Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 27 INDOOR AIR QUALITY AND VENTILATION in traditional building retrofit NICHOLAS HEATH W HEN PEOPLE think of retrofit, the first thing that springs to mind is often insulation. This is perhaps unsurprising, given the increasing and logical preference for a fabric- first approach to improving energy efficiency. However, for a retrofit to be thorough and low risk in the long term, insulation is just one part of the equation. To maintain the health of both buildings and occupants, insulation must be part of a wider, whole-building approach that also considers indoor air quality and ventilation, among other things. Much has been written about the energy efficiency of traditional buildings and construction materials. Rather than focussing purely on thermal performance, this article is concerned with the movement of air and moisture both in and around building fabric. This issue can be split into two broad areas: first, the moisture characteristics of the fabric elements themselves; and second, the junctions and spaces between the various building fabric elements. BUILDING FABRIC AND MOISTURE Many traditional building materials allow some degree of moisture movement within the fabric. Different terms are used to describe this, with ‘permeability’ or ‘breathability’ being among the most common, but these do not present the full picture. ‘Moisture open’ is a more holistic term, covering the following key areas: • vapour permeability – a material’s ability to allow water vapour to pass through it • capillarity – a material’s ability to draw up or transfer liquid water • hygroscopicity – a material’s ability to absorb, temporarily ‘store’ and then release water molecules from the surrounding environment as relative humidity changes (often called moisture buffering). When adding insulation to traditional building fabric, it is essential that these characteristics are considered and compatible systems are used in order to minimise the risk of excessive moisture building up in or on materials. Insulation and airtightness systems then need to be applied coherently to avoid similar moisture build-up issues as a result of either a) cold bridges at uninsulated areas or b) moisture building up within or on fabric. The unintended consequences of excess moisture build-up as a result of imperfect retrofit are becoming more and more widely recognised, and these include not only cosmetic and fabric damage but also decreased air quality and its potential negative health impacts. In extreme cases, the moisture balance between ingress (rainfall on the outer face of a wall, for example) and evaporation may be tipped, so a structure becomes progressively more cold and damp, which in turn attracts more condensation until saturation occurs. VENTILATION AND AIRTIGHTNESS Ventilation is defined in the current Building Regulations as ‘the removal of “stale” indoor air from a building and its replacement with fresh outside air’. It is needed not only to control internal moisture levels but also to get rid of pollutants and to maintain an indoor environment that is healthy for both the occupants and the fabric. In uninsulated older buildings ventilation is often largely uncontrolled and relies on a combination of gaps in the building fabric, window and door opening and more deliberate measures like chimneys and other vents. In many cases this ventilation can be excessive, particularly at colder and windier times of year, resulting in substantial and unnecessary heat loss. However, it is essential to maintain controlled, intended ventilation paths. Solid brick walls, bay windows and tiled roofs in a typical Edwardian terrace. Venting top and bottom, the original vertical sliding sashes (right) offer controlled and highly efficient air circulation when retrofitted with draught seals.