Hygrothermal Performance of Sandwich-Panel-Renovated External Walls

Abstract

The facades of many hall-type buildings, built in the 1970s or 1980s, are at the end of their service life. Facades must be renovated so that the value of the building will not reduce or the usability suffers. When repairing, it is reasonable to estimate the energy efficiency of the building, which can be improved by increasing the thermal insulation of envelopes. Because the share of new buildings is only 1–2 % of the total construction stock per year, it is important to pay attention to the energy efficiency of existing buildings. Commonly, energy renovations are concentrated on residential buildings, but hall-type buildings have a considerable energy saving potential. The purpose of the Master’s thesis was to develop hygrothermally functional renovation methods to exterior walls made of a concrete sandwich panels and an aerated autoclaved concrete (AAC). In the designed structures, slightly ventilated cavities were used which removes excessive moisture from an old frame and prevents mould growth inside the wall structures. An external thermal insulation with a slightly ventilated cavity creates challenges to the thermal behaviour of the wall because the air flow reduces the thermal resistance of the exterior wall, which also increases thermal losses. In this thesis, the impacts of different variables were considered to the functioning of wall structures. The variables were locality, orientation, test year, start point of repair, ventilation in a cavity, thickness of insulation and water leakages into the structure. The simulations were executed by Delphin 5.8.3 program. For the simulations, the airflow rates in slightly ventilated cavities were estimated based on literature and the ventilation rates were determined by using a measured hourly weather data. The considered structures were ideally airtight, thus the moisture loads of air leakages were ignored in the calculations. The initial conditions of old frameworks were found out by making test simulations to these structures and the results created the initial conditions to the renovated structures that were simulated. From the results can be noted that the structures contained a large amount of moisture that complicated the operation of the structures when airtight sandwich panels were installed on the surfaces of exterior walls. The initial moisture had to be removed by a drying period due to the moisture performance of the structures and the durability of the steel parts. The drying phase was carried out with an open ventilation interval, allowing for higher ventilation that removed excessive moisture. Initial moisture contents varied greatly during a year, especially with the AAC structures, so the starting time of renovation played an essential role in the hydrothermal operation on the exterior walls. In addition, the wind driven rain past the facade caused challenges to the hydrothermal performance of the wall structures. The excessive moisture load required ventilation of the wall structure to prevent favourable conditions for mould growth. Increasing rainfalls, caused by climate change, will raise the moisture load on structures in the future, which have to be considered in terms of the long-term performance of the structure. This was reflected with higher MMAX values on examination points when the cases were simulated with the future test years. The largest energy savings were generated by the facade repairs implemented with the AAC structures. Drying of the structure increased the thermal insulation capacity of the wall, whereupon even the plate claddings created energy savings compared with the original structure. On concrete sandwich panel walls, changes in heat losses of the wall structures were not as notable because the original heat losses on the concrete sandwich panel walls were lower.