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Condensation/ Dew point calculator

Image by Randy Laybourne

It is that time of the year again in NZ—having condensation on our windows. Here we will give you a quick explanation of the different terms of moisture, relative humidity, condensation and dew point. We follow this by providing quick examples of what can happen when we get it wrong. Then, we will offer a calculator for you to calculate your dew point. Lastly, we will give some tips on how to ensure you and your building are healthy.



Moisture or water vapour is the presence of water in the air. There is, depending on the weather, a varying amount of water in the air. In our homes, we increase this amount by breathing, cooking or having a shower.


Relative humidity

Relative humidity describes the amount of water relative to what a particle of air can hold. Imagine, for example, a cup as a particle of air. If we fill the cup half full of water, then we have 50% relative humidity (RH). The healthiest indoor air has a relative humidity between 40% and 60%. In other words, the cup is between 40% and 60% full of water.



When the air gets colder, it shrinks in size. Our cup (with 50% RH) could shrink to half its size. But the water content stayed the same and is now 100%, so the water overflows as the cup cannot hold the liquid anymore. The reason that condensation usually happens most visibly on our windows is that the windows are the coldest spots in the house—the point where our air cannot contain the water anymore as it shrank.


Dew point

The dew point describes the temperature at which the cup cannot hold the water anymore. Once your surface has cooled the air to that temperature, the air will release the water as condensation; on your window or window frame most often. The dew point is dependent on the temperature of the air and the relative humidity at that temperature. The end goal is to have our surfaces indoors and in the walls above the dew point, to avoid condensation.


Image by Fleur

What happens when we have condensation?


Condensation can happen not just in your windows but also as the air passes through your walls to the outside. Condensation and high relative humidity have the following drawbacks:

  • It could rot our window sill if it has not been taking care of properly.

  • It is time-consuming to wipe off condensation from windows day after day.

  • A building with high relative humidity can be up to 30% more expensive to heat.

  • Toxic mould can form in damp building materials, potentially causing asthma and other respiratory diseases.

  • condensation formed in our building frame (inside the wall) could get trapped and rot our timber frame or cause mould in the insulation.

  • Most insulation materials cannot deal with condensation or moisture and will loose drastically on its efficiency, sometimes over 30%.​

How can I avoid condensation on my windows and building structure?

There are ways in which we can alleviate condensation in our homes. 

In essence, we have to lower our dew point, the temperature at which the water in our cup starts to overflow. We can do this by reducing the relative humidity or by warming-up our building itself, though often both are required in cold damp homes. Research by BRANZ has clearly pointed out that our homes are not cold because they are damp, they are damp because they are cold. By heating up the test spaces to a minimum of 18 degrees Celsius, in less than 1% of the time high relative humidity was an issue in that study. Meaning that proper heating of spaces eliminated 99% of the humidity issues in homes (1).


Different climates, materials and living stiles require different products for best results. Get in touch for a free consultation- we help you find what works best for you according to science, not a marketing message.

1. Reduce the relative humidity

Reducing the relative humidity in our indoor air to a healthy level (between 40% and 60%) is an important step. It reduces our heating bills, keeps us and the building healthy and decreases the dew point. We can lower the relative humidity in several ways:

  • Use more natural building materials and finishes that buffer moisture: they take it in during times of high humidity and release the moisture again during the daytime, as the relative humidity in the room lowers. Examples of such products are sheep wool or wood fiber insulation, sheep wool rugs, solid wood furniture or clay based plaster. Different studies have found that homes with lots of moisture buffering materials can save you up to 25-30% of your energy costs.

  • Get more plants that minimize moisture in the air. Using plants like the Boston Fern, English Ivy, or the Peace Lily will help lower the amount of moisture in your air.

  • Get a ventilation system. A ventilation system will replace the stale air, which has high relative humidity, with fresh air and reduce relative humidity. Please ensure a ventilation system with a heat exchanger to be selected for this. While it is very common in NZ to have a ventilation system that draws air out of the roof cavity, those systems do not comply with regulations for the NZ ventilation standard for indoor air quality.

Let us be clear, to put 1 plant in a bedroom will not do the job, further it does not replace the need to either regularly open your windows or use a ventilation system. Using buffering materials just lessens the need for ventilation, which is particularly handy overnight if you do not have a ventilation system.


2. Warm-up your building

A different way and often used in conjunction with lowering the relative humidity is to warm up your building surfaces. In double glazing, for example, the cold temperature is blocked at the outer glass, keeping the inner glass pane warmer and often above the dew point (though there are cases where the condensation still happens due to very high RH or on the aluminium window frame). Another way to decrease condensation on windows are exterior shutters- shutting out radiant cooling from the sky and keeping the temperature of the windows higher.


Warming up our building envelope is a way to reduce or eliminate condensation in our walls. It is important though that the water vapour has a chance to escape the building and will not get stuck and absorbed within the structure to cause structural and health issues. Many natural building materials like magnesium oxide wall boards, natural paints and insulation will do this. 


Insulating our walls will keep the parts inside the wall warmer; this is especially important with steel framing as steel gets much colder than, for example, timber. You should also consider to use more efficient heating sources that heat surfaces, drying those out and reflecting the heat into the air. They will often save you on running costs and also create higher comfort in our homes. See here for more info.


Image by Gaelle Marcel


It is essential to keep the relative humidity in our homes at 40%-60%. This will keep us healthy, avoid unwanted things like higher heating bills, potential structural damage and health consequences like asthma.

When a particle of air cannot hold the water it contains, condensation occurs. This point is called the dew point. To lower the dew point in the buildings, it is essential to reduce the relative humidity to a healthy level and warm up our building. To achieve this, we should try to use natural building materials and finishes whenever possible and consider adding ventilation system, double glazing, better radiant heating and better insulation to our home.   


(1) BRANZ Study Report SR389; 'Could damp homes be too cold/ underheated?'; Author: Andrew Pollard, 2018

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