24 C O N T E X T 1 4 9 : M A Y 2 0 1 7 using EN 15026), but there is no robust protocol for using this standard in the UK. There are a number of difficulties with this approach at present: • WUFI is a very complex programme that needs a good understanding of building physics to use successfully. It can be easy to enter the wrong parameters, producing misleading or inaccurate results. • Detailed data on the heat and moisture transport properties of the materials making up the structure is needed. There is a database, which contains mainly German materials, in WUFI. It is difficult to know whether these are relevant to UK constructions. Many of these properties are complicated and expensive to measure, and would no doubt raise objections from insulation manufacturers. • Detailed external weather data from the location of the building is needed to runWUFI. • EN 15026 describes and models only one-dimensional movement of heat and moisture.TheWUFI programme can be two-dimensional but this is currently outside the range of EN 15026, and there is no commercially available version of WUFI for two dimensions. • WUFI does not model air movement in structures sufficiently accurately, and it can not deal with twodimensional junctions (such as eaves and reveals). Incorrect moisture predictions can lead to two types of failure. Predictions that problems may occur, when in fact the risk is negligible, may limit the installation of insulation unnecessarily. Failure to predict real problems and take appropriate precautions will lead to problems such as rot of timbers, frost attack to masonry, damp staining to interior finishes, and bad indoor air quality (due to mould and damp). The difference in building physics and construction between older and more modern buildings has been ignored in recent years. It is essential to choose the correct intervention so that these principal differences are taken into consideration.With the desire to improve buildings from a thermal performance perspective, the basic principle of ensuring that the building is in a good state of maintenance, and understanding the limitations in construction form, are rarely considered. It is important that no improvement or intervention restricts the passage of the moisture either to the internal or external surface, without very careful consideration and design.The possibility of unintended consequences must be considered fully. Amaintenance or improvement programme introducing a more modern material into the structure brings a series of risks which are rarely considered. Examples include sand-cement renders, non-breathing paints and silicone waterproofing layers. Recent research highlights the facts that moisture movement is multi-dimensional, and that any alterations or change in this free movement can result in a build-up of moisture, either on the inner surface of the wall (mould) or within the structure of the building (interstitial condensation). The underlying cause is the isolation of the wall structure from a source of heat and ventilation by a non-breathing element. In the long term this can result in wall failure or in early failure of internal coverings such as plaster. Any signs of disrepair, water ingress, damp or deterioration must be rectified before considering other measures. The overriding issue that has been identified with current working practices across the industry is an inconsistent approach to assessment. A sound approach is needed, using building pathology, checking and quality control. The main issues that need to be understood are: • Which areas need to be surveyed before insulation works are undertaken. • The impacts of cold or thermal bridging, and the importance of minimising the risk. • The importance of attention to detail and specification on site. • The impact of saturation on conductivity, condensation risk, temperature gradients, cold spots and convective looping. • Communication between site installation and quality control in a positive feedback loop to ensure that good practice is reinforced and bad practice not ignored. • The need to give talks on the principles of good practice before each project, with any key elements of the insulation system being clearly set out to installers. • More robust and evidenced quality control with well-trained staff. • The creation of more robust details, reducing the over-reliance on sealants and workmanship. If the effect of thermal bridging is not taken into consideration when calculating potential heat loss from a building, it is likely that the overall heat loss will be underestimated. It follows that if buildings are improved through insulation but thermal bridges remain, heat loss will be concentrated at the point of the bridge relative to the newly improved walls.While it is desirable to minimise this effect to help reduce heating costs, it can also be the cause of physical problems. Since heat will transfer out of the building more readily at the point of a thermal bridge, the internal surface temperature at that point will generally be reduced relative to the surrounding surface area. When the temperature difference reaches a critical ratio, measured as the temperature factor (f Rsi ) at the junction, condensation can form, which can lead to problems such as mould growth. Latest research indicates that when reveals and penetrations are not insulated, the temperature factor and subsequent cold bridge are actually worse than before the walls were insulated. This results in a concentration of risk in a two-dimensional junction that is least capable of dealing with a drop in temperature. Modelling indicates that this area contributes significantly to the heat loss from a building after improvements. The losses through cold bridging have been identified to be the cause of 19 per cent of heat loss through a building envelope.This lack of attention to detail is understandable in view of the desire to avoid bridging the damp-proof membrane or course, but careful treatment of this area can result in reduced cold bridging and subsequent risks. Colin King is director of BREWales.
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