Moisture excess in buildings constitutes a complex problem affecting indoor air quality, energy consumption and the lifetime of the building envelope. We investigate the effect on moisture transfer in structures as a positive pressure is applied inside the enclosure. It is found that, contrary to established belief, the positive pressure does not induce any negative effects on the structures’ moisture content in normally ventilated classrooms, even with high occupancy. Our case study consists of a school building in Finland, subject to temperature and relative humidity measurements after a small (5–7 Pa) positive pressure was realized through ventilation control. We first address analytically the moisture excess generated inside the classrooms for 14 days, using dynamical balance equations that account for both ventilation effects and occupants’ moisture release in the environment. It is found that the average moisture excess is very small, largely below 1 g/m3, even for ventilation rates that are half the design value. We also examine the moisture performance of the envelope, by addressing the moisture migration at upper and lower joints of the external walls for both measured and design values of the indoor absolute humidity (AH). A coupled numerical model of diffusion and convection shows that moisture accumulation in the envelope and the according stresses are negligible for any realistic AH values. This result is in agreement with field measurements at the school. In conclusion, it seems that applying a small overpressure in a well-ventilated school building during a standard service period resulted in no accumulation inside the external walls, even at high occupancy and with low ventilation. Remarkably, it slightly dried out the moisture content in structures under actual occupancy conditions. The positive pressure has accordingly no negative effects on moisture performance, and is capable to guarantee a good indoor air quality as well.

中文翻译：

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正压对学校建筑湿度性能的影响

建筑物中水分过多是一个复杂的问题，会影响室内空气质量、能源消耗和建筑围护结构的使用寿命。我们研究了在外壳内施加正压时对结构中水分转移的影响。研究发现，与公认的观点相反，正压不会对正常通风的教室中的结构水分含量产生任何负面影响，即使占用率很高。我们的案例研究包括芬兰的一所学校建筑，在通过通风控制实现小 (5-7 Pa) 正压后，进行温度和相对湿度测量。我们首先分析了 14 天教室内产生的水分过多，使用动态平衡方程来考虑通风效果和居住者在环境中的水分释放。发现平均水分过量非常小，大部分低于 1 g/m3，即使通风率是设计值的一半。我们还通过解决室内绝对湿度 (AH) 的测量值和设计值的外墙上下接缝处的水分迁移来检查围护结构的防潮性能。扩散和对流的耦合数值模型表明，对于任何实际的 AH 值，封套中的水分积累和相应的应力都可以忽略不计。这一结果与学校的实地测量一致。综上所述，似乎在标准服务期内在通风良好的学校建筑中施加小超压不会导致外墙内积聚，即使在高占用率和低通风情况下也是如此。值得注意的是，它在实际占用条件下略微干燥了结构中的水分含量。因此，正压对防潮性能没有负面影响，并且能够保证良好的室内空气质量。