Keep the Water out and the Relative Humidity Low

Have you ever pulled out your “once- a-year” suit jacket only to find it speckled with mildew?  Do the kids enjoying tracing in the ice that forms on the inside of the windows in winter?  Do your allergies or asthma give you more problems at home than anywhere else?  And scariest of all: Do we dare go downstairs into the basement, and if yes, should we wear Wellington boots?

Whether it’s a dirt floor, gravel, ledge, or a poured concrete slab, almost every basement in mid-coast and central Maine shares one thing: the foundations of our buildings are set upon hard packed marine clay.  In addition to our thousands of miles of beautiful Maine coastline, the last glacial period left us with this marine clay, a substrate with an incredible ability to absorb and hold moisture.

So, your house sits on a sponge (or, in some cases, a babbling brook), and since all that moisture has to go somewhere, much of it evaporates and diffuses into your home.  High levels of moisture in the house can lead to problems with corrosion, decay and rot, mold and mildew (mold growing on fabrics), and poor indoor air quality.

Most molds grow at temperatures between 40 and 100 degrees and at relative humidity levels above 70%.  Temperature is easy to understand – 40 degrees cold, 100 degrees hot – so let’s focus on relative humidity.  What is it, and why should we care?

Google relative humidity (RH) and your mind will be blown because psychometric charts, dry bulb and wet bulb readings, air moisture content in relation to temperature and vapor pressure are all complicated concepts.  Let’s keep it easy: When it comes to relative humidity, lower is better, and somewhere between 35%-60% is ideal.  As RH approaches 70%, mold sees the welcome mat at the door, and if 70% RH is your reality, mold has already moved in.

You can buy a buy a digital hygrometer to measure and track your home’s RH, but keep this in mind: We live in a coastal climate and therefore our humidity levels tend to be high.  That’s not going to change.

Our homes are leaky, so inside air is constantly being exchanged with fresh air.  Houses do need to breathe, but most homes depend on natural ventilation or air leakage. Air leakage means that in the winter, when we need it the least, our homes gets way too much ventilation and in the summer, when we need it the most, there’s almost none.  Natural ventilation has proved, for generations, that an inexpensive and ineffective strategy isn’t much of a solution.

So: High humidity and leaky houses make for high moisture loads inside.  This alone could be manageable but then there’s that dang sponge/river lurking in the basement.  Getting rid of that will make the building less saturated and give it a fighting chance against high relative humidity.  Once the major moisture sources inside are managed, tightening up the building – essentially plugging the holes that let air in and out – will alleviate mold problems and reduce the air leakage, giving you a warmer home that is easier and cheaper to heat.

But don’t tighten up anything until you’ve dealt with the moisture in the basement:

  • No more humidifiers in the winter.  Except for the very rare dry Maine house, adding moisture to the air, even in the winter, is a mold mass invite.
  • Address and repair plumbing leaks, roof leaks, rainfall runoff, and condensation.
  • Install drainage inside and/or out.
  • Understand the seasonal conditions of the basement and use a dehumidifier when necessary.
  • Reduce air leakage by tightening up the building envelope.
  • Use spot ventilation within the home where moisture is created – while cooking, showering, drying laundry etc. - and terminate these exhaust fans outside of the building (not just into the attic).
  • Install an air-tight vapor barrier – up to 12 gallons of moisture a day can be introduced to the air inside the building from the diffusion of moisture up through a dirt or porous concrete floor.

Keep the water, in all its forms, out and the inside of the home will no longer be a breeding ground for mold.  Then you can put on your jacket (no mildew), look out the window (no ice), take a deep breath (no mold spores) and enjoy the exceptional feeling of knowing you’re living in a truly safe and healthy home.


Jessie Davis is a Building Performance Institute-certified Building Analyst and an Energy Advisor for Evergreen Home Performance.  Jessie helps homeowners cut energy waste, improve comfort, and maintain healthy relative humidity with customized insulation, air sealing, and basement encapsulation projects.

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