Open windows for dry home

By - , Build 158

Recent BRANZ research has quantified one aspect of an important but often overlooked factor for achieving a dry home – occupant use.

Figure 1: Absolute humidity inside and outside the test room.
Figure 2: Temperature inside and outside the test room when a window was opened by 300 mm at 8 am.

A RECENT BRANZ case study demonstrated that many moisture problems seen in New Zealand homes can be solved by simply opening windows at certain times (see Build 156 New home, old habits). A few simple behavioural changes can make the difference between a damp and a dry home.

But how long do windows need to be open for? How much heat is lost? Is it enough to just have trickle vents?

Testing trickle vents and window opening

To start answering these questions, BRANZ looked into the performance of window opening, as stipulated in Building Code clause G4 Ventilation.

A north-facing room in the BRANZ test house was conditioned with heating and moisture to 25°C with a relative humidity of 70%. This equates to approximately 16 grams of liquid water for every cubic metre of air inside the room.

The windows were opened by 5 mm (similar to a trickle vent) and doors closed. Despite the room being heated to 25°C, condensation formed on the insulated walls and ceiling, as well as the single-glazed windows.

During experiments, the environmental conditioning was turned off at 8 am and a 1,030 × 735 mm awning window was opened by various amounts up to 300 mm.

Figures 1a and b show the decay of airborne moisture content in a room with the window left ajar at 5 mm and opened 300 mm. Both experiments were conducted under similar wind conditions.

Figure 1: Absolute humidity inside and outside the test room.

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Opening window wide is effective

Within 10 minutes of the window being opened by 300 mm at 8 am, the airborne moisture content had reduced by 14% (see Figure 1b).

After this initially rapid drying out, the humidity in the room decreases at a much slower rate, as the condensation on the walls, windows and ceiling evaporates slowly.

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Trickle vents alone not enough

For trickle vents, the airborne moisture content in the room reduced by only 4.5% in 10 minutes (see Figure 1a). It would take hours for the airborne moisture to reduce to the level reached by opening the window for just 10 minutes. This is despite the trickle vents being open overnight and during the dosing event.

On their own, trickle vents are not enough to manage high moisture loads, such as showering.

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Room retains some heat

Figure 2 shows the temperature in the room before and after the window is opened by 300 mm and the outside air temperature. Because the internal linings of the room continue to radiate heat after the heater has been turned off, they help to maintain the temperature in the room above the outside temperature.

The effect is that, even after opening the window for an hour with no heating, the air temperature in the room does not drop to outside levels. Solar gains during the measurement period also helped to maintain a relatively high air temperature inside the room.

Figure 2: Temperature inside and outside the test room when a window was opened by 300 mm at 8 am.

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Reheating costs are tiny

The cost to reheat the air in the room back up to 25°C after leaving the window open for 1 hour is approximately 2 cents of heating energy. This does not take into account any cooling of the wall linings and furniture, which will also cost a similar amount to reheat.

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Sustained heating needed if high moisture

If the house has experienced high moisture loads for some time, window opening alone may not be enough to get the house back to a dry state. Excess moisture gets stored in clothes, furniture and carpet and requires sustained heating to evaporate into the air. It can then be ventilated away by opening a window for 10–15 minutes.

Repeating this heating and ventilating process for 1–2 weeks is usually sufficient to bring a house or room back to acceptable moisture levels. Moisture levels can then be maintained by simply opening windows once in the mornings.

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Key advice from findings

Ideally, we want to eliminate airborne moisture quickly at source and return to healthy levels before the next moisture-producing event occurs.

Trickle vents maintain low moisture levels within a room but are less effective at removing large quantities of moist air quickly after showering or sleeping in a closed bedroom.

Opening a window fully for 10–15 minutes can provide sufficient ventilation after a moisture-dosing event to replace the majority of the wet air inside the room. Most of the heat stored in wall linings and furniture will still be retained in this time. While the windows are open, the door should be kept closed to prevent the damp air from being carried throughout the rest of the house.

After 10–15 minutes of open windows, the gains are not as great.

When condensation occurs, the room must first be heated to evaporate any moisture stored in linings or furniture before it can be ventilated away by opening a window. This process may need to be repeated regularly for 1–2 weeks.

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This is part of a BRANZ study answering common questions around the energy efficiency of moisture removal. Technical removal solutions will also be investigated.

Download the PDF

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

Figure 1: Absolute humidity inside and outside the test room.
Figure 2: Temperature inside and outside the test room when a window was opened by 300 mm at 8 am.