By following some easy to implement practices, occupants in a new centrally heated home were able to reduce high levels of internal moisture and enjoy a comfortable indoor environment.
RECENTLY, A BUILDER called BRANZ to ask why a 2-year-old home had excess indoor moisture issues with water condensing on the double glazing.
The 180 m2 home has central heating and is occupied by two adults and a school-aged child. The occupants opened some windows a little at various times of the day.
An array of five sensors were set up to monitor the temperature and relative humidity in bedrooms, bathrooms and living areas.
Checking airtightness of buildings
Measuring airtightness at 50 pascal is an agreed method to compare the airtightness of different houses (see The nitty gritty on airtightness on page 86).
The airtightness of this house was measured to 3.5 air exchanges per hour at a pressure difference of 50 pascal. This lies well within the airtightness distribution of new homes.
As the house is in a relatively sheltered location, the wind-driven background air infiltration was insufficent to ventilate adequately. In normal pressure conditions, the house will not achieve an air exchange of 3.5 times its volume/hr, only an estimated air change of about 0.15–0.2/hr. A sufficiently ventilated building should have an air exchange rate of 0.3–0.5 per hour.
Moisture sources identified
Figures 1 and 2 show measurements from 1 day. The temperature over the day ranged from 16–22°C.
The absolute humidity (see Figure 1) reveals sources and sinks of moisture better than the relative humidity (see Figure 2), as it is only weakly affected by the air temperature.
Moisture from the bathroom is travelling into other parts of the building (see Figure 1). This is because the bathroom ventilation can’t cope with the moisture and the bathroom door is either left open or opened after use.
The night-time moisture in the bedroom is also a visible moisture source in Figure 1 with the absolute humidity staying almost constant in this location. In other parts of the house, the moisture content of the air drops due to condensation on cold surfaces.
Relative humidity high
The relative humidity is high throughout the 24 hours with occasional spikes close to 100% for activities such as cooking or showering.
To gauge the drying potential of the outdoor air and effectiveness of extra ventilation, the relative humidity of the outdoor air was calculated at the temperature in the kitchen. The bottom trace in Figure 2 at around 50% shows the relative humidity of the outdoor air at indoor temperatures. A substantial gap of around 10–20% is clearly observable.
This indicates that the moisture problems were due to inadequate ventilation and can be improved with, for example, a manual ventilation scheme.
Solution – contain and ventilate
The results were shown to the owner and the following manual ventilation scheme and source containment discussed:
- Close bathroom door while using bathroom.
- Operate the bathroom fan.
- Heat the house.
- Open windows and doors wide in the morning for 10–15 minutes, enabling a full air exchange before leaving the house.
If possible, containment is the most effective way to reduce moisture. This keeps the space to be ventilated relatively small. The internal doors should be closed, the windows opened and the bathroom ventilation switched on.
Heat, then open windows to dry house
Heating is important as warm air carries more moisture than cold air. To dry out a house, the air has to be heated to drive out moisture in furniture, carpets and wall linings. The windows should then be opened to exchange moist air with the drier outdoor air.
Keep the heating off while the windows and doors are open. This cycle of heating and releasing air ensures that the house returns to a normal moisture regime over a relatively short period.
Significant recovery in just 3 weeks
With the manual ventilation scheme in place, the house returned to a normal moisture level in just 3 weeks. To measure this, the probability of encountering a relative humidity of less than 65% was looked at. This is the level where ventilation should be used to prevent moisture from getting too high.
Probability graphs for 0, 8 and 20 days since the start of the intervention are overlaid in Figure 3. This shows that the indoor air is likely to be below 65% relative humidity for:
- less than half of the time (44%) before the changes
- 62% of the time after 8 days
- 80% of the time after 20 days.
Before the changes, there was very high relative humidity of over 80%. After the 20 days, the maximum relative humidity had fallen to well below 80%.
Comfort levels quickly achieved
The changes to the manual ventilation had significant benefits in just 3 weeks. All that was needed was heating and manually opening windows to do a flush ventilation and achieve a near full air exchange of the house in about 10–15 minutes.
In this case, heating is needed to remove the stored excess moisture. The added benefit is that dry air can be heated better and improves the comfort level. The electricity cost for a flush ventilation of a fully heated house with an indoor-outdoor temperature difference of 10°C is around 1 kWh, depending on house size.
Articles are correct at the time of publication but may have since become outdated.