Field test of dehumidifiers for avoiding condensation in unheated historical wooden houses

The UNESCO world heritage “Decorated Farmhouses of Hälsingland” represent a well-preserved Swedish regional timber building tradition from the 18th and 19th centuries, featuring wall paintings of high cultural and artistic value. The houses have remained unheated and naturally ventilated over centuries, and have relatively leaky building envelopes. Recent indoor climate measurements and observations, however, have identified occasional condensation on indoor surfaces during unfavourable weather changes in wintertime. Such condensation poses a risk of degrading the wall paintings and other valuable objects, although low winter temperatures prevent mold growth. To mitigate condensation risk, sorption dehumidifiers – working also at temperatures below 0 °C – were installed in one of the UNESCO farmhouses during a winter season. The dehumidifiers were programmed to limit the indoor air relative humidity (RH) at maximum 80 %, and their dried air was distributed to all rooms via a flexible ductwork. Additionally, climate loggers and passive tracer gas technique were employed to measure temperature, RH, and air change rate in all rooms. By comparing with measured indoor climate in other similar farmhouses in the region, the results indicate that the dehumidifiers chiefly managed to limit RH at 80 %, thus preventing condensation in all rooms, despite a relatively high mean air change rate of around 0.8 ACH. However, locally and temporarily, enhanced RH peaks occurred, possibly due to unfavourable transient wind and/or stack conditions. The study also provides some practical installation guidance.