Avoiding condensate and mould Tips, duty of information and sample cases for avoiding damage in existing and new buildings

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1 Dipl.-Ing. (FH) ift Rosenheim Tips, duty of information and sample cases for avoiding damage in existing and new buildings 1 Introduction The ift Rosenheim s Centre of Technical Experts and those manning its telephone hotline have noted that users today still frequently complain of condensation on windows and glass. More worryingly still, this condensation can lead to the formation of mould on window surfaces and reveals. In situations involving condensation and mould there are always several possible causes, and several people who could be held responsible. The causes and responsible parties must therefore always be determined for each case individually. The legal and normative requirements governing the analysis of condensation and mould formation are somewhat ambiguous, and can be interpreted in a variety of ways. The various parties involved are often rather rigid in their outlook: construction workers tend to see the way that the occupants and users heat and ventilate their buildings and live their lives as the main cause of problems involving condensation and mould. Users, on the other hand, will insist that they have done everything perfectly. It is therefore the courts and hence the expert assessors role to identify the real causes of the damage. Any complaint can result in a lawsuit, which will cost both parties considerable time and stress. The next section will look at the main causes of condensation and mould damage, and at who is responsible in each case. Fig. 1 Condensation on a bottom corner of a window due to the thermal bridging effect of the spacer, and details from DIN of situations in which condensation is acceptable ift Rosenheim Page 35 of 166

2 2 Physics of condensation The extent to which condensation occurs on component surfaces is essentially determined by: the absolute water content of the indoor air (water vapour partial pressure), and the surface temperature of the component. As soon as the surface temperature reaches/falls below the dew point of the indoor air, condensation will occur. Both of these variables fluctuate dramatically: the absolute water content of the indoor air varies throughout the day as moisture is released (e.g. during cooking) and extracted (e.g. by ventilation). Surface temperatures vary according to the temperature of the outdoor air, the amount of heat being transferred by convection, and what thermal insulation is in place. They also vary over time and according to their location. Mould starts to form as soon as both sufficient moisture and sufficient nutrients are present. For some years it has been known that this moisture does not need to be present in the form of liquid water: mould can grow without water if the relative humidity of the surrounding air is just 80% or more. Nutrients can take the form of organic materials such as woodchip wallpaper or a thin layer of dust. Because mould is generally the result of condensation occurring over an extended period, the rest of this paper will deal with condensation. 3 Signs of a risk of condensation A glance at expert opinions and expert advice from recent years reveals a number of particularly common causes of condensation. These are described below. 3.1 Structural quality In the situations described below, the responsibility lies mainly with the planners, construction workers, window designers and/or installers. a) Built situation of windows in the external wall In monolithic external walls (i.e. walls made from just one material, such as brick), the windows should be installed in the middle third of each wall. In wall constructions containing an insulating layer, the windows should ideally be positioned in the insulation zone. The nearer to the outside and the further from this optimal position the windows are located, the more likely it is that condensation and mould will occur. Note: The standard DIN stipulates that, for window reveals, evidence must be provided of the temperature factor (f Rsi ). Only for the built situations described in DIN 4108 Supplement 2 is this not necessary. b) Insufficient window coverings on the outside In densely-populated areas, and cities, it makes sense to use external window coverings (roller shutters, venetian blinds or window shutters) to provide privacy in the evenings and night-time. As well as blocking the view of prying eyes, these coverings also provide temporary thermal insulation during the night. Users who do not have this kind of window covering installed will be forced to fit opaque curtains on the indoor side of the window to protect their privacy. However, hangings on the inside of windows make a huge contribution to encouraging condensation on the surface of the window! c) Type of space heating The main source of heat in new buildings is underfloor heating. This is beneficial for ensuring uniform air stratification and keeping the supply temperatures for the heating sys- Page 36 of 166 ift Rosenheim

3 tem low. However, this type of heat distribution encourages condensation on windows and facades, because it involves virtually no convection of the hot air (unlike radiators). It should be stressed to users that furniture and carpets must not be allowed to pose an obstacle to heat distribution in the critical zones in front of windows. The heating loops should be reinforced around the edges at the bottom of floor-toceiling windows, particularly in the case of bay windows and glass-to-glass corners. d) Installation of windows in existing buildings Where new windows are fitted in older residential buildings, there is an increased risk of condensation and mould occurring, particularly on the side reveals. Because these buildings were typically made from solid materials (e.g. solid brick, or concrete blocks), flanking insulation is virtually always necessary if the requirements of DIN are to be adhered to. 3.2 User behaviour Unfortunately, condensation on windows is often due to the behaviour of the building occupants. The following causes in particular are worth mentioning: a) Convective heat transfer Heat must be able to reach the surfaces of components by radiation or convection. Windows typically provide less insulation than external walls, so in these areas it is particularly important to ensure that the heat transfer is not impeded. Floor-to-ceiling opaque curtains, large interior window sills covered with an abundance of pot plants, and chests of drawers, sofas and kitchen units in front of floor-toceiling window units, are all frequent causes of severe condensation. Windows with deep profiles and overlap seals on the indoor side have virtually monopolised the market, while insulating glass units with thermally-optimised spacers continue to establish themselves as the standard. If condensation occurs on a triple insulating glass unit with a thermally-optimised edge, this is a clear indication that the indoor air is too humid. In many flats and houses, it is no longer sufficient to ventilate the dwelling purely by opening the windows by hand. It makes eminent sense to install externally mounted air transfer devices to provide a basic level of ventilation and air change. To ensure that any ventilation devices work properly and are accepted by users, it must always be carefully explained to building users, building managers, etc. how to use them correctly. Fig. 2 Kitchen units in front of a casement door. Condensation and mould are virtually inevitable along the lower edge of the glass (circled in red). ift Rosenheim Page 37 of 166

4 b) Indirect heating ( flashover of heat and moisture) Building occupants often fail to heat bedrooms, pantries and rarely-used rooms sufficiently, or indeed at all. As soon as the indoor air from rooms that are heated and used to a normal extent is allowed (or deliberately encouraged!) to enter these areas, condensation is very likely to occur. c) Increased moisture in air (user behaviour) In many cases, condensation was the result of unusually high humidity. The following causes were noted: Hanging washing indoors Large numbers of pot plants, large aquariums, etc. Too many people living in the dwelling Air humidifiers/pots of water on woodburning stoves. Some building biologists have proposed a maximum relative humidity of 60%; from a structural point of view, however, this is much too high. 4 Recommendations for preventing damage If, prior to construction work being carried out, it becomes apparent that some of the risks described in section 3 are present, this should be pointed out. One effective way of actively preventing damage is to thermally optimise the windows as far as possible. be as airtight as possible, with the seals arranged according to the principle of tighter on the inside than the outside, in order to prevent condensation and mould formation in the rebate. As a further way of actively reducing condensation the application of DIN should also be insisted on, because user-independent ventilation can considerably reduce the humidity of indoor air. However, users still need to be informed about the various mechanisms at work, and encouraged to cooperate in ensuring that the necessary preventive steps are taken. Bibliography [1] DIN : Thermal insulation and energy economy in buildings Minimum thermal insulation requirements. Beuth Verlag GmbH, Berlin [2] DIN Fachbericht (Technical Report) : Thermal insulation and energy economy in buildings Avoidance of mould growth in residential buildings. Beuth Verlag GmbH, Berlin [3] DIN : Ventilation and air conditioning Part 6: Ventilation for residential buildings General requirements, structural design requirements, execution and labelling, handover/acceptance (inspection and approval) and maintenance. Beuth Verlag GmbH, Berlin For example, the use of thermally-insulating glass with a thermally-optimised edge seal is strongly recommended. Triple insulating glass units with warm edge edge seals are the most effective solution currently available for avoiding condensation on the glass edge (when used in conjunction with normal frame designs!). In addition, the sealing plane on the indoor side should Page 38 of 166 ift Rosenheim

5 Useful information in brief Basics of assessing the risk of condensation 1 Because windows and facades are very thin compared to external walls, they are always the weakest point on the surface of a building in terms of preventing heat from escaping. It is therefore impossible to completely eliminate the risk of condensation on these components. 2 The U W value is always the average for the individual components. Condensation and mould formation, on the other hand, are always the result of geometrical and material-specific thermal bridges for example the spacers around the edge of the glass in insulating glass units. It is therefore not possible to determine the likelihood of condensation from the U W value of the window. 3 The risk of condensation and mould formation can be evaluated by thermal simulation ( isotherm calculation ). The first parameter required for this is the surface temperatures on the indoor side, which, according to the mould criterion, should ideally be 12.6 C. 4 The second important variable determining the likelihood of condensation is the relative humidity of the indoor air. The relative humidity and temperature of the indoor air fluctuate dramatically, so one-off measurements are not suitable. Temperature and humidity data should be collected over a period of several weeks, during the heating season. It is advisable to use data loggers for collecting these data. 5 For documentary evidence on the basis of the standard, the indoor climate of 20 C temperature and 50% relative humidity given in DIN is used for the component design. It is often suggested that these values are reliable averages for a typical indoor climate but this is not so. Unfortunately there are no laws or standards specifying a maximum permissible indoor humidity. 6 It has been noted that the indoor air temperature in situations where condensation or mould damage has occurred is often 23 C or more. As well as resulting in considerably higher transmission heat loss, high temperatures also increase the ability of the air to absorb moisture, thus raising its dew point. At 24 C and 50% relative humidity, condensation occurs at surface temperatures as high as 12.9 C (at 20 C/50% => 9.3 C). The user therefore has an important role to play. 7 By taking full advantage of all potential for optimisation currently available (good thermal insulation; avoidance of thermal bridges; high level of airtightness; defined, user-independent ventilation) condensation problems can be (more or less) eliminated. ift Rosenheim Page 39 of 166

6 Dipl.-Ing. (FH) Born in 1976 in Balve (Märkischer Kreis) 1996 Allgemeine Hochschulreife (diploma from German secondary school qualifying for university admission or matriculation), Ursulinen-Gymnasium Werl Civilian national service Vocational training for the skilled profession as carpenter, certificate by the Chamber of Crafts in Soest Course of studies at the University of Applied Sciences in Rosenheim Diploma degree in the faculty of "Wood Building and Construction" since 2004 since 2010 Assessor at ift Centre of Technical Experts Assistant Head of ift Centre of Technical Experts Page 40 of 166 ift Rosenheim