Published 22 October 2019

Understanding the numbers on your hygrometer

A Hygrometer is a handy tool that allows you to record and read temperature and humidity levels in your home. But what do the numbers actually mean when it comes to keeping a healthy home?

We asked our resident Home Performance Advisor Craig to help break it down for us.

Relative humidity is a measure of the amount of water vapour in the air.

Comfortable levels and a realistic target of relative humidity is between 40-60%. Relative humidity is considered to be high when it constantly exceeds 70% - which is actually very common in Wellington. This along with consistent temperatures of less than 16 degrees Celsius will result in mould growth.

The World Health Organization recommends that main lounge living areas are heated to 18-21 degrees while occupied, and bedrooms to be no lower than 16 degrees overnight (18-20 degrees if young or elderly are staying in the room.)

Relative humidity can be controlled in 3 ways:

1. Reducing the moisture at the source. Use kitchen and bathroom extraction fans, ducted to the outside; dry clothes outside, ensure your clothes drier is ducted to the outside, do not use un-flued gas heaters, minimize indoor planting and fish tanks. Finally, keep ground moisture away from the house. Are there any drainage issues or broken gutters that may dampen the exposed soil under your house? In many cases, a moisture barrier is recommended under the house.

2. Increasing the air temperature via heating and heat retention. Warmer air can support more moisture, reducing condensation on windows.

3. Ventilating via mechanical systems such as Smart Vent or simply by opening windows at opposite ends of the house for 1/2 hr a day. Allow a good breeze through. Outside air, although cooler, is usually drier. You’ll find that cold, dry air takes less energy to heat than warm, damp, moisture-laden air.

A bit more science about relative humidity- Relative humidity is expressed as a percentage figure. For example, relative humidity at 60% means the air contains 60 per cent of all the total water vapour the air can possibly hold at that temperature. If the amount of water vapour in the air stays unchanged, but you increase the air temperature of the room with additional heating, you will notice the relative humidity drop.