Wood rot

Wood rot can occur in wooden elements in constructions, in cases of excess moisture. Therefore, decayed wood is an indication of moisture problems in the building, and all moisture problems should be mitigated prior to installation of internal insulation. Wood decay is initiated by three types of rot fungi; brown rot, soft rot and white rot. Some fungal species can transport moisture over large distances, thus the moisture source feeding the fungal attack may not be located at the decayed wood. Wood rot yields reduced strength and mass loss in the wood. The consequences can be detrimental, and ultimately lead to collapse if the supporting system is attacked. Furthermore, some fungal species give off unpleasant odours, and some may even be the cause of respiratory symptoms for some residents.

Fungal attack is generally caused by a favourable combination of moisture content in the wood above the threshold value for a certain period of time with favourable temperature conditions. Furthermore, previously attacked wood is more prone to new fungal attacks when compared to new, sound wood. Figure 4–12 exhibits examples of wood rot in buildings.

Figure 4–12: Example of wood decay in a storey partitioning beam (L), and rot fungi attack on beam near the roof (R).

As the wooden elements at risk in historic constructions are mainly embedded wooden parts, it will in most cases not be possible visibly to detect wood decay. This includes e.g. embedded wooden beam ends, half timbering and wooden framing for insulation systems. Therefore, wood in the construction and possible excess moisture in the construction should be detected. Moisture in the construction in general is described in Section 4.2.2. Table 4‑6 sums up what to look for, and where to pay special attention with regard to wood rot.

Table 4‑6: What to look for, and where with regard to wood rot.

The critical positions in the building envelope are where wood is present and especially where the moisture load is high. This is illustrated in Figure 4–13 based on (RIBuild Deliverable D1.1, 2015). Not only the position in the envelope is critical: the moisture load on wooden constructions is highly dependent on the structural details and material composition of the detail as well as the characteristics of the materials, including also the wooden species and quality. These factors determine e.g. the drying potential.

In historic masonry facades, wood is mostly used for half-timbering in external walls. Although structural floors are not a part of the wall, wooden beam ends and supporting laths may be placed in the external walls and therefore in direct contact with bricks or stones in the external wall. Consequently, the moisture content of the embedded timber will be dependent of the moisture conditions in the wall.

When an external wall with wooden elements as half-timbering or beam ends is insulated internally, temperature and heat flux through the wall will decrease and the moisture content therefore is expected to increase in the wall including the wooden elements. Furthermore, if the building is internally insulated with systems that contain wooden materials e.g. wooden framing, there is a risk of rot if condensation can occur due to insufficient vapour barrier or if water from driving rain is trapped in the internal insulation.

Fungal growth can - depending of the species - result in unpleasant odour and emissions, which must be considered as an indoor climate problem. Depending on the exposure and the immunological reactivity of the inhabitants the inhalation of airborne micro-organisms and their metabolites of some species may cause respiratory symptoms (Singh & Singh, 2015).

A rot attack causes wood decay, resulting in reduced strength and ultimately collapse of the wooden construction. How fast a rot attack develops depends on the available moisture. If moisture supply is stopped the attack stops, but will return if moisture again becomes available. Some fungal species need long time exposure to high moisture content before the wooden construction weakens substantially, other species can weaken the wooden constructions fast if the conditions especially favours these species (Munck, Kock, & Larsen, 2003). Damage caused by the first kind of attack could be seen as lack of maintenance, as a slowly developing attack should be discovered before it becomes critical.

Water activity is a prerequisite for fungal growth, and the growth will initiate when the moisture content in wood exceeds a threshold value. The threshold value depends on different factors;

• Time of wetness, i.e. time above the certain threshold value

• Previous attacked wood has a lower threshold value than sound wood

• Temperature

For most fungal species the threshold value is in the over-hydroscopic range; condensation or liquid water sources e.g. penetrating rain are therefore usually a prerequisite for fungal growth. In general, the threshold value for wood rot is higher than for mould growth, and therefore, the threshold value for mould is more likely to be the limiting factor in critical positions where mould growth is not accepted.

Rot can attack all wooden constructions. Some species need lime as well as moisture to grow (Bech-Andersen, 1995). Lime is often used in historic buildings and therefore present for fungal growth.

There are several models for predicting rot or phenomena associated to rot (RIBuild Deliverable D2.2, 2019). Of the evaluated models in the RIBuild project, the model given by (Viitanen, et al., 2010), seems to be the most applicable for the use in RIBuild, as it describe a relevant parameter (mass loss) by using hygrothermal parameters (temperature, relative humidity) in a nuanced way, instead of e.g. defining a general critical value for moisture content independently of temperature. Material parameters describing e.g. water uptake in sound and damaged wood respectively, would be relevant as well, but difficult to determine. In case of simulations where these parameters are relevant, general assumptions must be made.

Remedial actions if wood rot is identified

Some remedial actions are quite simple, such as repairing joints and leaks from downpipes, while others are more complicated, ensuring that sufficient parts of damaged wooden beam ends are replaced.

The most important measures to minimize or prevent rot attacks are:

• Controlling the moisture, ensuring the moisture level will not exceed the threshold value, coupled with a temperature threshold

• Prevent water ingress into the wall; e.g. make sure joints in brick walls are filled, and with no leaks from rainwater drainage systems or through roofs

• Limit the use of wood in critical parts of the envelope, especially less robust species

Moisture is the most important factor in limiting the risk of wood decay in existing constructions containing wood. If parts of the original structure are renewed e.g. due to rot attack in the existing construction is it possible to choose other materials. Replacing wooden beam ends with concrete beams is one possibility. When parts of the construction are replaced, not only the damaged wood is removed; sound wood must also be removed to create a safety zone. How much sound wood that should be removed depends on the fungi species.

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