FEATURE: DAMP – HOW TO PREVENT IT USING CLEVER SUSTAINABLE DESIGN
25th November 2017
It is ten years since my first enlightening read of Neil May’s fascinating paper ‘Breathability – the key to building performance’*. It clarified and amplified many things I knew about using materials cleverly to prevent damp-related problems, and spurred me to design buildings with his solutions in mind. Being an architect in Scotland, water is always a significant factor in any building design.
Neil May’s paper is essentially about water and how the individual qualities of materials can be used to build healthy robust long-lasting buildings, and how if we get it wrong there are risks to human and building health.
May’s paper came back to me recently on a visit to Chamonix, where I saw the cut timbers pictured being used as a decorative finish in a hotel. End grain timber has the highest rating for water vapour storage (Hygroscopic capacity) - 54KG/m3 - of all building materials that I could find. In the high Alps, heated interiors often become too dry in winter for good health, so the use of a modest ventilation air change rate and some moisture storage in materials such as end-grain timber can be beneficial. It looks great, too.
May says we need to design airtightness as a feature in order to achieve energy efficiency, otherwise insulation becomes almost pointless. This means controlled ventilation, either natural or mechanical, is vital to ensure healthy indoor air quality. In particular indoor air with relative humidity levels of between 40% and 60% is needed to avoid mould, dust mites and other unpleasant phenomena. This can be achieved with ‘moisture buffering’, that is using materials such as this end-grain timber to absorb moisture and then to give it off again as conditions require. This naturally clever solution actually makes a building more robust.
A second aspect of May’s paper relates to a recent Thomas Robinson Architects project upgrading a stone building on a wet Scottish Island. It’s the application of a woodfibre board to the inside face of 600mm-thick stone walls. This utilises the thermal contribution of the existing wall. Other methods that include a ventilated cavity do not, effectively wasting the thermal performance of the existing structure. It also manages the moisture cycle and permits drying of the stone wall in both directions, as the external face has been newly lime rendered. The biggest risk in this sort of traditional building upgrade are the embedded timbers of the joist ends and lintels. If you get the upgrade specification wrong you may inadvertently create conditions for condensation to occur which can cause the dreaded dry rot. Here, detailed expert knowledge is key.
I’ve adopted another of May’s recommendations in a timber frame specification on another project. Building in the West Coast of Scotland we assume that everything will get soaked during construction. With the standard Scottish timber frame system of a sheathing board on the outside of the timber frame panel you effectively have a vapour control layer on the wrong side (the cold side) of the insulation. There will also be a vapour barrier on the warm side of the insulation in the form of a polythene or more sophisticated membrane. The result of this is that the timber studs are never able to dry out. In fact, they may continue to get wetter as internal vapour barriers are rarely installed perfectly. The timber is at risk of rotting if moisture levels reach 18%. May’s suggested alternative, which we use often, involves putting the sheathing board on the inside (the warm side) of the frame and then installing a breathable hygroscopic insulation between the studs and over the outside of the studs all from the outside. This build up allows for ongoing drying of construction acquired moisture and is robust enough to prevent problems if the vapour barrier on the warm side is damaged.
Of course, it’s not up to clients to be aware of these issues. It’s our job to know the risks and to solve them during the build. Human health, building performance and the longevity of our clients’ financial investment in buildings is always at the forefront of our minds. As architects we know how to protect our client’s interests in these areas. We don’t expect them to know what to ask for with different types of construction and products. When we explain our recommendations to clients however, most get on board with our philosophy, as investing in health, building performance and value are easy choices to make.
*Neil May is managing director of Natural Building Technologies (NBT) and a leading light in the responsible retrofit movement which is increasingly informing government policy and industry practice on sustainable refurbishment.