Out of concern for the environment, the traditional polyethylene vapour barrier in lightweight constructions has sometimes been replaced with more diffusion open materials, such as polypropylene fabric. In order to evaluate how this may affect the risk of condensation in the constructions, in-situ measurements of the moisture content and temperature in the walls, roofs and floors of three houses have been conducted. Furthermore, the measurements have been used to establish a model that can be used to predict the moisture performance for design moisture supply loads. Measurements of moisture content and temperature in the outermost part of the insulation of the constructions have been carried out every three weeks over a two-year period, during which the indoor temperature and relative humidity were continuously registered. Based on indoor climatic data as well as information from a nearby weather station, the moisture conditions have been calculated using a one-dimensional transient diffusion model that takes into account the sorption properties of the materials.

The results of the measurements indicate that for the low moisture supply loads in the three houses there is no immediate risk of moisture damage. The results of the simulations show good correlation with measurements for the walls and a floor. The calculation model was therefore used to simulate the conditions for a design moisture supply of 2 and 4 g/m3 using a reference climate for Stockholm from the literature. The results show that even for the lower design value, moisture would accumulate and condense in the wall construction. Since high relative humidity would for long periods of time coincide with temperatures feasible for mould growth, the wall construction is not to be recommended. A similar simulation of the piled floor construction shows that even for moderate moisture supply loads the risk for condensation is high. A correlation for the roof construction could not be achieved without adjusting parameters and therefore the model was not considered valid for the roof. There are strong indications that this is due to ongoing settling of the loose fill insulation in the roof construction, since no evidence of convection could be found from the measurement results.

• experiment: laboratory : Moisture in lightweight construction
• insulation
• moisture in buildings
• WRB weather-resistive barrier, house wrap