How to Use Humidity Monitors to Find Mold

Humidity Monitor

Humidity Monitors are an excellent tool to help find concealed mold sources. Fortunately, they are also inexpensive, so there is no excuse not to monitor your humidity. Costs start at as low as $9-$11. CHECK humidity levels in each room. Having several monitors might be a good idea. Humidity monitors are readily available through online stores like Amazon or Walmart, and other similar outlets, and home improvement or big box stores. Notice differences after rain or snow events.

Here’s one similar to mine:

An affordable humidity monitor

Some more expensive versions can compare outdoor and indoor humidity levels, which can also be very helpful. We run our air conditioning sometimes just because the humidity is high outside, even though the actual temperatures might be similar. A properly sized and functioning air conditioning system should take excess humidity out of the air. If the system is not dehumidifying, then have the HVAC system checked. In this case, all the rooms may have higher than desirable humidity levels.

➡️ Find my favourite tools for moisture, mold, humidity and more RIGHT HERE

A more expensive humidity monitor

Optimal humidity levels vary based on the season and the climate

A good goal number is 40% humidity, however in warm, humid climates or during warm, humid weather, up to 55% humidity might be acceptable.  In cold climates and during cold weather, (when heating is being used), 40% should be the acceptable high number. Know that consistent and excessively high humidity levels, increase the risk for having mold anywhere, but especially in ductwork, as the potential for condensation increases with humidity levels.

How to check humidity 

Check humidity levels for each room separately and look for anomalies or differences that are odd or unusual. In my house, I found a bathroom that was consistently measuring 70% humidity in the winter with the heat running. No other room was ever that high at the time. Even when we didn’t use the bathroom/shower, the humidity number stayed about that same level. That was my first clue that there had to be a moisture source hidden somewhere to keep the humidity high. That hidden source was raising the humidity in that room alone as the accumulated moisture constantly tried to dry into the space, and added moisture to the air.

It’s a bit like having a soaking wet towel in a closet that is trying to dry. With leaks, that towel keeps getting wet again, but if it stays in the closet, it can never fully dry out. Eventually that towel will get pretty nasty because it can’t dry out fast enough. Sometimes, the leak stops (perhaps if it is from a rain event) and then the area can dry out, but dead mold spores that accumulate (like that stinky towel) remain. 

My personal experience

My leak didn’t get to the point of smelling bad, as we caught it early. I actually had noticed the bathroom door sticking (swelling from high humidity) but never could ‘feel’ the humidity difference in that room. We had no exposed mold or exposed water damage and had seen no leaks. My ERMI/HERSTMI score was 2.4 and our air test showed nothing unusual despite the fact that we would later find very toxic mold.

What we found was very small – about 2 inches of mold, but still very toxic. The tile guy, who uncovered it was sick for two days after removing it. My whole family was also sick for two weeks after we found it despite sealing off the area well and taking precautions to avoid cross-contamination. I was the only one who went in to look at the damage, with a full respirator on , but I still got sick along with everyone else. My guess is that some spores got out with me, when I went into the room and came out.


NEVER do work that might uncover mold without encapsulating and closing off the area very diligently. 

Know that just a respirator might not be enough for mold sensitive people like me. I’d recommend a Tyvek suit (pretty cheap) and goggles (also cheap) along with creating a vestibule to take them off in, as an upgrade from what we did. We did have negative air pressure and an air scrubber running and very good encapsulation, but it wasn’t enough to protect us from even that small amount of mold escaping. How we dealt with cross-contamination will have to be another post!

Cheryl Ciecko is an ALA, LEED AP, Licensed Architect who helps homeowners and building professionals create safe homes through online education to prevent mold, water damage, and poor indoor air quality. 


You can find her video masterclasses, online programs, and education subscription options at .


For detailed answers on your specific situation or building project, schedule your appointment for an individual consultation.




The information contained in this presentation is for educational purposes only and is not intended to diagnose, prescribe, treat, or cure any disease, ailment or injury to the body. It is not medical advice. FDA regulations prohibit the use of medical claims in conjunction with the sale of any product not approved by the FDA. Statements made herein have not been evaluated by the FDA. 

Any products, techniques, and/or personal usage tips referred to are not suggested as a replacement for proper treatment from a licensed health care professional. I am not a licensed health care professional and the decision to use or not to use any of this information is the sole responsibility of the listener and/or reader. Everyone is an individual with different body types, different blood types, different body chemistries, and it is important to remember that what works for one person may not work for another person.