Is Your Home Too Humid? The Numbers That Can Increase the Risk of Mold Growth

You do not need a burst pipe or flooded basement to have a moisture problem.

Sometimes the problem is already floating in the air.

Indoor humidity can contribute to damp conditions, condensation and mold growth—especially in areas such as crawlspaces, basements, bathrooms and around HVAC systems.

So what humidity level should homeowners actually be watching?

The Number to Remember: 60%

According to the U.S. Environmental Protection Agency, indoor relative humidity should be kept below 60%, and ideally between 30% and 50% when possible.

That means a humidity reading consistently around or above 60% should get your attention.

It does not automatically mean you have mold. It does mean conditions may be more favorable for excessive dampness and biological growth if the moisture is not controlled.

Mold by the Numbers

30–50%The EPA's ideal indoor relative humidity range when possible.

Below 60%The EPA recommends keeping indoor relative humidity below this level to help control mold growth.

24–48 HoursWet or damp materials should generally be dried within this period to help prevent mold growth.

These numbers all point back to the same issue: moisture management.

Why Does Humidity Matter So Much?

Mold spores are already part of our environment.

The CDC explains that mold exists indoors and outdoors and can enter homes through doors, windows, vents and HVAC systems, as well as on clothing, shoes, bags and pets.

Trying to eliminate every mold spore is not realistic.

Instead, homeowners should focus on preventing those spores from finding the moisture they need to grow.

High humidity can provide that moisture.

Where Does All That Indoor Humidity Come From?

Some sources are obvious. Others are surprisingly ordinary.

The EPA notes that cooking and showering can add moisture to indoor air. Humidifiers, dryers and other moisture-generating appliances can also contribute to increased humidity.

Other potential contributors around a home can include:

• Damp crawlspaces

• Poor drainage around the foundation

• Plumbing leaks

• Roof leaks

• HVAC condensation or drainage problems

• Bathrooms without adequate ventilation

• Dryer venting problems

• Basement moisture

• Outdoor humid air entering the structure

In Western North Carolina, homes with crawlspaces deserve particular attention because moisture underneath the home can affect wood framing, insulation and other building materials.

Your Crawlspace May Be Telling You Something

A crawlspace does not have to contain standing water to have a moisture problem.

Watch for signs such as:

Drooping or damp insulationMoisture can affect insulation and the materials surrounding it.

CondensationWater droplets on ductwork, pipes or other surfaces indicate moisture is condensing from the air.

Musty odorsPersistent earthy or musty smells may warrant further investigation.

Visible growth on joists or subflooringWood provides an organic surface on which mold can grow when sufficient moisture is present.

Damaged or missing vapor barrierExposed soil can be one contributor to crawlspace moisture.

What About Condensation?

Condensation is another important warning sign.

When humid air contacts a sufficiently cold surface, water vapor can turn into liquid water. The EPA specifically recommends acting quickly when condensation or moisture is collecting on windows, walls or pipes.

That little bit of water may seem harmless.

But if a surface repeatedly becomes damp and never has an opportunity to dry completely, you may be creating conditions favorable to mold growth.

Should You Buy a Humidity Meter?

Monitoring relative humidity can be a simple way to learn more about the conditions inside your home.

Humidity meters, often called hygrometers, allow you to monitor relative humidity in different rooms or areas.

Pay attention to patterns rather than a single reading.

Does humidity stay elevated?

Does it rise significantly after rain?

Is the crawlspace considerably more humid than the living area?

Do certain rooms consistently have condensation?

Those observations may help point toward a larger moisture issue that needs to be addressed.

Do Not Just Treat the Mold—Find the Moisture

This is one of the most important lessons for homeowners.

If mold is present, removing visible growth without correcting the source of moisture may leave the conditions that allowed the problem to develop in the first place.

The CDC states that when mold is growing in a home, the mold should be cleaned up and the moisture problem corrected.

That could mean correcting drainage, repairing a leak, addressing HVAC condensation, improving ventilation or dealing with crawlspace moisture.

The solution depends on the source.

Think You Have a Moisture or Mold Problem?

Visible mold is not the only reason to investigate.

Persistent musty odors, high humidity, condensation, water staining, damp crawlspace insulation or previous water damage can all be reasons to take a closer look.

If you're concerned about mold or moisture in your home, CleanTech provides professional mold inspection and remediation services throughout Western North Carolina and surrounding areas.

CleanTechMold Inspection & Remediation(828) 437-4474cleantech911.com

And if you've recently experienced a leak or water damage, read our companion article: “How Fast Can Mold Grow After Water Damage? The 24–48 Hour Window Homeowners Should Know.”