Properties of Traditional Building Construction

Liquid water is transported within permeable materials by capillarity or gravity. This is the primary mechanism of moisture movement through a permeable material.

Most traditional permeable materials are capillary active. They can temporarily absorb liquid water in their near-surface pores and then dry out through evaporation. This process can happen both internally and externally.

The external face of a wall is subject to continuous wetting and drying cycles, with rain wetting the surface, and wind and sun promoting evaporation. In traditional construction, highly capillary active lime renders and limewash finishes were a common protective external finish, for example on rubble masonry walls or timber framed buildings. They reduce rainwater penetration into the wall thickness and aid drying by maximising evaporation across the whole surface.

In most circumstances, permeable wall assemblies prevent rainwater penetration to the inside. However, when liquid water content within the pores of a permeable material is high enough to create flow paths between the outside and the inside, rainwater can be drawn in and carried through to the internal face. This can happen more readily if the building fabric is in a poor state of repair.

When building components are exposed to large volumes of water (due to broken or defective drainage, leaking pipes or flooding), liquid moisture will flow through permeable materials under the influence of gravity.

Permeable materials tend to be hygroscopic. They manage their moisture content to reach equilibrium with the surrounding environment by absorbing water vapour from the ambient air and releasing it through evaporation when environmental conditions allow.

Internal traditional finishes, such as timber or appropriately coated lime and earth plasters, provide this buffering effect during fluctuations in indoor air temperature and vapour pressure. In most cases, hygroscopic materials help to maintain a healthy indoor environment by regulating relative humidity levels. This minimises the risk of surface and interstitial condensation and diminishes the risk of mould growth and fabric decay.

Even when hygroscopic materials are used, it is essential that internal relative humidity is also regulated through appropriate ventilation.

Water vapour can also move through the building envelope in two ways:

• Via air pathways
  Air containing water vapour will move through any unobstructed air pathways, such as cracks or gaps, due to differences in vapour pressure either side of the building component. This can result in the concentrated locations at which movement of moisture as water vapour occurs. Traditional buildings were not built to be draughty, but sometimes cracks or unintended air pathways appear. They may be caused by:
  • the inherent movement of traditional materials (such as timber shrinkage or the wetting and drying cycles of masonry)
  • a lack of maintenance (for example, when external joinery in poor condition)
  • the presence of inappropriate past alterations and/or repairs (for example, when external cement render cracks due to thermal movement and incompatibility with the substrate)
• Via diffusion
  Water vapour can be transported through the pore structure of a permeable material by diffusion, again driven by differences in vapour pressure. In the UK, moisture vapour normally travels from the inside (higher vapour pressure) to the outside (lower vapour pressure). However, when elevations are exposed to sunshine after being wetted by rain, moisture vapour can move towards the inside and condense within cooler parts of the building fabric (reverse condensation).

Moisture transport by diffusion is extremely slow. Thus, the primary mechanisms of moisture movement are via air pathways or, through a permeable material via liquid water transport.

The thermal resistance of traditional construction is often underestimated (the ability of a material or system to resist the flow of heat by conduction), causing many people to believe that these buildings are not thermally efficient. However, some traditional materials, such as cob (earth) and thatch, are inherently good insulating materials. In situ testing has shown that the thermal conductivity (the ability of a material or system to conduct heat) of brick and stone is often lower than presumed. This means they are often more thermally efficient than is assumed and additional insulation will not provide the levels of benefit anticipated, contributing to a 'performance gap'.

Depending on their inherent properties, building materials can absorb heat (by exposure to solar radiation or warm air), store it, and then release it when the surroundings are cooler.

In certain buildings of traditional construction, mass solid walls and floors help regulate the internal temperature in relation to external fluctuations. They can, for example, reduce the likelihood of occupants feeling discomfort during the increasingly hot summers caused by climate change, when paired with adequate opportunities for night-time purge ventilation. Buffering is governed by the following properties of building materials and systems:

• High thermal mass (they can hold heat within their thickness)
• High thermal inertia (they heat up and cool down slowly)
• Low thermal diffusivity (there is a low rate of heat transfer through the material)