Managing moisture: why we must learn the four Cs

This article was originally published in issue 30 of Passive House Plus magazine. Want immediate access to all back issues and exclusive extra content? Click here to subscribe for as little as €10, or click here to receive the next issue free of charge

We usually photograph buildings in the sun – look at any featured project in this magazine. So, I was surprised, on a recent training course, to be shown a picture of a building during heavy rain. The picture showed an abutment of a pitched roof to a wall, and the rain was bouncing of the roof and running in a torrent down the line of the flashing. The point was that buildings must withstand serious and sustained attack from water, and much traditional construction and detailing is attuned to that task. There are ancient skills involved – look at the outside of any medieval cathedral, where every detail of the stonework is designed to throw rainwater off the building and stop it running down the face of the masonry. Those buildings have lasted for eight hundred years, so the masons clearly knew what they were doing.

It took me nearly ten years working on domestic retrofit projects to realise that retrofit risk management is all about controlling and balancing moisture. Not only keeping the rain out but also dealing with rising damp, plumbing leaks, and the moisture occupants create through respiration, bathing, cooking and laundry. Not only liquid water but also ice and water vapour. Not only moisture outside the building but also moisture inside the building (in the air) and in the building fabric. Not only moisture itself but the consequences of having too much of it in the wrong places: damage to building fabric and finishes, and mould, with associated risks to health. Even new buildings are at risk: there are reports of mould in new homes – a consequence of airtightness combined with under-ventilation.

Buildings must withstand serious and sustained attack from water, and much traditional construction and detailing is attuned to that task.

The series of Breaking the Mould articles in Construct Ireland (Construct Ireland is the former title of Passive House Plus magazine) set me thinking about these issues. Those articles have led to the current Hygrothermal Risk Assessment module in the Building Performance course at the Technical University of Dublin. In the UK, much work has been done by the Sustainable Traditional Buildings Alliance (STBA), leading to the development of the acclaimed online ‘Guidance Wheel’. Publication of the seminal BSI ‘White Paper’ Moisture in Buildings: an integrated approach to risk assessment and guidance has led to the establishment of the UK Centre for Moisture in Buildings (UKCMB) and to the forthcoming update of BS 5250 Code of practice for the control of condensation in buildings. UKCMB now offers CPD courses in Understanding and Managing Moisture Risk in Buildings and is working towards qualifications for building moisture safety professionals similar to those already established in Sweden.

At UKCMB and elsewhere, researchers are improving our understanding of building moisture risks and developing the tools to manage them. The BSI White Paper sets out four key principles of moisture risk management, embedded in a holistic approach covering the planning, design, construction and operation of buildings. We know these principles as ‘the four Cs’:

Context

The building context includes not only its location, orientation and exposure (to sun, wind and driving rain) but also its construction, historical and cultural significance, its state of repair, its use and its energy efficiency. All these factors are important considerations.

Coherence

The coherence principle is about ensuring that the technical, hygrothermal approach to a building is consistent, in design, detailing, construction and use. It distinguishes two fundamentally different approaches to moisture control. A ‘moisture closed’ approach, as found in most new buildings, attempts to exclude moisture from the building fabric, using impermeable materials. A ‘moisture open’ approach, as found in most traditional buildings, manages the dynamic equilibrium between moisture inside the building, moisture in the building fabric and moisture outside. A moisture open approach has greater capability to deal with building defects and provides capacity for drying.

Capacity

Where there is uncertainty about the moisture performance of a building, capacity should be built into the processes of design, construction and use. Current and future uncertainties include changes of occupancy and the effects of climate change. For example, ventilation systems should always be capable of providing more than the minimum ventilation rates required by Building Regulations (and demand control can maintain energy efficiency by matching ventilation demand, room-by-room).

Caution

The cautionary principle is well known. There is a lot that we do not yet know about how moisture interacts with buildings, especially with modern moisture loads, materials, construction methods and standards of heating and ventilation. The unknowns, uncertainties and complexities about moisture risks necessitate cautious specification and ongoing watchfulness to mitigate possible problems. Despite contrary economic pressure, buildings should not be over-optimised.

These four principles are part of the legacy of the late Neil May. The more I apply them to building projects the more I am convinced of their effectiveness. With luck, the four Cs will soon be widely applied by building professionals, we will pay more attention to moisture risks, and we will confidently take more photographs of our buildings in the rain.