Home Humidity: What Causes It, What To Do About It

For those of us in the Southern Hemisphere, we are in the midst of a very humid summer (hello rain!). You know that feeling when, although you’ve opened the windows to ‘air’ the house, everything feels damp? That is caused by indoor humidity! However Northern Hemisphere friends, this is not just a summer issue, indoor humidity can increase from a myriad of things at any time of the year, leaving our homes susceptible to condensation and thus, mould.

So what are indoor humidity level goals you may ask? According to the National Asthma Council, relative humidity of 30% to 50% is generally considered to be a comfortable and healthy room climate. Mould thrives in a 65% + humidity although, in favourable conditions, can grow at 55%.

You may be wondering what factors increase indoor air humidity? Being someone hit by mould illness myself I understand the importance of knowing what to look for when it comes to building, buying, or renting your next home. I’ve put together a list of things that increase indoor humidity, as well as 7 humidity hacks to reduce the incidence of mould.

1. You live in a humid climate

Warm air can hold more water than cool air, this means that if, like me, you live in a warmer climate when the air temperature increases, sadly its capacity to hold moisture also increases. 

Without proper ventilation, the increased water in the air is the perfect breeding ground for mould. 

Note: get to know the humidity changes through your seasons as sometimes your summer might be humid but you might have a dry winter or vice versa and by knowing these variations, you’ll know what times in the year you need to be more proactive at reducing humidity in the home. 

2. Household Activities

Everyday domestic activities can be the biggest culprits of high indoor humidity levels, especially without proper ventilation. The following activities increase indoor humidity:

Cooking: Cooking creates a lot of excess heat and moisture. Ducted range hoods are the most effective at helping reduce excess moisture in the air from cooking because it is venting the air out of the kitchen. Ductless range hoods – the ones that are just there for show, recirculate air back into the kitchen space which may increase humidity levels.

Laundry: Vented clothes dryers, without proper ventilation, is one of the biggest culprits for moisture and mould. I walked straight out of a house inspection after seeing a completely black covered ceiling in the laundry and my heart broke for the people who just don’t understand how unhealthy that makes their living space (and clothes!) but also how in time that will lead to requiring costly remediation and wall replacements. This can be entirely avoided and remedied by opening the window or door (if you have an outside laundry or venting your dryer directly outside or ducting it directly outside or at least using a dehumidifier for several hours during and after the use of the dryer. 

Alternatively, next time you are investing in a clothes dryer, consider a condenser dryer which removes the humidity in the dryer via a condenser. You can then use the water in your garden or for your indoor plants. Also, if you dry your clothes inside using a clothes horse, you could be increasing the humidity levels so you definitely need a dehumidifier on for that unless you live somewhere with a dry climate and great cross ventilation. 

Heating: Unflued gas heaters not only release toxic gases throughout the home, they can also cause condensation and moisture due to lack of ventilation. It’s always a good idea to leave a window or two open a crack if you have gas heating on. It’s also a good idea, I’m on a tangent here, to avoid gas heating if you have MTHFR gene polymorphisms. Refer to the work of Dr Ben Lynch to explore this further. 

Bathroom: Showers and baths without using an exhaust fan or having a window open can increase indoor humidity dramatically within just a couple of minutes. 

3. Aluminium Window Frames 

Thermal transmittance, also known as U-value/R-value, is the rate of transfer of heat through a structure, the higher the rating the lower its ability to insulate, aluminium frames have a high U-value/R-value. This means they transfer heat well. If it gets cool outside, indoor heat escapes through the metal frame of the window. Often the aluminium frame becomes colder so when warm air comes in contact with the cold frame, the air is cooled and moisture is released, causing droplets to condense on the window and window frame. Excess water can lead to mould growth along the bottom of the window. 

4. Single-Pane Glass

Similar to aluminium frames, condensation occurs when cold dry air hits the exterior of single-pane glass windows, at the same time that warm humid air inside your home comes in contact with the interior of the window, or when it’s hot and humid outside and air conditioning is used indoors. Double-pane glass on the other hand has a small space between the two glass panels designed to keep a consistent temperature. As with aluminium frame windows, single-pane glass can cause pools of condensation to pool along the windowsill which could lead to mould. 

5. Air Flow

Poor airflow and lack of ventilation results in high indoor humidity resulting in an increased risk of microorganisms such as mould and bacteria. Relative humidity which is greater than 50% can increase dust mite levels, causing an increase in allergies. Worst buildings for airflow include semi-detached houses, renovated homes that haven’t rectified old issues, older apartments/units.

6. Living in a bubble… the downside to energy efficiency 

Unfortunately when you build for energy efficiency, you also inadvertently create added moisture in your ‘airtight’ indoor space which some experts suggest is contributing to the huge problems we’re seeing with relatively new builds and mould growth. Nicole Bijlsma from Building Biology, says a growing number of people are experiencing the symptoms of sick building syndrome (SBS) because new buildings are limiting the amount of fresh air coming inside. 

I believe the way forward will be to marry energy-efficient, airtight constructs, with the installation of ‘lungs for the house’ as per the wonderful German PassivHaus design method, where filtered air is constantly entering the house and stale old humid air is leaving the house 24/7. You can learn more about that with the podcast we’ve done with Amelia Lee, Casey Grey and Blue Eco Homes

Sometimes waterproofing and drainage aren’t done properly. 

Check out this nifty table that nerds out on exactly how much moisture these common activities in our day to day add to our indoor air moisture: 

Typical quantities of water vapour produced in the home: 
Breathing (active)  0.2 Per Day (per person)
Cooking 3.0 Per Day
Showers and Baths  1.5 Per Day (per person)
Clothes Drying (unvented)  5.0 Per Day
Gas Heater (unflued)  1.0 Per Day
A typical home may produce in excess of 20 litres of water vapour per day

Source: https://www.agwa.com.au/documents/item/60

 7 Humidity Hacks

Now that we’ve talked about things that can increase indoor air humidity, let’s look at a few ways to decrease it.

  1. Get to know your indoor air humidity by having a hygrometer, which you can pick up at the hardware store. 
  2. Turn on your ceiling fans on warm or humid days to help circulate air. Consider installing them if you own your property or have investment properties that you want to protect your investment for and ensure air can circulate well. These are relatively inexpensive home improvements that return big on their investment. 
  3. Use your clothesline where possible or invest in a heat pump or condenser dryer like this one by Fisher Paykel. 
  4. Use a dehumidifier to keep moisture at bay. I use one every day to help air out our mattresses and whenever I’ve been somewhere with window condensation, I’ve used for the first 2-3 hours of the day to ensure the moisture doesn’t sit there and also for 2-3 hours after the family showers in the bathroom. Check out our favourites from AusClimate – we have a compact 16L and a larger 25L unit as our mix for our 2 bd apartment which can suck moisture at a speed of 2 x full in a day during the humid season or when it’s super rainy – and they give our community 10% off all year round with the code LOWTOXLIFE (valid all of 2022/2023)
  5. Or if you want a dehumidifier and air filter in one, Philips have a 2 in 1 that is quite good.
  6. Open the windows to let fresh air in regularly (as long as it’s not too humid outside). You can check on your weather app what the external humidity is vs your indoor humidity measured on your hygrometer to help you know before building your confidence in just knowing. Often wind picks up just before storms and that can be a great time to open everything up for an hour and get gusts of fresh air into your home.
  7. Turn on your range hood when cooking and if it doesn’t flue to the outside, use a dehumidifier in the kitchen, especially when making slow-cooked stocks, stews, soups and broths that generate a lot of steam.
  8. Take a cooler and shorter shower most days to reduce steam, and consider putting on the dehumidifier after the morning showers are all done, to ensure moisture is abated quickly – this will save you struggling to clean grout and silicone because the mould won’t be able to grow into it in the first place.

Has this list helped? I’d love for you to share your experience of trying to find your perfect safe home below!

Alexx x

Comments 1

  1. So I’m confused. Which dryer is best if my main goal is to keep humidity down? Is a condenser dryer my best bet or should I be looking at a heat pump dryer? Our dryer is being installed against an outer wall so we can put a vented dryer in if that’s better but for some reason that’s a no go – intuitively I would have thought that venting the hot air outside would be the most optimal solution so I’m very confused.

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