The rate of mold growth is directly proportional to the relative indoor humidity. Therefore, a rise in the relative indoor humidity increases mold growth rate.

Although mold spores are virtually everywhere, including the rooms and air, a higher humidity level increases their concentration, making them unsafe. So what are safe humidity levels to keep the house at so mold doesn’t grow?

Maintain between 30 and 50 percent humidity to prevent mold from growing in your home. If indoor relative humidity (RH) exceeds 60 percent, it creates a conducive environment for mold and mildew to grow and thrive. Besides mold growth, such a high humidity level attracts pests like cockroaches.

The EPA has some great info in much more detail on the reasoning for these exact numbers. You can visit that here if you like.

In the rest of this article, I’ll discuss the impact of climate on mold growth and how to know if you have high indoor humidity. I’ll also discuss reducing indoor humidity levels to protect your home from mold. Let’s dive in!

What Is the Normal Mold Count?

As I already mentioned, mold spores are virtually everywhere. However, problems arise when these molds increase in number to become visible. So, what is the standard mold count?

The average mold spore count is between 200 and 500 per cubic meter of air (sp/m3).

Therefore, anything beyond 500sp/m3 is unsafe and exposes your family to the dangers of mold. If you notice such a high level of mold spores, it means your home has high indoor relative humidity.

A point worth noting is that even a low mold spore count in your home can harm people with some health conditions.

For instance, if you have an allergic or asthmatic person in your home, you should remediate any mold issue, even at low concentrations. Exposing people with such conditions to mold can be life-threatening.

The Impact of Climate on Mold Growth

Mold levels growing on wooden home foundation
Warmer, moister clients facilitate mold growth like this.

One of the things you may be wondering is if the climate you live in impacts mold growth at your home. To understand this better, let’s look at how molds grow.

Mold reproduces through tiny spores dispersed by wind or water.

The spores can only germinate and grow if they land on surfaces with optimum conditions.

According to Science Direct, these conditions include sufficient moisture, warmth, oxygen, and food.

Now that we know the conditions necessary for mold growth, what role does climate play?

Climate determines the temperature and concentration of moisture or humidity in the air. The higher the humidity, the greater the chances of mold spores germinating and reproducing.

Hot and humid climates have the optimum conditions for mold growth.

Therefore, if you live in a hot and humid climate, you should be extra careful about controlling your indoor humidity levels. Untreated mold growth can lead to severe health complications.

Cool and rainy places are also not spared. The presence of rain means these climates have sufficient moisture, a necessary condition for mold growth.

Hot and dry climates are less likely to be affected by outdoor mold growth. However, you can still encounter indoor mold growth in such a climate if you don’t pay attention to your indoor humidity levels.

a man standing in front of bare walls with mold on them after pulling cabinets off during our netzero home renovation project
In my first netzero home renovation, I found a lot of mold behind the kitchen cabinets after removing them. This is common due to a lot of moisture getting trapped. The humid Florida environment doesn’t help either!

It’s also worth mentioning that mold growth can increase even in dry climates due to the moisture emanating from cooking and showering.

Health Risks Associated With Mold

High mold levels at home can be catastrophic to a family’s health.

However, people react differently to mold–while some are highly sensitive to mold, others may be asymptomatic.

Molds produce irritants and allergens that cause allergic reactions in sensitive individuals.

These reactions may include sneezing, coughing, itching, wheezing, watery eyes, runny nose, skin rash, and throat and lung irritations.

According to the Centers for Disease Control and Prevention, asthmatic people may react more intensely when exposed to molds.

Their reactions may accompany upper respiratory tract symptoms, wheezing, and coughing.

Moreover, people with weak immune systems are more likely to develop health complications when exposed to mold spores.

Woman wearing green sitting outside on her computer sneezes and reaches for a tissue
In some people, mold exposure can lead to symptoms like sneezing, wheezing, and a runny nose.

Based on the above discussion, the following categories of people are most at risk for mold-related health problems:

● People with allergies

● People with underlying lung diseases

● Those with chronic respiratory diseases like chronic obstructive pulmonary disorder

You should monitor your home for mold if you or a family member falls in any of the above categories. Allowing mold to grow and spread in such a home can be fatal.

Causes of High Humidity Levels at Home

Besides climate, some things can increase your home’s humidity level, making it more prone to mold growth. Some of these are:

● Leaky pipes and faucets

● Improper ventilation in basements, attics, bathrooms, and kitchens

● High humidity due to activities such as showering and cooking

● Using humidifiers without proper maintenance

● Using air conditioners without regular servicing

● Drying laundry indoors

How to Know If You Have High Indoor Humidity

Teal blue painted corner infested with green mold
If your walls look damp, that’s one sign of many that you have mold.

Since humidity is the main culprit behind mold growth, the earlier you can identify it, the better. But how do you know if your home has high indoor humidity?

Fortunately, several signs indicate high indoor humidity. They include:

● Damp walls and ceilings

● Musty smell in some parts of the house

● Visible mold growth on furniture and walls

● Foggy condensation on windows

● Crumbling of materials like wood and stucco

● Intense allergies

● Presence of pests like cockroaches and dust mites

● Intense night sweats

What should you do when you notice these signs?

Test your home’s humidity levels to confirm they’re within the stipulated range. You’ll need a digital hygrometer for testing.

A digital hygrometer is an excellent option because it indicates temperature and humidity readings for easier comparison.

The digital hygrometer measures the moisture percentage in the air and tells you exactly how much water vapor there is in your home.

Mold growth and wood rot behind the wall in our second Net-Zero home
In my second netzero home renovation, here’s the mold we found in the wall after tearing out the bathroom fixtures. It was everywhere!

Ideally, the humidity level should remain 30 to 50 percent for maximum comfort.

Alternatively, you can use sampling techniques to determine the amount of humidity in your home. These techniques include the following.

Air sampling, which involves taking a sample of air using a spore trap and sending it to the lab for analysis.

Surface sampling or swabbing a section of the affected area and sending it to the lab for analysis.

Bulk sampling, which is sending a sample of the affected material to the lab for analysis.

Unlike the sampling techniques where you have to wait for lab results, using a hygrometer is a time-conscious approach.

It’s worth mentioning that your home’s humidity level may fluctuate as seasons change due to variations in outdoor temperatures. Thus, the table below shows the recommended humidity level based on the outdoor temperature.

Outdoor TemperatureOptimum Indoor Humidity Level
More than 50°F (10°C)Maximum 45%
25°F to 50°F (-3.89 to 10°C)Maximum 40%
0°F to 25°F (-17.78 to -3.89°C)30% to 40%
-20°F to 0°F (-28.89 to -17.78°C)20% to 30%
Below -20°F (-28.89°C)15% to 20%

Table 1: Optimum indoor humidity level based on the outdoor temperature. Source: hvac

How to Lower Your Home’s Humidity Levels

Reaching the ideal indoor humidity range is necessary to keep your family from the dangers of mold. Here are some ways to reduce indoor humidity levels.

Invest in a Dehumidifier

Dehumidifer running in living room with plant and leather couch
A dehumidifier makes a big difference in how humid your home is.

The primary role of a dehumidifier is to remove excess moisture from the air.

A dehumidifier is essential if you live in a humid climate. It also comes in handy if your home has recently experienced water leakage or flooding.

The best place to put the dehumidifier is closest to the moisture source. For a multi-level home, you can place the dehumidifier in the basements.

Use an Air Conditioner

Most people only use air conditioners to cool indoor air. However, these devices also reduce indoor air humidity.

For maximum efficiency, you should set your air conditioner to around 23.89-25.56°C (75-78°F). With time, you’ll notice a significant difference in humidity levels in your home.

Improving Ventilation

Improving airflow throughout the house is one of the most effective ways to reduce indoor humidity.

Sufficient ventilation disperses water vapor from areas of high concentration, like the kitchen, laundry room, and bathroom.

Luckily, there are many ways to improve ventilation in your home. Some of them include:

● Opening windows and doors whenever possible

● Investing in fans to help circulate air

● Using a ventilation system to regulate airflow

● Installing vented exhaust fans in the kitchen, bathrooms, and laundry room

Checking and Repairing Plumbing

Leaking faucets and pipes adds water to the air, raising humidity levels.

If you suspect your home has a plumbing issue, address it immediately. A professional plumber can identify the source of the leak and perform any necessary repairs.


A door with a white edge being fitted with black rubber weatherstripping.
Weatherstripping is an eco-friendly solution for the home that can also combat mold.

Tiny space around the doors and windows facilitates the escape of cool and warm air. Unfortunately, these spaces also encourage the entry of excess humidity into the house.

Weatherstripping is an excellent way to reduce these air leaks and keep your home at a comfortable humidity level.

You can invest in weatherstripping materials like V-strips, foam tape, felt, or door sweeps. Although the installation process is easy, you can hire a professional if you don’t have time to do it yourself.

You should weatherstrip all door and window openings to get the best results.

Moreover, you should check the seals around your attic hatches, dryer vents, and wall outlets and ensure they are intact.

Finally, you can install vapor barriers to protect the walls.

Check Your Landscaping

If water enters the house from outside when it rains, your landscaping needs to be more efficient to keep your home dry.

If so, install a water drainage system or regrade the soil around the foundation. Doing so will prevent water from rain and snow runoff from entering your house.

Additionally, trimming trees and shrubs near windows can help limit water droplets from entering the house.

You should also check your gutters and downspouts to ensure they are clear of debris and functioning properly for sufficient water drainage.

Final Thoughts

Mold spores are ubiquitous. However, they germinate and multiply in areas of high humidity, sufficient oxygen, and warmth.

When mold spores reproduce, they increase mold levels at home, making it unsafe, especially for asthmatic and allergic people.

Therefore, the best way to keep your home free of molds is by maintaining indoor humidity at 30 to 50 percent.

Some excellent ways to lower indoor humidity levels include using a dehumidifier or air conditioner or improving indoor air circulation.

If you plan to insulate your home, you may wonder whether mold can grow on the insulation. Check out this article to learn if mold can grow on insulation.

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