Salvatore della Vecchia, Samy Clinchard, Rick Aller, Ulla Haverinen-Shaughnessy 720 Degrees Oy
ABSTRACT
In this study, continuous data of indoor temperature (T) and relative humidity (RH) from 84 Finnish office buildings were coupled with weather data on outdoor conditions over the year of 2016, and used to assess excess indoor water vapour content (v). The results were compared to the humidity class 2 based on standard EN ISO 13778. The result indicated that v was usually very low (mean -1.1 g /m3, SD 1.1, range -5.9 - 7.09). The critical level was exceeded in 14 buildings on 72 occasions out of 19018 data points (0.4%). Most important determinants for exceeding the critical level was humidification (in two buildings, 84% of the occasions) and low outdoor T (<0 oC). Based on continuous monitoring, building operation can be notified to take actions in order to prevent a situation where critical water vapour content is exceeded for a prolonged period of time.
INTRODUCTION
Different activities generate moisture indoors. These include moisture generation from occupants due to respiratory process, showering, cooking, dishwashing, laundry and drying /1/. In addition, indoor air water vapour content (vi) can be increased by humidification. On the other hand, indoor water vapour can be removed by ventilation and dehumidification processes. It is often recommended that indoor air should be kept
“dry and cool”, which help to lower material emissions and microbial growth on building materials.
At certain temperature (T), higher moisture content of the air corresponds with higher relative humidity (RH). Likewise, maximum water vapour content of air (without condensation) is depended on the temperature. Condensation occurs, when RH reaches 100%. Therefore, certain thermal conditions, including low T and high RH, associate with risk of condensation in buildings, which is not desirable from the point of view of building dynamics and indoor air quality (IAQ). However, there exists a limited amount of data about such conditions from different building types. For example, offices are different from residential buildings in terms of occupant density and their activities, which is also likely to impact moisture generation indoors.
Standard EN ISO 13788 (2012) provides calculation methods and classification of critical surface humidity and interstitial condensation /2/. Based on the standard, humidity class 2 is applicable for both offices and dwellings with normal occupancy and ventilation.
Current technology enables continuous monitoring of IAQ, including T and RH. Whereas these two parameters themselves provide useful information pertaining to thermal comfort, they can also be used to assess indoor air water vapour content and, together with corresponding information from outdoor T and RH, excess indoor water vapour content (∆v) resulting from indoor air humidity. The aims of this study were to assess ∆v
in Finnish office buildings over time (one calendar year), to compare results with the standard /2/, and to assess possible risk factors for exceeding the critical level.
MATERIAL AND METHODS
Continuous data (measurement interval 15 s) of indoor T and RH from 84 Finnish office buildings (1-12 sensors per building) were coupled with local weather data on outdoor T and RH over the year of 2016. First, daily average water vapour content (g/m3) in indoor air (vi) and outdoor air (vo) were calculated, followed by calculation of excess indoor ∆v (vi-vo) resulting from indoor air humidity, according to the empirical formulas presented by Nevander & Elmarsson /3/.
The sensors’ temperature measurement range was between 0 and +50°C with an accuracy of ±0.3°C and the RH measurement range was between 0 and 90 RH% with an accuracy of ±3 RH%. The precision of weather data is higher (e.g. ±0.1°C for outdoor T).
However, there may be some spatial differences in outdoor T and RH, for example due to topography /4/, causing uncertainty in the estimates.
RESULTS AND DISCUSSION
Table 1 shows descriptive statistics for indoor and outdoor T, RH, and v by month.
Table 1. Descriptive statistics for indoor and outdoor T, RH and v by montha in 2016.
Mo Mean, SD (Rangeb)
9 22.4, 1.2
Based on the results, average indoor water vapour content is slightly below outdoor air vapour content, which indicates that indoor moisture generation is smaller than its removal. There are no records about possible use of dehumidification, which is generally assumed not be commonly used in Finland. Inaccuracy of +3% RH at 22.4 oC and 26.8 % RH (i.e. the annual average values) would result in about 0.6 g/m3 difference in vi. However, even if 0.6 g/m3 was added to vi, ∆v would still remain negative for the most part of the year.
Figure 1 shows weekly average excess indoor ∆v vs. outdoor T in buildings with and without humidifiers. Critical level is exceeded only in buildings with humidifiers.
Figure 1. Weekly average excess indoor ∆v vs. outdoor T in buildings with and without humidifiers.
We have also used continuous carbon dioxide (CO2) data (collected from the same buildings simultaneously with T and RH data) to estimate ventilation rates, which appear to be relatively high. The results regarding ventilation rates will be reported elsewhere in
detail, but as a summary, most of the building are exceeding the criteria for Class 1 based on classification for indoor air climate in Finland throughout the year /5/. High
ventilation rates may partially explain low vi.
It should also be noted that monthly average indoor RH is below 20% during the winter months, approaching 10% in January, which was the coldest month of the year. Based on epidemiological, clinical, and human exposure studies, low RH plays a role in the increase of reporting eye irritation symptoms /6/. Dry and irritated mucous membranes of the eyes and airways are common symptoms reported in office-like environments.
Average indoor T is above 22 °C for each month except January, indicating that dropping the temperature by 1-2 °C could help keeping indoor RH more acceptable. Such a decrease should not negatively impact relative performance of the office workers /7/.
Several field studies have been performed to define typical ∆v in dwellings, usually finding values between 2 and 3 g/m3 during the heating seasons, and critical levels of about 4 g/m3 during the cold period (Outdoor T < +5 °C), corresponding to humidity class 2 /8, 9/. Based on the data presented in this paper, ∆v appears to be on much lower level in offices, which could be due to occupant activities less frequently including cooking, bathing, etc. From the point of view of removing excess indoor water vapour, ventilation requirements could be adjusted accordingly.
It is generally considered that ∆v is relatively constant during the colder part of the heating season (Outdoor T < 0 °C) and it decreases when the outdoor temperature increases /8/. In addition, the results from this study indicate that in the monitored offices, the weekly average ∆v exceeded humidity class 2 only in cases where humidifier is used and outdoor T is low (Outdoor T << 0 °C). An advantage of continuous monitoring is that building operation can be notified to take actions in order to prevent a situation where critical water vapour content is exceeded for a prolonged period of time.
CONCLUSIONS
Based on the data from 84 Finnish office building continuously monitored over 2016, the most important determinant for exceeding the critical level for indoor water vapour content was use of humidification and low outdoor T. Overall, the indoor water vapour content was usually below outdoor air water vapour content, which indicates that indoor moisture generation is smaller than its removal.
REFERENCES
1. Zemitis J, Borodinecs A, Frolova M. (2016) Measurements of moisture production caused by various sources Energy Build. 127: 884–891.
2. EN ISO 13788:2012. Hygrothermal performance of building components and building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation. Calculation methods.
3. Nevander L E, Elmarsson B. (1994) Moisture handbook, theory and practise. (in Swedish). Svensk Byggtjänst.
4. Finnish Meteorological Institute, http://ilmatieteenlaitos.fi/lampotila-ja-kosteus#17.
Accessed 15 Jan 2018.
5. Sisäilmastoluokitus 2008. Sisäympäristön tavoitearvot, suunnitteluohjeet ja tuotevaatimukset, in Finnish. RT 07-10946, https://www.rakennustietoshop.fi/en/rt- 07-10946-sisailmastoluokitus-2008.-sisaympariston-tavoitearvot-suunnitteluohjeet-ja-tuotevaatimukset/103675/dp
6. Wolkoff P, Kjærgaard S. (2007). The dichotomy of relative humidity on indoor air quality. Environment International 33 (6): 850-857.
7. Seppänen O, Fisk W. (2006) Some quantitative relations between indoor
environmental quality and work performance or health.HVAC&R Research 12(4):
957-973.
8. Geving S, Holme J. (2012). Mean and diural indoor air humidity loads in residential buildings. J. Build. Phys., 35(4), 392-421.
9. Kalamees T. (2006). Indoor humidity loads and moisture production in lightweight timber-frame detached houses. J. Build. Phys., 29(3), 219-246.