Improving indoor climate and comfort with wooden structures

In this report, the moisture performance of a bedroom in a wooden apartment building is studied numerically using hourly weather data from 4 different cities (Helsinki, Finland, Saint Hubert, Belgium, Holzkirchen, Germany and Trapani, Italy). The bedroom is occupied for 9 hours by two adults during the night (22:00 to 7:00), the volume is 32.4 m3 and the wall surface area is 60 m2. With the basic input parameters (moisture production of 60g/h, ventilation rate of 0.5 ach and a permeable internal coating on the ceiling and walls) the moisture transfer between indoor air and the building structure is very active. With these parameters, the moisture transfer between indoor air and structures can significantly improve the indoor climate and air quality compared to the case where the internal coating is vapour tight. Moisture storage in wood based materials can reduce the peak humidity during the night and this moisture can then be removed by ventilation air during the following day. In general (at a ventilation rate of 0.5 ach), the indoor humidity is close to the outdoor humidity when the occupants enter the room (22:00) for all structures and materials. The increase in absolute humidity during the night is quite independent of the climate, but the amount of time when the indoor climate and air quality are unsatisfactory is very dependent on the climate. Passive methods of controlling the indoor climate are naturally more successful in moderate climates than in hot and humid climates, even though they provide benefits in all climates. With the basic input parameters, there are many materials that can realise an enhanced moisture performance. For example, either a hygroscopic wallboard or hygroscopic insulation can provide good performance. However, when there is a hygroscopic wallboard, the insulation behind the wallboard has little effect on the performance. Therefore, the indoor moisture level of a room with a hygroscopic wallboard is quite insensitive to the hygroscopicity of the insulation and the vapour resistance of the elements behind the wallboard. When there is hygroscopic insulation behind a non-hygroscopic and permeable wallboard (most wallboard materials have some hygroscopicity), the performance is only slightly worse than when there is a hygroscopic wallboard. These results are for the basic input parameters and the difference between different materials and solutions becomes more important when: the moisture production increases, the ventilation rate decreases, the active area decreases, the vapour resistance of the paint increases or during long term weather changes. With the basic parameters, the risk of mould growth is low, but the risk increases as the moisture production rate increases. The simulation results in this report demonstrate that thermal mass and solar shading are important for moderating indoor temperatures in northern and central European climates, but even a structure with a high thermal mass performs poorly in southern Europe when there is no heating or cooling. A room with a massive wooden floor and ceiling (200 mm) has a similar thermal performance as a room with a concrete floor and ceiling (200 mm). Also, moisture transfer can help cool the room when the outdoor temperature increases. The sensitivity of the ventilation rate is analysed and the results show that ventilation is very important for removing moisture, especially when an impermeable coating is applied. The increase in humidity during the night becomes greater as the ventilation rate decreases for all cases. With a permeable paint and a ventilation rate of 0.1 ach, the indoor air humidity increases on average by 7.4 g/h during the night, which is equivalent to the humidity increase when the ventilation rate is 0.9 ach and the paint is impermeable. Nevertheless, the amount of time that the indoor humidity exceed 60% RH during occupation, decreases as the ventilation rate decreases because the indoor temperature increases as the ventilation rate decreases. The thermal comfort and perceived indoor air quality at the end of occupation can be similar with 0.1 ach and a permeable paint as with 0.25 ach and an impermeable paint. As the moisture production increases, the fraction of the produced moisture that is stored in the wall increases very slightly. The moisture removed by the ventilation air, the moisture removed by the hygroscopic structure and moisture that remains in the indoor air are nearly linearly dependent on the rate of moisture production.