide, water and the energy for growth. However, the lignin is not metabolised and this gives rise to the darkening in colour of the wood. A number of wood destroying fungi other than dry rot also decay the wood in the same manner, leaving the lignin untouched. The characteristic darkening of the wood by these fungi give them the loose title of ‘brown rots’; dry rot is one of the brown rots. When the wood is broken down and utilised for food, shrinkage, loss of weight, loss of strength and cracking occur. It is the shrinkage which causes the typical ‘cuboidal’ cracking (cracks to form small cubes) of dry rot and the other brown rots. Indeed, it is this shrinkage and cracking which is often the first sign of a problem.


The essential requirements for any fungal decay to take place are both food and water, especially the latter at a sufficient level. Fungal decay is generally initiated in several stages. First the water penetrates the wood and this allows bacteria and micro fungi to colonise. These break down part of the cell structure but do not cause weakening of the wood. Instead, the wood becomes more porous which allows it to become even wetter. Provided that the wood is now sufficiently wet and remains wet and that other conditions are suitable the wood rotting fungi such as dry rot can colonise.


It is essential to understand that water is absolutely fundamental to the growth and survival of not only dry rot but all wood destroying fungi; wood decay cannot occur, exist or survive without it!

Spore germination: To initiate growth from a spore the wood must be physically wet; in other words it must be subject to a source of water ingress, e.g., leaking gutters, wood in contact with damp masonry, etc. In practical terms the wood must have moisture content in excess of 28-30%. Spores will not germinate on dry surfaces or surfaces which are not suitably wet. In other words, unless the wood is wet dry rot cannot become initiated. Growth: Whilst timber needs to be wet for growth to be initiated, at moisture contents of around 22% existing mycelial growth ceases and the fungus will eventually die; decay just above 22% is likely to be very minimal. However, for practical purposes when dealing with fungal decay as a whole, moisture contents of 20-22% should be taken as the threshold figure and assume moisture contents in excess of this level put the timber at risk. The fungus flourishes under humid, stagnant conditions; hence growth tends to be secretive and hidden and is therefore often extensive before it becomes evident.

Summary Table of Bugs, Mould and Rot in Buildings
Summary Table of Bugs, Mould and Rot in Buildings


3. Understanding rot

Moisture thresholds (dark blue text from Joseph Little’s Tech 15 report)

To avoid rot, it is generally recommended to ensure that moisture content levels are maintained below a threshold of 20 % of the mass of the timber.

“Decay generally requires wood moisture content at fiber saturation (usually about 30%) or higher and temperatures between 10 and 40°C. Because wood moisture content can vary widely with sample location, a local moisture content of 20% or higher may indicate fiber saturation elsewhere.” (ASHRAE, 2009, p. 25.15)

A threshold level of 20 % is also recommended in Fraunhofer IBP (2011), whereas Singh (1996) suggests using a lower threshold of 16 to 18 % for subsurface moisture content level. Ridout (2000) describes the different effects that two different water content levels (both greater than 20 % by mass) can have on timbers built into masonry walls. The first may only be relevant at roof leaks and the likes. The latter could well be relevant behind internal insulation installed to certain types of solid masonry walls.

“If the walls are made of brick or any other porous material and excess water penetration has caused free water to fill the large pores, then the water will travel easily along the end grain of the timber. The fungi will consume the entire component end as far as the water entering the wall and evaporative loss will allow. This form of decay will continue for some time, even after the supply of water is halted, because until the large pores of the wall are empty of water there is plentiful supply of free water within the wall.

“However, if the wall is only damp then much of the water it contains will be held by capillary forces in the small pores, and the amount of water that is available to the fungus is limited. The timber will still wet and rot, but only the bearing within the wall will normally be lost. Decay caused by small leaks is therefore usually restricted, and will cease rapidly when water penetration is halted.” (Ridout, 2000, pp. 129-130)

To reduce the absorption of moisture from the masonry into the timber, the latter could be isolated from the masonry.

“Timber should be isolated from damp masonry by air space or damp proof membrane, and free air movement should be allowed around timber in walls, roofs and suspended floors.” (Singh, 1996)

The purpose of isolating the timber from the masonry is twofold: to avoid moisture transport through capillary action and to increase the potential for evaporative drying. However, isolating timber from the masonry also results in an increased oxygen supply, potentially causing mould growth and thermal bypass.

CLR research to date suggests that designing to promote a timber moisture content of ≤15% at critical positions within retrofitted assemblies would seem a comfortably safe target. This is certainly possible in some situations – but likely to be more challenging in others. Certainly a maximum moisture content of ≤18% should be aimed for where vulnerable materials are present, and if considering this less challenging target solutions and products providing enhanced drying and moisture handling should be considered to temper the resultant humidity levels resulting from the higher moisture content being anticipated. More on this later in the course, including whether to consider Relative Humidity or Moisture Content as the key factor in decision making or monitoring – useful where expert hygrothermal analysis is not available.

4. Understanding mould growth on substrates

Mould growth

Mould is a fungus, ubiquitous in nature. Thousands of known mould species exist. Mould reproduces via spores, which are common components of household and workplace dust.

“Moisture in buildings arises from several sources: if not properly controlled it can lead to mould growth and condensation – problems which affect about 15% of homes in England to some degree.” (BSI, 2011, p. 5)

Mould growth in buildings occurs generally on the surfaces of building materials. It can often be smelt, frequently identified as ‘a damp smell’, before it is seen. Where surfaces are visible, mould can be perceived due to its colour as surface discolouration, also referred to as ‘ghosting’ or staining, or, in more extreme cases, as fuzzy layer of growth on surface. (Figure 41) Mould growth can also occur interstitially, where it is not as readily noticed.

“Excessive mould growth in buildings has multiple consequences. Discoloration due to the onset of mould can lead to higher maintenance costs, to the economic devaluation of buildings where mould is persistent and, in the case of historic buildings, to damage of historically important finishes. At worst, mould growth releases an abundance of spores and volatile organic compounds in the air that can lead to health problems for building occupants, potentially causing allergic reactions and respiratory problems.” (WHO Europe, 2009)

However, mould growth can only occur under suitable environmental conditions. Mould growth should therefore be avoided by ensuring that environmental conditions are created that don’t encourage mould growth. These threshold levels and the methods to assess them are discussed in the following. At design stage steps should be taken to design-out the potential for mould to occur.

JL Fig 41 L5.1

Above: severe mould growth on the non-hygroscopic surfaces of a coated window frame (left) and a plasterboarded window reveal (right).

Mould risk evaluation

As living organisms, moulds require certain conditions to grow. In particular, there must be sufficiently high temperature and moisture levels and nutrients available. Moisture does not have to be present in liquid form for mould to grow. If the relative humidity is high enough, it creates environmental conditions sufficient for mould growth.

The British Standard BS 5250, concerned with condensation control in buildings, states, regarding the relationship of indoor humidity levels and mould growth:

“Large numbers of mould spores are always present in the atmosphere. In order to germinate those spores require warmth, a source of nutrition, oxygen and moisture; because they are hygroscopic they do not require liquid water. Many mould spores can germinate if the relative humidity at the surface exceeds 80%. Once established mould spores can continue to grow at a moisture level lower than 80%.

“Buildings provide many sources of nutrition, and oxygen is always present, consequently the growth of moulds depends on moisture conditions at internal surfaces. In winter the internal surfaces of the external walls can be colder than the air in the room and the relative humidity at the face of the wall is about 10% greater than that in the room. As a result, if the relative humidity of the room stays at 70% for long periods of time the external wall surfaces will be sufficiently humid to support the growth of mould.

If the relative humidity of the air in a room exceeds 70%, the surface relative humidity of an external element is likely to exceed 80%. If that occurs for more than two or three days, mould is likely to develop on the surface. Surface relative humidity is determined by the internal vapour pressure and the surface temperature of the external element, which depends upon the nature of the construction. The presence of thermal bridges such as those around doors and windows produces lower local temperatures.” (BSI, 2011, pp. 15, 23)

80 % RH is often used as a rule of thumb threshold to stay below in order to avoid mould growth on surfaces.

There are primarily three factors that influence the amount of moisture available for mould growth:

1. High vapour resistance in building assemblies which allow moisture to build up at interfaces between different construction layers / components.

2. Liquid water infiltration from outside as a result of a leaky building envelope or structural failure. This includes groundwater introduced via rising or penetrating damp.

3. Moisture condensation on mould susceptible substrates, which originates from water vapour inside, outside or below the building.

As stated previously there are hundreds of thousands of mould species on the planet. Of these, a few hundred have been found in buildings, and their growth patterns studied in detail. (Sedlbauer, 2001) Each mould species is slightly different and, therefore, has slightly different growth conditions.

In summary, for mould to grow in buildings requires:

  • oxygen
  • nutrients
  • sufficient moisture – over a long enough period of time for spore germination and mycelium growth.

Key facts:

  • Spores germinate in about 5 – 7 hours
  • Once the mycelium begins to grow it generates its own water supply from the surrounding environment
  • Mould is sensitive to heating and more so to desiccation (typically when relative humidity below 65%)
  • Lower temperatures inhibit the various moulds’ rates of growth
  • Mould mycelium may die off, but spores remain viable for new growth when conditions are suitable.

Substrate classes

Different materials provide different conditions for mould growth. Building materials are generally grouped into three substrate classes:

  • Class I are easily biodegradable materials, such as wall paper and gypsum plaster- board and materials for permanently elastic joints
  • Class II are less degradable, porous materials, such as plasters and mineral building materials and some timbers.
  • Class III are building materials that can be neither degraded nor contain nutrients non-degradable materials, such as glass, metals, foils and tiles.

Effect of temperature on mould growth and biological decay


Buildings are typically maintained at a temperature of 18ºC to 24ºC, which is hospitable to many moulds, some of which can survive at temperatures below 10°C or above 50ºC. At relatively low temperatures (10ºC to 15ºC) spores take longer to germinate, and growth is slower.

We look in more detail at critical thresholds of temperature and effects on rates of mould growth, for different moulds, later in the course.

Nutrient Sources

There are numerous sources in today’s buildings to satisfy the nutritional needs of fungi, including materials containing cellulose, such as:

  • wallpaper e.g. ‘woodchip’
  • gypsum wallboard
  • wood paneling
  • plywood
  • oriented strand board (OSB)
  • pre-cast panels and ceiling tiles
  • fabrics and carpets
  • upholstered furniture
  • and other porous materials where fungi break down the material itself or use organic debris that has collected.

Some traditional construction and finishing materials contain natural chemicals that retard biodeterioration; for example, the heartwood of some tree species contains terpenes and other substances that inhibit fungi growth.

Man-made products such as ceiing tiles, fabrics, carpets, draperies, upholstered furniture, some insulations, gypsum wallboard covered in paper with cellulose and pressed wood products with added binders and resins are susceptible to mould growth as they lack natural antimicrobials and provide a nutrient source.

5. Examples of moulds, bugs and rots in action

Wet rot, dry rot, beetle damage
Photos of wet rot, dry rot, beetle damage

Above,left to right: Wet Rot, Dry Rot, Beetle


Table 15atabshot1a

Table 2



Table 3 : Potentially vulnerable timbers identified prior to IWI:




Above image: suspended timber floor with shallow crawlspace over damp earth subfloor. The assembly was removed and replaced with a solid insulated floor as a moisture robust retrofit of the existing assembly was considered technically too risky. In another project, a homeowner who was about to remove an old suspended timber floor (not a radon area) instead experimented with removing some floorboards, laying cheap PIR foamboard ‘factory rejects’ on the earth subfloor and – using a small amount of closed-cell spray foam – filling the remaining gap up to the underside of the floorboards. The foam encapsulated the joists and provided a capillary and vapour ‘barrier’ with the earth subfloor – except at the floor perimeter. The existing slow, ongoing (perhaps speeded up) decay of joist ends and possibly floorboard edges was part of an accepted level of risk based on a ‘planned obsolescence’ strategy. The level of decay at floor perimeter will be observed over time, including evidence of rots and moulds. This approach, like some other similar experiments is intriguing in its low cost DIY approach, but does not deal at all with the airtightness issues at the floor to wall junction.

Below image : a suspended timber floor over a damp basement with low headroom was replaced with a thin structural steel/concrete floor slab and insulated above with woodfibre insulation continuous with woodfibre IWI. The post retrofit risk to floor timbers from the effects of the adjacent damp brick / stone masonry walls and humid basement air was judged too high for insulation to be fitted between the joists, as the free flow of ventilation air at joist end bearing points would be lost. In addition the client wanted to maintain maximum headroom in the basement without lowering the basement floor and having to underpin the walls: the slim insulated structural slab solution minimised risk and maintained adequate head height whilst also avoiding thermal bridges at the floor to wall junction.



Below: the perhaps relatively minor risk of creating a linear thermal bridge (cold, higher RH voids) adjacent to a nutrient source (woodfibre) creating a potential habitat for silverfish at this IWI / floor insulation junction was minimised by filling with expanding foam before the floor insulation was placed. Generally though, a no-void policy appears to be a robust rule of thumb when applying insulation, particularly IWI. For IWI that relies on a ventilated cavity between the insulation and the (usually masonry) outer leaf, ventilation must be of a sufficiently high level to remove moisture. Typically this might suggest an externally ventilated cavity, with sufficient vents/airbricks, perhaps 75 – 100mm wide. A CLR case study relates to this ventilated cavity issue – see module 6.

anti silverfish fills 49 harold

Examples of other types of timber components identified as being potentially at risk after retrofit may include those belonging to neighbouring properties:




Images courtesy AECB/Simmonds.Mills.

Above image top: ‘sub-standard’ outshot extension being demolished, exposing a neighbouring gable wall.

Above image centre: damp wall base and embedded timbers exposed in the wall.

Above image bottom: Opportunities to improve the thermal and moisture performance of the neighbour’s wall were not taken up due to uncertainty around any future liabilities that might arise from such measures. The extension was rebuilt with a low energy fabric incorporating a ventilated gap to the neighbour to try to minimise changing the old wall’s hygrothermal condition.


In the case above, a ventilated cavity was maintained between the new insulated extension and the existing neighbouring wall in order to avoid any potential liabilities arising as a result of the new works.



Briefly think about your home or a retrofit project that you know well….

  • List the type and location of biological decay that you have already found.
  • Do the same for areas that you cannot see, but where you anticipate that biological decay might be found.
  • For different areas of the building, do you have some idea of the conditions close to the wall / in places where decay might occur – and what kind of decay might you anticipate?


This lesson has covered:

  • An introduction to Moisture in Buildings
  • Biological decay (Rots, Moulds, Bugs)
  • Understanding rot
  • Understanding mould growth on substrates – critical thresholds
  • Examples of rots, moulds and bugs in action
Lesson tags: bugs, damp, moisture damage, mould, penetrating damp, performance gap, rising damp, rot, ventilation
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