Wood in service is principally prone to deterioration by different wood depolymerising fungi in temperate climates. A key prerequisite for fungal degradation of wood is the presence of moisture. Conversely, keeping wood dry is the most effective way to protect wood from wood degradation and for long term binding of carbon. Wood is porous and hygroscopic; it can take up water in liquid and gaseous form and water is released from wood through evaporation following a given water vapor pressure gradient. During the last decades, the perception of wood-water-relationships changed significantly and so did the view on moisture-affected properties of wood. Among the latter is its susceptibility to fungal decay.
Most bio-based building materials such as lignocellulose are hygroscopic and tend to absorb water molecules from the surrounding atmosphere. Moisture can refer to bound water that is absorbed on the interior surface of the material or free liquid water filling pores.
The material moisture content (MC) is the ratio of water taken up by a material and its dry mass and can be calculated as follows:
MC = material moisture content [%]
mw = mass wet [g]
m0 = oven dry mass [g]
The material moisture is in equilibrium with the air humidity through sorption processes. The interrelationship between relative humidity (RH) and the equilibrium moisture content (EMC) of a material is usually presented as sorption isotherm diagrams. Hereby one needs to distinguish between adsorption and desorption since the EMC differs between moistening and drying. The difference between the two MC values is called hysteresis and can be explained through the different mechanisms when pores of different shape and size are filled or emptied respectively.
Starting from the fresh state of a plant, the MC decreases during drying until free water has been fully evaporated, but water is remaining in the cell walls (cell wall water). During drying cell lumens release liquid water and cell walls arrive in the transition between a saturated and an unsaturated state, i.e. the so-called ‘fiber saturation point (FSP)’ or ‘fiber saturation state’. The term as such is somewhat misleading since fibers are not saturated with water, but the cell wall is. ‘Cell wall saturation (CWS)’ would therefore better describe the phenomena attributed with this state. Such a state can be reached only theoretically. More likely, adsorption and desorption are temporarily and spatially ongoing processes which never attain an equilibrium.
However, within the hygroscopic range between absolute dry state and cell wall saturation many different wood properties are significantly affected by moisture changes, such as swelling and shrinking, strength and rheological properties, electrical, thermal and acoustic properties. Above cell wall saturation the cell lumens are filled with liquid water, which does not further affect the properties of the cell walls but does for instance affect growth conditions of decay organisms, hydraulic and electrical properties of the entire tissue including voids and cell lumen.
Moisture transport depends on several factors including the material, the anatomical direction, the actual moisture content. Apart from wood and bamboo that form solid, but porous bodies, many other bio-based building materials have a fibrous structure and consist of small single elements that are used as a conglomerate. Furthermore, treated bio-based materials as well as composites contain extrinsic substances that can be more or less hygroscopic than the bio-based material itself. Consequently, to describe the MC of chemically modified solid wood and fibers, wood polymer composites, reed, straw, or hemp further aspects need to be considered.
With respect to polymer composites and chemically modified lignocellulose one can distinguish between the material MC and the MC of the bio-based substance. Therefore, knowledge about the constitution of the specific material is required, e.g. the polymer percentage of a composite or the weight percent gain (WPG) of a chemically modified material. Apart from changes in mass the impregnation with modification agents can also lead to changes in sorptivity. Similar effects can be observed when bio-based materials are impregnated with preservatives, e.g. different salts with high sorptivity. The moisture performance of materials that are usually applied as conglomerates rather than single elements strongly depends on the moisture content of the entire conglomerate.
Moisture is omnipresent since water molecules can be absorbed from surrounding air, but the relative humidity of the air is affected by a range of factors, including the season, daily temperature variations, heating regimes within buildings. Materials can also get wet in direct contact with liquid water. Precipitation in various forms such as rain, hail or snow has the potential for wetting. Wood used outdoors without shelter is therefore frequently exposed to precipitation, which can be intensified in form of wind-driven rain and splash water. Almost permanent wetting is provided when organic materials are in direct contact with freshwater, sea water, or moist soil.
Wood can also be subject to moistening when exposed indoors or in the building envelope. Failures in pipes, roof membranes, and other elements that protect a building from the outdoor environment such as windows, doors, and claddings, have the potential to cause severe wetting of load-bearing elements and insulation materials such as mineral wool or organic fibers. In contrast to wetting outdoors, the event of moistening indoors is usually unexpected and therefore instant measures for draining and re-drying are not provided. Direct contact between inorganic materials that got wet and bio-based materials can lead to moisture related problems with the latter even though it might be rather remote from the leakage. High relative humidity inside of buildings in combination with the transport of humid air from the interior to the exterior can lead to condensation problems if wall mounting is incorrect. However, both, condensation and leakage do also occur outdoors.
Fungal decay is one of the most prominent risks for all bio-based building materials and requires attention. It is commonly agreed that the moisture conditions for decay fungi become critical when free water is available exclusively in the cell walls, but not in the cell lumens anymore. It is assumed that decay fungi require liquid water for transporting low molecular weight compounds and enzymes involved in the wood depolymerization and therefore the moisture content of wood and other lignocellulosic materials needs to be above cell wall saturation. However, results from several studies indicated that the minimum threshold MC might be significantly below cell wall saturation. Further, the actual thresholds depend on the exposure conditions, for instance the accessibility of an external moisture source (e.g. moist bricks or insulation material), availability of nutrients, the substrate, and the decay organism itself.
The process of wood infestation by decay fungi can be divided into different phases. The authors suggest the following: 1.) spore arrival, 2.) spore germination, 3.) mycelial growth, 4.) wood metabolism, 5.) autolysis of fungal hyphae, and 6.) formation of fruiting bodies and sporulation. It is assumed that the requirements regarding moisture and other physico-chemical parameters (e.g. pH, temperature, nutrients) differ between the six phases. But the six phases of fungal infestation will overlap in the wood substrate because of spatial colonization, and the required physico-chemical factors can also overlap between phases of wood decomposition. Most relevant for wood in service – especially in above-ground situations - and therefore in the focus of wood pathologists are the phases of spore germination, mycelial growth and metabolizing of wooden cell walls. Wood exposed in soil contact is often in direct contact with developed fungal mycelium and the phases of spore arrival and spore germination is only relevant for the transition zone between soil and air.
To determine the moisture requirements of wood and decay fungi is challenging. Besides the various limitations with fungal experiments and the difficulties to determine wood MC accurately, it appeared that the most challenging task is the interpretation of the test results. Rather often the origin and the exact location of water in wood stays unclear. The latter is closely related to the relationship between air humidity and the EMC of wood. However, different physico-chemical processes are involved in wetting and drying of wood. Hence, the moisture requirements of decay fungi cannot be reduced to static wood MC values, but need to be seen in the context of dynamic processes including adsorption, diffusion, capillary condensation, desorption, and active moisture transport by the fungus itself. Usually, only an average wood MC (global MC) is measured and MC gradients between different locations in wood (local MC) are barely ever accounted for. Finally, fungal degradation of wood itself supplies moisture.
Usually, wood protection by design is used synonymously with ‘moisture protection’ to reduce moisture-induced risk for decay or any other kind of biotic attack. Hereby, two principle rules are followed: 1.) keeping water away from the structure and, if this is not possible, 2.) removing water from the structure as fast and as effectively as possible. It is agreed in general that biocidal treatment is applied for wood only if wood cannot be protected by construction measures.
Besides numerous design solutions, water can be kept away from bio-based building material by application of coatings and covers (physical protection). Coatings have long been used efficiently for moisture protection of timber products such as window joinery or claddings. However, their efficacy strongly depends on their own robustness and climate. As soon as a defect occurs in a coating, its protective character can be reversed since defective coatings act as water traps.
Most wood-based building materials are susceptible to moisture, and wetting is a prerequisite for biotic decay. Hence, moisture protection is a key issue for any wood-based building material. Keeping water away from the structure is the first and most important step in any protection concept by design. Moisture can derive from rain, hail or snow - in combination with wind considered as wind driven rain. Furthermore, moisture derives from air humidity, and water vapor can condensate indoors, outdoors and most critically within the building envelope where redrying is usually inhibited. Finally, rising water can derive from the soil or any other porous foundation.
If water uptake is unavoidable or the respective protective measures do not coincide with the overall design concept, other measures need to be taken to assure fast redrying, that is, drainage and ventilation measures.
Different methods are used to characterize the moisture performance of wood. Moisture measurements can be performed directly or indirectly. Direct methods are based on removing all water from the material; indirect methods measure a certain characteristic related to the MC of the material. Furthermore, they can be distinguished by characteristics like continuous or periodical measurements and measuring local or global MC. Each measuring method has certain peculiarities that require consideration when designing a test set up and conducting MC measurements in laboratory or field experiments. Among the most commonly used techniques in research are the following:
Quantification and localization of capillary and cell wall water – especially in the over-hygroscopic range is considered crucial for determining minimum moisture thresholds of wood decay fungi. The limitations of the various methods and experimental set-ups to investigate wood-water-relationships and their role for fungal decay are manifold. Hence, combining techniques from wood science, mycology, biotechnology and advanced analytics is expected to provide new insights and eventually a breakthrough in understanding the intricate balance between fungal decay and wood-water relations.
Service life planning with timber is another area of research where moisture plays a key role and moisture measurements can help to characterize exposure conditions and resulting decay hazards in a quantitative way. Finally, wood protection by design usually aims to reduce moisture-induced decay risks. Consequently, the achieved effectiveness of protective design actions can be quantified best based on moisture measurements.