Relative Humidity and Mold

May 29, 2020

One of the more interesting projects that I’ve worked on in recent years involved a church and mold growing on the pew cushions of the sanctuary.  The client was concerned not only about the mold, but how the mold managed to get there.  Mold needs moisture in order to grow, but there wasn’t a flood in the church.  How was it able to grow?  The answer – elevated levels of relative humidity (RH)!

What effect can relative humidity have on mold growth?

When relative humidity is above roughly 60%, there is a risk of mold growth– with risk really increasing when the RH exceeds 80%. Elevated relative humidity can also lead directly to condensation.  When humid air is in contact with a cold surface, it can create moisture or condensation.  Some surfaces can provide nutrients for mold growth, which is another important factor. 

In Chicagoland, humidity can be a problem just about any time of year. Oftentimes we think about humidity during the winter months because the humidity levels are too low and the air feels dry.  To improve comfort, some people will use a humidifier to add humidity (sometimes too much!) to the air during the winter.  But humidity is an area of concern during the summer months as well.  

Relative Humidity and Mold on cold surfaces

Again, when high levels of relative humidity hit a cold surface (that is below the dew point temperature) this creates condensation which in turn helps facilitate mold growth.  I’ve seen this happen when some clients had been away for weeks at a time only to discover mold throughout their basement drywall when they returned.  Someone accidentally left the air conditioning running constantly so that the walls were cold.  This occurred during the summer when the levels of relative humidity were high.  This combination created condensation, which in turn led to mold growth.  

The Test

After my initial inspection of the church sanctuary, I installed a HOBO temperature and relative humidity data logger to get an idea of what the levels were during roughly a week and a half of church activities.  The results can be viewed on the graph above.  The top blue line shows the relative humidity, which is measured as a percent.  The bottom black line shows the temperature.

Temperature and humidity

The graph above shows an interesting pattern during the data logging.  During the times that church services were occurring, the relative humidity levels were quite high.  Conversely, during these same periods the temperatures in the sanctuary were at their coldest.  In interviewing the client, we found that the air conditioning would be on to cool the space during sanctuary use.  We further discovered that the church doors would be opened more often which could allow humid air to enter into the building.  Ironically, when the room wasn’t in use, the temperature would be high and humidity would be at its lowest point. Holding all else constant, a warmer room has a lower relative humidity.

An added wrinkle   

In doing our assessment of the church, we discovered another odd fact.  The mold growth on the seat cushions appeared to be primarily on the pews located on the far sides of the room.  Why wouldn’t this issue occur in the center seats?  I had the chance to talk to the janitorial staff and their answer was revealing.  Typically, young families would sit in these areas.  After the service, they would leave behind the remains of snacks and juice bottles.  In addition to the condensation, the food remains could provide additional nutrients to aid in mold growth in the seat cushions.  


Whether it be for individual homes or commercial spaces, I believe that there is a huge benefit in measuring both the temperature and relative humidity.  Relative humidity should never exceed 60% for an indoor space.  During these church services, we observed that the humidity levels would spike to over 75%!   

In the end, we advised whenever possible to use dehumidifiers to reduce the humidity to acceptable levels.  We further advised the church to clean areas where young families would gather to help prevent mold growth.   

A trained professional can aid you or your company in helping determine if your property has a mold or moisture problem.  But the best advice that I could give to anyone is to invest in relative humidity monitors.  This can help ensure that the indoor environment is both comfortable and healthy.