Positive pressure effect on moisture performance in a school building

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Positive pressure effect on moisture performance in a school building. / Ferrantelli, Andrea; Vornanen-Winqvist, Camilla; Mattila, Milla; Salonen, Heidi; Kurnitski, Jarek.

In: Journal of Building Physics, Vol. 43, No. 2, 01.09.2019, p. 121-142.

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@article{ab8b17e7253d49c6ad8313a29ec6a824,
title = "Positive pressure effect on moisture performance in a school building",
abstract = "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/m 3 , 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.",
keywords = "indoor air quality, indoor air ventilation, Moisture performance, moisture transfer, positive pressure",
author = "Andrea Ferrantelli and Camilla Vornanen-Winqvist and Milla Mattila and Heidi Salonen and Jarek Kurnitski",
year = "2019",
month = "9",
day = "1",
doi = "10.1177/1744259119837144",
language = "English",
volume = "43",
pages = "121--142",
journal = "Journal of Building Physics",
issn = "1744-2591",
publisher = "SAGE Publications Ltd",
number = "2",

}

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TY - JOUR

T1 - Positive pressure effect on moisture performance in a school building

AU - Ferrantelli, Andrea

AU - Vornanen-Winqvist, Camilla

AU - Mattila, Milla

AU - Salonen, Heidi

AU - Kurnitski, Jarek

PY - 2019/9/1

Y1 - 2019/9/1

N2 - 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/m 3 , 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.

AB - 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/m 3 , 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.

KW - indoor air quality

KW - indoor air ventilation

KW - Moisture performance

KW - moisture transfer

KW - positive pressure

UR - http://www.scopus.com/inward/record.url?scp=85063957549&partnerID=8YFLogxK

U2 - 10.1177/1744259119837144

DO - 10.1177/1744259119837144

M3 - Article

VL - 43

SP - 121

EP - 142

JO - Journal of Building Physics

JF - Journal of Building Physics

SN - 1744-2591

IS - 2

ER -

ID: 33289021