The Building Conservation Directory 2023

P R O T E C T I O N & R E M E D I A L T R E AT M E N T 4.1 137 C AT H E D R A L C O M M U N I C AT I O N S T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 3 | C E L E B R AT I N G 3 0 Y E A R S can radically change moisture movement and bring considerable hygrothermal risks. Since damp walls are less thermally efficient, drying out a wall can improve its thermal performance by up to 30 per cent. When selecting the appropriate measure to improve the efficiency of a historic or period building, a holistic or whole- building approach must be undertaken. Interstitial condensation is a significant risk when installing IWI. Generally, the warmer the room, the more water vapour will be held in the air. This increases the likelihood of the water vapour permeating outwards into the masonry or into the roof void. The concern then is that the embedded timber structure becomes highly vulnerable to decay. Decaying timber, in turn, may result in structural issues and unnecessary loss of historic fabric. BUFFERING Peak moisture levels may be ameliorated by buffering, an essential component of a traditional building envelope’s performance. As the building fabric naturally adapts to fluctuations in temperature and humidity, it creates a stable internal environment. The use of natural or hygroscopic building materials can positively impact the condition of the embedded timber structure. Traditionally built structures tend to have thick masonry walls, which create high thermal inertia. The wall density warms and cools slowly, depending on the external conditions. This mass creates a stable environment that reduces temperature fluctuations creating an exceptional thermal buffer. Buffering internal humidity is critical when creating a stable internal environment, although it can be complex. Two crucial aspects contributing to buffering internal moisture are the hygroscopicity of the building materials and the frequency of air change. Limiting air change can be challenging in a building that relies on ‘breathability’. However, a balance between heat loss rate and the number of air changes per hour can be achieved by reducing air infiltration from the outside by sealing windows and doors, while allowing the porous material to function to its maximum capacity. Targeted ventilation measures should also be introduced to remove moisture-laden air, preferably using passive stack ventilation supplemented with mechanical ventilation where necessary. Hygroscopicity is the capacity of a material to attract or absorb water from the air or surrounding environment as water vapour and then release it back into the air. This process might be considered more important than the ability to discharge water through the wall. Therefore the capacity of a building envelope to buffer moisture and release it with the aid of ventilation is fundamental to creating stable humidity. Consequently, internal wall insulation with the capacity to buffer humidity is a critical aspect of the overall health of the embedded timber structure. TIMBER DECAY Timber is one of the most versatile and sustainable building materials used in construction. In a historic building, timber provides valuable information on past building methods and construction practices. Therefore, we should preserve as much original timber in a building as possible. Traditional buildings can contain an enormous amount of timber as part of their original construction. Timber is typically found in the most critical positions in a building’s envelope, where moisture tends to be at its highest. This makes embedded timber vulnerable to decay and puts it at high risk when retrofitting a building with wall insulation. Structural elements such as timber lintels, beams and joist ends embedded in the masonry are highly vulnerable to changes in moisture, so the long- term effects of any retrofit measures must be assessed. A thorough analysis of the embedded timber structure should be carried out in advance of any work. The moisture content of the masonry and embedded timber should be reviewed as part of this analysis. When insulating a suspended timber ground floor, a holistic approach is required. Some conflicting views exist on the relationship between timber decay and subfloor ventilation. However, installing insulation will likely decrease evaporation through reduced vapour pressure or drying potential. High external ground levels, damp subfloors, leaking gutters and other pre-existing sources of moisture may also exacerbate decay where drying potential is reduced or the humidity is increased. When an external wall envelope is insulated on the inside, temperature and heat flow through the wall decrease, so its moisture content is likely to increase. As internal insulation creates a steep temperature gradient, there is a risk of the temperature reaching dew point at the interface between the wall and insulation. This can be significant as it may create an environment for timber to decay. High moisture content at the wall surface may allow sufficient conditions for fungus and woodboring insect outbreaks. Reducing airflow or increased airtightness and the likelihood of elevated internal humidity contribute to timber decay risk. Since timber can be up to three times as conductive along the grain as it is perpendicular to the grain, interstitial condensation is a concern deeper into the wall. Masonry decay is a similar concern. If the masonry’s heat flow is reduced, the drying potential will be reduced. Therefore masonry such as soft brickwork may be vulnerable to decay and lead to long-term deterioration of the building fabric. HOLLISTIC APPROACH A holistic approach is essential to understanding the building environment, thermal performance and construction before deciding on the best approach to reviewing the timber structure. When examining a building, location, shading and orientation to the prevailing weather must be considered. Think about the original use of hood mouldings over windows and doors, and cornices at the roof level can help a building shed water. Physical investigation or opening- up works are advisable where there is a risk of decay. This can give a better understanding of the actual in-service condition of the embedded timber structure. This exploratory analysis of the building materials can map and understand the source of moisture, construction methods and estimated lifespan of the structure. The use of non-destructive decay-detecting micro-drilling technology can help analyse large embedded timbers that cannot be accessed easily. Most people want to take the low-risk approach when dealing with embedded timber that will become concealed behind new insulation. This can result in cutting or removing the embedded timber from the wall and supporting it on steel. Thermal images can provide graphic evidence of vulnerable fabric – in this case, clearly showing where insulation is missing from a ceiling.