This blog was originally posted on the ech2o blog.
The way moisture moves in buildings is seen as quite obscure. Yet we often experience the effects of water in walls. Since I graduated, I have been working as a researcher in moisture and mould in buildings. Every time I explain to somebody what I do for a living, people start sending me pictures of their mouldy showers, damp walls and water dripping from windows soon after. Even the flat I live in has an interesting floor-to-ceiling efflorescence which reminds me that my work – and the work of many colleagues – is much needed.
The British building stock is a peculiar one. The industrial revolution triggered the construction of a large number of buildings with thin solid brick walls, which have been left unrendered often with the aim of cutting costs. Although the wall thickness was regulated by the Building Act of 1774, developers tended to build thinner walls than required, often one-brick thick. This was not only a structural hazard, but a risk from a moisture point of view.
Cheap fuel burning in the many fireplaces was helping keep the walls fairly dry. However, this is just one of many factors affecting the life of a solid wall.
Firstly, wind-driven rain reaches the building fabric when absorbed by the porous bricks and mortar. This is particularly true in case of south-to-west-facing walls (as south-west is the prevailing wind direction) and off the west coast of the UK, where winds are stronger. Poor pointing, cracks between bricks and mortar, blocked gutters and drains contribute to increase rain penetration into the wall.
Secondly, groundwater can rise through the wall, forced by the wicking power of certain bricks, and benefiting from the lack of a continuous damp-proof course at ground level. In the picture on the right, the water saturates the area around wall sockets, causing a safety hazard.
Last but not least, people live in the building – a detail we tend to forget. We cook, shower, use the kettle and breathe. All these activities release water in the air in form of vapour.
All this water builds up in the walls, but it’s balanced by drying. Increasing the wall temperature will let water evaporate. After evaporation, water vapour moves out of the building fabric following a mechanism called diffusion. This forces vapour to move from wet to dry areas, and pushes vapour towards both sides of the wall – towards the room and outdoors.
HERE COMES THE SUN
The warmth of a fireplace or of its modern counterpart, central heating, is the first type of drying that comes to mind. However, one of the most powerful drying mechanisms we have is sunlight.
When the sun’s rays hit a surface, the surface temperature increases causing evaporation of any water trapped in the wall. But for the radiation to have a stronger effect, the surface needs to be dark.
Solar-driven evaporation also requires a reservoir to be effective, such as rainwater sitting in the external side of the brick wall. Considering that most unrendered solid brick walls are dark and the importance of rainwater wall reservoirs, solar-driven evaporation is ironically more relevant a mechanism in the UK than in sun-kissed Italy, where I am from.
THE IMPORTANCE OF INSULATION
As is the case when the drying process is artificial, water evaporating because of solar radiation moves towards drier areas of the wall. Since a room is drier than its walls, vapour is attracted from the wall to the indoor environment. But at the same time, wind blowing on the wall outside the building helps the most external layer of the wall to dry, attracting vapour out of the wall surface.
This solar-driven process occurs in the majority of traditional solid walls in the UK; its intensity depends on the physical properties of the masonry and its exposure to wind-driven rain and sun. It is a valuable process for keeping moisture levels low in walls. Some vapour inevitably reaches the indoor environment; this is usually removed via ventilation, together with the vapour generated indoors by people living in the building.
Because of all these processes, adding insulation to the wall changes the wall’s moisture balance.
If the insulation is added internally, heating up the indoor environment will not keep walls dry anymore – unless the insulation has serious gaps. Also, depending on the type of insulation used, insulation can inhibit the drying process of the wall from its wet core to the drier room. Solar-driven vapour movement will occur anyway, but vapour will find a barrier in the new insulation and accumulate, leading to mould growth behind that barrier and potential health issues for the occupants.
A careful selection of insulation strategies can mitigate this risk. A building we are monitoring in Maidenhead shows that a wall insulated with a moisture-safe system is able to dry out effectively with solar drying (http://bit.ly/299lkkC). It’s hard to define what a moisture-safe system is, given the many factors influencing the moisture balance in a wall. However, we can say that a system is moisture-safe when it is selected on the basis of a thorough moisture risk assessment. In the assessment, the hygrothermal interactions between the wall and the surrounding environments are evaluated against various combinations of factors that lead to damage.
Solid-wall buildings are currently 20% of the UK dwelling stock. They need to be insulated not only to comply with building regulations on energy efficiency, but also to improve the comfort of occupants. Adding internal wall insulation is perceived as a risk many are not willing to take for their buildings. However, risk can be considerably reduced by understanding how water moves through insulated walls.
What can we do to address those issues? University College London and other partners have recently set up the UK Centre for Moisture in Buildings (www.ukcmb.org) to develop a moisture-safe built environment in the UK. Hopefully, this will lead to a better understanding of the issue of water in buildings, and eventually bring about more effective solutions that will prevent mould from forming. Until that day, I will continue receive pictures of mouldy walls from friends.
Written by Valentina Marincioni, Research Engineer in Building Physics – Moisture, UCL Institute for Environmental Design and Engineering.