Hygrothermal Risk Assessment - Retrofit of External Wall by the Application of Interior Insulation

Inside insulation of external walls of timber-framed construction is an adequate retrofitting measure in cases where there is an interest of preserving the existing facade. According to certain recommendations, and for the purpose of minimizing the work efforts, the additional insulation is placed directly on an existing wall, leaving the existing vapour retarder in the area that is substantially colder than before the retrofitting and thus increasing the risk of mould growth in the wall. The hygrothermal conditions inside the retrofitted wall are investigated for various indoor and outdoor conditions, and with and without indoor air intrusion in the wall. According to the results, 32% of 500 simulated scenarios obtained an annual average of relative humidity larger than the critical value for mould growth at the most critical spot inside the wall. The corresponding ratio was 43% in the wall assembly with an assumed air intrusion. Suggestions for the improvements of the moisture performance of the wall are also suggested. The used methodology of risk assessment is fully presented in the paper; it is of a general character and can be used in other retrofitting studies.

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