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How to identify & resolve basement mold issues

Why basements are problematic

Basements are prone to musty odors and mold growth.  This isn’t surprising.  If you dig a giant hole in rain soaked dirt, the water will quickly fill the cavity.  We really shouldn’t be surprised when water enters our basements – instead we ought be amazed it doesn’t happen every week.  In the Pacific Northwest, a basement is essentially a concrete boat attempting to float in a muddy pond…  It can work, but we shouldn’t be startled by the challenge.

Moisture wants to move from areas of high concentration to low concentration.  If the soil surround your home is wet and your basement is not, the laws of physics are compelling the water to enter your home. 

Water and mold in basement

Ok, but what if you live on well-drained lot and your basement never floods?   Do not fear – basement failure is not so limited…  Here’s the problem – no one heats their basements to the same temperature as the rest of the home.  Basements are often poorly insulated and expensive to heat.  They’re also used much less than the rest of the home.  Most homeowners sensibly respond by closing the heat registers in the basement.

But what happens when a body of air cools down?  The relative humidity increases.  The volume of air expands and contracts dramatically with temperature changes.  Water vapor does not.  Therefore, as the air cools the ‘relative’ amount of moisture goes up.  Because the volume of moisture has not changed, the moisture now occupies a greater percentage of a given volume of air.  This is why it’s called ‘relative’ humidity.  The actual quantity of moisture hasn’t changed, it’s just become relatively greater in comparison to the air.

Below are two graphs – one showing the typical conditions inside a well ventilated home in the winter.  With the temperature at 68F, the RH is down at 35%.  This is perfect.  Mold would have a very difficult time growing in these conditions.    The second graph shows what happens to the relative humidity when you take the exact same air and simply cool it down to 55F.  The humidity hits 55%.   In climates with damp winters such as Seattle, the starting conditions will likely be significantly higher, which will exacerbate the basement problem.


Basement humidity graph 1

Nice low humidity at 68F

Basement Humidity without heat

Lower the temp down to 55F and the humidity jumps to 55%

The lesson?  Keep your basement warm.

Specific health concerns with basement mold growth

Basement mold problems are significantly more likely to affect the indoor air quality, and subsequently the health of sensitive occupants, more than other areas of the home.  There are a few reasons for this.  As noted above, basements often have less ventilation than the rest of the home.  Either the registers are closed to save money or in many cases, the hvac system simply wasn’t designed to fully heat and ventilate the basement.  Bathrooms in basements are also more likely to have an old or non-existent exhaust fan.

Basement mold can be reduced with a good filter

If the furnace isn’t routinely exchanging the basement air, the furnace filter is less able to pull the mold spores from the air.   And if bath fans are not present, humidity and pollutants will linger longer.  Also, due to the stack effect (warm air rises), contaminated air from the basement will often infiltrate the upper portions of the home.

The potential for basement mold to cause health effects also depends much on the building materials used on the flooring and walls.  For example, an unfinished basement with bare concrete walls and flooring, will provide far fewer food sources for the mold. However, a finished basement with insulation, sheet rock and carpeting provides dramatically more food sources.

Basements & carpeting

Concrete is much more porous than you would imagine.  Depending on how it cured and other factors, large quantities of water vapor can pass through the concrete slab.  In a basement with bare concrete, the water vapor is absorbed into the adjacent air before any major problems can occur.

Of course, most people don’t want to walk around on bare concrete.  Hardwood floors are expensive and risky, so many homeowners opt for carpeting.  But now the moisture has nowhere to go.  Technically, the moisture can still move through the carpet and pad, but these materials slow the movement dramatically.  If the quantity of water vapor is very small, the moisture will eventually make its way into the adjacent air.  But once the rate of moisture transport exceeds this dry out ability, mold growth will occur.

Basement with no visible mold growth or flooding

basement mold from vapor

But beneath the carpet?  Clear evidence of vapor emissions through the concrete slab

Moisture coming through basement slab      Moldy carpet in basement

A nearby carpet without a rubber backing showed no signs of moisture buildup.

Carpet without mold

Basement mold & moisture investigative techniques

Identifying the exact cause of mold and moisture issues in a basement is difficult.  Also, the cost of a mistake (your recently remodeled basement flooded with 6 inches of water) is quite expensive, it’s typically best to hire a professional.  The following are techniques we’ll use to pinpoint the source of the moisture.

  • Vapor emissions testing (measure the amount of moisture moving through the slab)
  • Air sampling for mold to identify if the air quality has been affected
  • Exterior drainage investigation
  • If present, check the interior footing drain / sump pump
  • Moisture testing of carpet, padding, tack strip and sheet rock.


Long term moisture and mold prevention

Moisture management and temperature control are the two key factors for creating a mold free basement.  More specifically, your goal is to create a warm and dry environment free of ground water moisture intrusions and excessive humidity.  Though it may seem obvious, identifying the source of the moisture is not always straightforward.  Humidity based moisture issues can look very similar to ground water intrusion (and vice versa).

Source identification

Liquid moisture includes issues such as:

  • Water from entering through the cold joint
  • Flooding from periodic high water events
  • Leaky foundation walls
  • Water infiltration through cracks in the concrete slab

Airborne moisture includes:

  • Excessive relative humidity
  • Vapor emissions through the concrete slab


And remember, it can always be worse…