School indoor moisture

This Issue This is a part of the Schools feature

By and - , Build 158

BRANZ has been measuring moisture levels in a school’s classrooms. This information is providing a sound basis for understanding the indoor climate and designing moisture-safe long-span roofs in schools.

Figure 1: Relative humidity and temperature for a school week in winter.
Figure 2: Absolute indoor and outdoor moisture content for a Friday and Saturday in late September.

IN 2015, BRANZ published Initial guidance on the moisture design of large-span roofs for schools. The aim was to minimise damage from condensation in new roof designs. It had a preliminary conservative design process and outlined a more refined process for use once some research questions were answered. Since then, BRANZ has been filling the gaps.

Filling gaps for design calculations

One input into the roof design process that needed further research was the temperature and humidity profile for the school classrooms. Without measured data, assumptions have to be made about the indoor climate that can have an important bearing on the level of moisture in the roof structure.

Recently, measurements from a school have helped develop data to enable the selection of an appropriate interior climate. Airflow through ceilings (see pages 82–83) provides data on how much of this indoor air might be transported through the ceiling due to airflow. This information can be used in a design calculation such as that in Build 151 (pages 60–61) for determining roof ventilation.

School conditions monitored

To provide more reliable indoor climate data, BRANZ monitored the conditions in a Wellington primary school over a half-year period including winter and summer months.

The 3-storey, southwest-facing concrete building originally consisted of three classrooms on each level. However, the change to innovative learning environments means each level now operates as one large classroom with up to 60 students in the space.

The temperature and relative humidity were recorded every 15 minutes at various locations on each floor. Figure 1 shows the out-of-phase relationship between relative humidity and temperature for a school week in winter 2015. Central heating is activated around 6 am Monday to Friday. The relative humidity stays at a higher, more constant level over the weekend when there is no heating.

Figure 1: Relative humidity and temperature for a school week in winter.

Quantifying the moisture students add

Beyond simply recording the indoor climate, the aim was also to quantify the amount of moisture the students released into the classroom. To do this, absolute moisture content was measured because, unlike relative humidity, this is not dependent on temperature.

The absolute moisture content of air is defined as the mass of water vapour contained in a mass of dry air. For example, if there was 6 g of water vapour for every 1 kg of dry air, the absolute moisture content would be 6 g/kg.

Water content was calculated for the coldest and warmest period on Tuesday 23 June as 4.4 g/kg and 6.0 g/kg respectively (red circles in Figure 1). Although the relative humidity had decreased, the absolute moisture content had risen. This was mainly due to the students (with some moisture released from furniture and carpets at higher temperatures during the day).

Weekday compared to weekend

Figure 2 shows the absolute indoor and outdoor moisture content for a school Friday and a weekend Saturday at the end of September 2015. In the early hours of Friday, the absolute moisture drops slightly, likely due to condensation on various surfaces as the dew point temperature is reached.

Figure 2: Absolute indoor and outdoor moisture content for a Friday and Saturday in late September.

Heating kicks in

Once the central heating activates at 6 am, the moisture content rises again as some condensed moisture is released back into the air. The outside moisture rises shortly afterwards with the warmer temperatures removing dew and releasing it into the atmosphere. The absolute moisture content is further increased when pupils arrive.

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Windows and doors open and moisture drops

Friday 25 September was a cold day – the highest outside temperature was around 10°C. Windows were probably closed. The dips in indoor moisture content around 10.30 am and 1 pm coincide with breaks when the students leave the classroom. Doors will have been opened, providing some ventilation.

This changes after 3 pm when the indoor moisture level drops continually, reaching a level similar to the outside. The two graphs in Figure 2 are then nearly identical on Saturday when the classrooms are unoccupied.

Because of added moisture from the students, the indoor moisture level is kept significantly above the outdoor level. Like our data during June, the added amount lies between 1.5 and 2.0 g/kg.

Seasonal variations

Table 1 shows the seasonal variation of the difference between indoor and outdoor moisture – let’s call this the excess moisture. A trend towards smaller differences can be seen towards the end of the year.

Table 1
Excess moisture level in classrooms during school hours
  (g water vapour/kg dry air) 
July 3.7 1.9
September 2.7 1.4
December 0.9 0.0

During the winter months, the excess moisture can reach values close to 4 g/kg with average values around 2 g/kg. The difference between indoors and outdoors is almost absent during December. The moisture source – the students – is the same, but during winter, the ventilation rate is likely to be much less due to windows being shut.

Although this data is from one school only, it provides a more robust basis for a design calculation than previously.

These and more data sets will be added to the BRANZ guidance so it becomes a key resource for anyone involved in the construction of school roofs.

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For more

Initial guidance on the moisture design of long-span roofs for schools is available from

Download the PDF

Articles are correct at the time of publication but may have since become outdated.

Figure 1: Relative humidity and temperature for a school week in winter.
Figure 2: Absolute indoor and outdoor moisture content for a Friday and Saturday in late September.