Posted by: SherlockHomesSY | 10 Dec 2010

Common Complaints: Damp Basement

Wet Basement Clues

(c) Carson-Dunlap, Illustrated Home

Damp Basement

Typical causes
– Drainage Through Foundation Walls
– Evaporation through Foundation Walls
– Evaporation Through Slab
– Plumbing Leaks
– Malfunctioning Air Conditioner Drains

Damp basement odor is all too familiar.  My clients often try to explain their concerns about basements as “that damp basement smell”.

Damp basements are almost always caused by water or water vapor coming through foundation walls or the slab.  Sometimes it’s the result of plumbing leaks or malfunctioning air conditioning systems.

About 50-60% of all basements leak.  However, 95% of all basements use dampproofing, instead of waterproofing for the foundation walls.  Are builders living in denial here?  Perhaps so are, but the error usually isn’t with the foundation wall itself; rather with the way storm water runoff and ground water around the foundation is handled.  Most foundation moisture issues can and perhaps should be controlled by better water management.

My personal preference is to do both, but we’re talking about existing houses here.  Digging up all the dirt around a foundation wall to waterproof it and install footing drains may not be feasible or cost effective.

Hydrostatic Pressure. The amount of water in the soil close to the house applies the additional weight of water against the foundation wall.  Add freezing temperature and this wall my fracture or collapse.  Oops!  Besides controlling the damp basement smell, this the major concern.  Any solutions must not only keep water out of the basement, but it must also limit hydrostatic pressure.

Drainage Through Foundation Walls. Water flows from areas of high concentration to areas of low concentration.  If there’s water built up against the foundation wall, it will easily flow through any crack in it.

Sometimes the moisture is produced by groundwater.  Other times it’s caused by letting surface water runoff flow toward the house.

Redirecting surface water is relatively easier.  A favorite “fix” is to use gutters.  It doesn’t solve the problem, but it does redirect water—until the gutters are overwhelmed, either because they’re clogged or the downpour is heavy.  Gutters only work if the problem is caused only by the water runoff from the roof.  The solution is better landscaping with designed drainage.

Foundation cracks?  Expect them to happen!  They usually occur while the concrete is curing—most of the time within the first 30 days.  Over 3 years, the house dries, so the house moves a lot, creating stress at foundation weak points.  Other natural forces can crack walls too.  Most of these cracks do not significantly affect the structural integrity of the house, but now the holes are there.

BTW: Did you know the design life of most foundation dampproofing is 25 years?

When groundwater is an issue, It’s time to call in a foundation specialist.

Evaporation Through Foundation Walls. Water flow through walls is relatively rare compared to evaporation through them.  First, water vapor easily travels trough concrete walls because they’re porous.  Water vapor also likes to flow from wet to dry.  Basements should be drier than outdoor soils, so the direction of travel is into the basement.

First, control the amount of water collecting outside the foundation wall.  Then consider a number of strategies to block vapor flow.  Solutions will work better and last longer with less moisture at the outside wall.

Evaporation Through Slab Floors. What was said about evaporation through walls applies to floors.  Water vapor treats them as equal opportunities.

Slabs are supposed to have a vapor barrier under the concrete.  Ideally, they should be installed on a bed of gravel over the soil.

Evaporation may slow over time, but will never stop.  Soil wicks up water, some 16 feet.  The soil under the foundation needs to remain moist to avoid slab cracks too.

Somehow, a vapor barrier needs to be installed with a durable walking surface on top of it.  Using a dehumidifier will work to keep the basement dry if the problem is small, but it doesn’t address the problem.

Plumbing Leaks. Most people easily recognize plumbing leaks—especially if they’re sudden or interrupt service.  But what if it’s a slow leak?  Then, what if it’s in a hidden area?  Then, what happens if the basement isn’t visited much?  This is when plumbing leaks get to be the “damp basement” problem.

Malfunctioning Air Conditioner Drains. An interesting problem haunts those who live humid areas in the summertime.  In the Southeast, it’s not uncommon for central air conditioning systems to wring a cup of water per hour out of the air.

This condensate is supposed to drain outside, either by gravity or a pump.  If the air handler (inside unit) is in the basement, a condensate pump is frequently used to avoid running drain lines across the floor.

However, if the drain clogs or pump fails, the condensate is supposed to fall into a pan beneath the air handler.  Guess what?  The building code has this loop hole that HVAC contractors drive semi-tractor trailer size trucks through.  The code requires a pan if water damage is not expected when the condensate hits  the floor. Builders interpret this to mean structural damage. There is no consideration for personal items at all.  I guess they don’t expect us to ever live in or store things in our basements.

First, avoid the problem in the first place.  There are floor water sensors (water alarms) that detect the presence of condensate water on the floor that will shut the unit off until the problem is addressed.  When a new air handler is put in, insist on a drain pan with a float switch in it to shut the unit off.

Otherwise, make daily trips to the basement air handler.  This will limit the amount of water on the floor to 24 cups (1.5 gallons).  People have gone on vacation, only to find their basement flooded on return.


If the basement wasn’t damp before or the cause is uncertain, and the problem isn’t obvious, it’s time for a moisture investigation.  First, find the source of moisture, and then fix it while removing any mold.  Moisture investigations and mold testing are described in detail on my mold inspection and mold testing website.

Our blog posts here about drainage have a lot to say.

In Conclusion

Everything said here about a basement applies to crawlspaces, as they are mini-basements.  Crawlspaces normally have additional features to consider, so we’ll talk about damp crawlspaces next!


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