## 1 Introduction  
The energy efficiency of buildings has become increasingly important due to the greenhouse gas emission reduction and the acceleration of climate change. The endeavor for energy efficiency in the building sector has promoted the use of more effective thermal insulation in the building envelope to reduce conduction heat loss. The change in the building industry to use thicker insulation layers has raised the question of possible moisture problems in these structures. Mistrust in the hygothermal behavior of highly insulated (HI) structures is not a new issue, since it was a topic already in the 1980s when there was a similar interest in buildings with low energy consumption [1]. The hygrothermal behaviour of energy efficient structures has been largely questioned [2; 3; 4]. In [PERSON]'s study of highly insulated walls with probabilistic methods, the hygrothermal performance was satisfactory for timber walls without potential deficiencies [2]. However, the performance decreased substantially with moisture sources as low as 0.5% of WDR (wind driven rain). Another study found that the drying ability might weaken due to the lower heat flow towards the outer part of structures [3]. The researchers added that a structure affected by an occasional moisture leakage is not able to dry out as fast as before. The study suggests that the overall fault tolerance is weakened with thicker insulation. The research was based on numerical simulations in selected reference years. A recent study of the hygrothermal behavior of highly insulated walls focused on the weather barrier material and its thickness, in the Norwegian climate [4]. The resultsfrom this study imply that there is a risk for mold growth in highly insulated mineral wool structures with built-in moisture. The results further indicate that the increased thermal resistance of the weather barrier layer can reduce the risk for mold growth. These aforementioned studies imply that the risks involved with highly insulated structures are possibly connected to the high instantaneous moisture loads or significant levels of built-in moisture. Other studies suggest that there are no significant risks with highly insulated structures [5; 6; 7; 8]. [PERSON], for example, studied the influence of thermal insulation material on the drying efficiency of highly insulated wall structures [5]. His numerical study showed that a vapor open thermal insulation allows faster drying of built-in moisture compared to a more vapor tight insulation material. Moreover, [PERSON] estimated that the drying potential (water vapor pressure difference) between the outer surface of the wall and the outdoor air does not change significantly between wall structures with thermal transmittances (U-values) 0.24 W/m\({}^{2}\)K and 0.09 W/m\({}^{2}\)K [6]. A study by [PERSON] used a stochastic approach to simulate heat, air, and moisture (HAM) transfer in wall structures with high and conventional insulation levels [7]. The study used climate conditions from two cities in Canada. It showed that the mold growth risk is usually lower for a mineral wool wall with a 216 mm insulation than for a wall with 140 mm of mineral wool. To examine highly insulated structures, [PERSON] simulated the hygrothermal behaviour of highly insulated timber frame walls in the Estonian climate [8]. He concluded that it is possible to design moisture safe timber frame walls with 400-600 mm insulation thickness. However, he emphasized that a careful selection of materials is required. These studies, that suggest there is no risk in highly insulated structures, are unsatisfactory because they lack experimental results on the drying ability of highly insulated structures under high moisture loads. As the literature shows, there is no general agreement about whether the increase of the thermal resistance (R-value) in envelopes can lead to the degradation of their hygothermal behaviour. Previous studies on highly insulated structures rarely deal with high instantaneous moisture loads.