Condensation Risk Assessment

The methodology afrisk analysis and assessment is reviewed and applied to study the reliability of condensation control measures in lightweight building envelopes. It is generally recognized that airtight construction is an essential part of condensation control. Nowadays, different air barrier systems are developed and documented to prevent air leakage and moisture accumulation in the envelope. But does this mean that the condensation risk is sufficiently minimized and that the protective system is reliable? Considering the high occurrence of human error in the building process, the possibility of air barrier defects during the service life of a building envelope may be high. To define the reliability of the condensation control system, the consequences of air barrier failure are quantified using a twodimensional numerical control volume model for the calculation of combined heat, air, and vapor transfer in multilayered building envelope parts. A set offailure modes and design calculation conditions is definedfor an exemplary woodframe insulated roof, and afailure effect analysis is peiformed in order to predict the condensation risk as a result of air barrier defects. The effectiveness of redundant design measures to improve the reliability of the condensation control system is studied. RISK ANALYSIS METHODOLOGY Risk analysis, as applied in this work, is a method to study the effects of uncertain events or unintentional actions on the serviceability of a design in order to prevent or reduce the negative outcome of these effects (Ang and Tang 1984; Ansell et aI. 1991). In this definition, risk not only refers to the probability offailure but also incorporates a notion of the magnitude of the failure. The techniques to identify and reduce risks were developed in the 1960s in the nuclear and chemical industries (KIetz 1992). The analysis provides a systematic and structured framework with three main stages: I. Risk identification 2. Risk assessment 3. Decision making The first step to be addressed at the design stage of a project is the recognition and identification of the range of risks to which a system is subject. The ultimate aim of risk identification is to link the possible problems that a system might experience directly to external factors and events involving component failures and human errors. The second stage implies the use of probabilistic calculation and assessment methods in order to predict the system performance and the probability of failure as a result ofthe unintentional events. In industrial risk analysis, risk is generally determined as the product of the frequency and the size of the consequence of the event (Ansell et aI. 1991; Kletz 1992). Risk is accepted when events that happen often have no or low consequences or when events involving serious problems are rare. When the predicted risk cannot be justified, decisions should be taken to eliminate or minimize the risk, either by preventing the event or by reducing the consequences in case of occurrence. These decisions often imply redesign of the system by the introduction of protective measures. There are generally three types of strategies, with decreasing order of effectiveness: inherent, engineered, and procedural safety measures. The reliability of protective systems is often improved through some duplication of components. This is called "redundancy" if the protective components are the same or "diversity" if they are different. In a nonredundant system, the failure of the protective component leads inevitably to the failure of the entire system. Arnold Janssens is a researcher and Hugo Hens is a professor at the Laboratory of Building Physics, K.U. Leuven, Belgium. Thermal Envelopes VIIiMoisture Assessments-Principles 199 Consider the example of fire risk prevention in buildings. A building is constructed with inflammable or fire-retarding materials whenever possible and affordable (inherent safety). Recognizing that fire risk is rarely eliminated, a sprinkler installation may be designed and installed in the building (engineered safety). Finally, to prevent casualties, a fire alarm and escape may be added (procedural safety). The reliability of the fire protection may be increased by installing two or more fire alarms (redundancy) or adding both sprinkler and alarm systems (diversity). There are similarities between the design of a hazard protection and a building envelope system. The building envelope may be regarded as a multicomponent protective system to eliminate or minimize the discomfort of the environment. The application of the redundancy concept is a common element of building envelope design. For instance, the reliability of rain control in the building envelope is often improved by adding redundant components, such as drained cavities, multilayered membranes, etc. Because of these similarities, the methodology of lisk analysis may well be applied to the performance analysis of building envelope systems. In some recent publications on moisture control, the concepts of risk analysis are implicitly present. It is recommended that designers incorporate moisture control measures to ensure the performance of the envelope system in the case that something goes wrong during construction or operation, for instance, by accidental wetting or by human error. This subsystem is called "the second line of defense" (Lstiburek and Bamberg 1996). UNCERTAINTIES IN CONDENSATION CONTROL Condensation Control Systems TenWolde and Rose (1996) presented two major approaches to moisture control in the building envelope. The first is to design and construct the building envelope for a high tolerance for moisture. The second is to limit the moisture load on the envelope. Designing the envelope for a high moisture tolerance implies the use of measures to control the migration of moisture into the construction, to control moisture accumulation in building materials, or to enhance removal of moisture from the building assembly (Lstiburek and Carmody 1993). The limitation of the moisture load often involves control strategies on the level of building design and operation, e.g., ventilation, dehumidification, or depressurization. Table I categorizes potential condensation control strategies for the building envelope. Table 2 lists measures to restrict the moisture load by proper building design and operation. The individual measures may be combined to create an effective condensation control system. The problem is to find out which combination of condensation control measures is the most reliable to reduce the condensation risk. 200 TABLE! Condensation Control Strategies for the Building Envelope* Strategy Aim Measure Control of Eliminate air leakage Air barrier moisture access Restrict vapor diffusion Vapor retarder Control of Raise temperature of Insulation outside of moisture condensing surface condensing surface accumulation Allow harmless High moisture capacity accumulation at condensing surface Removal of Promote drying Vapor permeable layers moisture Capillary active layers