Heritage Retrofit

28 BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION When a building is insulated, it is likely to ‘behave’ differently as a result. In particular, its internal conditions are likely to change. Relative humidity may be more prone to increases, for example, particularly where occupants are unaware of the change in building conditions and do not adjust their ventilation or other habits accordingly. The more comprehensive the retrofit, the more likely it is that such changes will occur. A common contributor to such changes is an increase in airtightness, often as an unintentional by-product of adding insulation which blocks up previous air leakage routes. Without adequate ventilation, moisture in the building is now less able to escape, and this can cause problems even where moisture-open insulation systems are used (although such systems should considerably ease the moisture transfer process). This is exacerbated where insulation is partial (leaving some cold surfaces) and where intended as well as unintended ventilation routes are blocked up, leading to problems such as moisture build-up in cold voids (roof spaces and cellars for example) or deterioration of air quality in the occupied spaces. More holistic retrofit projects often deliberately target improved airtightness, with the aim of sealing up unintended ventilation routes and thereby reducing unwanted heat loss. This is a sensible strategy, but must include consequential measures such as additional controlled ventilation to ensure that indoor air quality remains good. Deliberately making a building more airtight and then having to add more ventilation may seem like a paradox, but the key issue here is one of controllability. Uncontrolled ventilation can cause excessive heat loss, uncomfortable draughts and locally poor air quality; controlled ventilation keeps indoor air quality good while minimising heat loss and increasing comfort levels. A COHERENT APPROACH The aim, then, is to: • reduce heat loss via insulation and airtightness • retain a moisture balance in the building fabric via a coherent, thorough application of appropriate systems and through additional intentional ventilation where necessary • retain good indoor air quality via an adequate, fool-proof ventilation strategy. A successful retrofit considers insulation, airtightness and ventilation as integrated parts of a whole-building approach. ASSESSING VENTILATION NEEDS If considering a retrofit project on an older building, particularly a deep retrofit that aims to insulate all parts of the building and increase its airtightness, it is essential to consider the ventilation requirements at the outset. Perhaps the best piece of advice is to seek the services of a reputable, independent ventilation expert with experience of retrofitting traditional buildings and an understanding of the issues covered in this publication. As part of the assessment process, it is helpful to establish current airtightness levels and ventilation provision in the building, as well as any residual moisture or likely future moisture load. Informal initial checks should include intended ventilation routes (such as gaps below doors, wall and window vents, chimneys, extractor fans, roof and sub-floor vents) and unintended ventilation routes (such as structural cracks, poorly-fitting windows and doors, gaps between floorboards and at floor perimeters), and can be simply and effectively informed by occupant experience. For a more formal measurement, airtightness may be measured by a fan pressurisation test, a fairly simple measurement of the building’s air permeability (AP, measured in m3/hr/m2@50Pa). It is then necessary to identify the airtightness level being targeted by the retrofit project. This is also commonly measured in terms of AP. For context, current Building Regulations require new-build homes to achieve an AP of 5, while the default assumption for an older home is likely to be much worse. The table above provides an example of different ratings and what they mean. (N.B. This table is taken from the recent retrofit publication A Bristolian’s Guide to Wall Insulation , which provides detailed guidance on many of the principles outlined in this article.) Identifying the baseline performance will help identify air leakage routes that should be targeted for improvement and intended ventilation paths that must be maintained, while identifying the target AP will help inform the amount and type of ventilation provision likely to be needed. Once a retrofit project starts, repeat fan pressurisation tests can be very helpful, both during the retrofit (to check that planned airtightness works have been effective) and afterwards (to identify the actual airtightness of the retrofitted building). As well as understanding baseline and post-retrofit airtightness and ventilation performance, there are a number of other issues which require consideration at the planning stage to ensure that a healthy indoor environment is maintained: • Moisture buffering – the use of a fully moisture-open insulation system will support the performance of the ventilation system, providing a greater ‘buffer’ for moisture management when needed. • Airtightness method – as well as coherent design, the manner in which airtightness is to be achieved merits consideration. The more complex the system (a moisture-closed insulation system, for example, or one that relies on extensive use of tapes and/or TABLE 1 Air permeability ratings for existing homes Band Air permeability (m³/hr/m²@50Pa) Described condition A Less than 3 Very airtight B Between 3 and 5 Fairly airtight C Between 5 and 10 Acceptably airtight D Between 10 and 20 Not airtight – a leaky building E Above 20 Very leaky (Source: A Bristolian’s Guide to Solid Wall Insulation, see Further Information) Shutters on the ground floor of a Georgian terrace in Spitalfields, London: commonly used on the continent to keep interiors shaded and ventilated during the day, here the focus was on privacy and security.