Crawlspace Moisture Threats – How to Identify Them

They’re known for spiders and crickets and snakes, for rodents and marsupials and fungi.  But under-house crawlspaces are my bread and butter.  As a home inspector, I know that some crawlspaces are just that, you have to crawl through them.  I love to inspect a tall crawlspace, but more than a few are squeeze spaces, wriggle spaces, or bump-head scrape-back spaces. 

The worst, though, are wet crawlspaces.  What causes a wet crawlspace?  Moisture can come from three sources:  Condensation from the air; Infiltration through the foundation walls;  Leaks from plumbing pipes.  Rarely moisture comes from sources under the house like a spring or seep, or from roof leaks. 

What’s so bad about moisture in crawlspaces?  Actually, a lot.  High crawlspace moisture allows destruction of wood by fungi, insects, and termites.  If the wood stays dry the wood destroying organisms have no interest in it.  Have you ever visited the cliff dwellings in the Southwest?  There are wood roof beams and ladders there that have survived almost a thousand years in the very dry climate. 

Even the so-called “dry rot” requires high moisture levels in the wood to be active.  Sometimes it can be completely invisible.  I once was inspecting a crawlspace where I noticed some plumbing leaks that had allowed a steady stream of water to flow across the ground.  All of the joists looked fine and I had no question about their integrity.  But then I reached up to steady myself, grabbed a joist, and it crumbled in my hand. 

Wood lumber bought at the lumber yard is kiln dried to below 19 percent moisture, and if the wood moisture stays at that level or lower no decay will occur.   It’s fortunate that most agents of wood decay, be they termites or fungus, are relatively slow moving.   It’s also fortunate that if the wood begins to deteriorate because the moisture level goes up, it will usually stop deteriorating once the moisture level goes back down below 19 percent.

Moisture in crawlspaces can lead to disaster, but most of the time it’s relatively cheap to keep moisture out. 

Let’s look at the three sources of moisture and how to prevent each one.

First is condensation. I live in North Carolina, where we have very humid summers.  We use air conditioning to cool the air, but air conditioning saves houses because it also removes humidity – moisture – from the air inside the house.  Often the ducts that carry the conditioned air through the house are located in the crawlspace.  The ducts, and in fact the whole crawlspace, is cooler than the outside air.  Now what happens when you expose a cool surface, like, say, a glass of iced tea or an air conditioner duct, to warm humid outside air?

Condensation!  Moisture from the humid air condenses on any surface that’s below the dew point.  In crawlspaces that’s every surface – floor joists, insulation, air conditioner ducts, the plastic vapor barrier on the floor.  In July and August I often emerge from a crawlspace with soaked coveralls.  All the wood in the crawlspace just soaks up that moisture like a sponge.  Condensation can also occur in the winter, when the crawlspace is warmer than the outside, and the condensation forms on the band joists, the outermost part of the floor framing.

What can you do about condensation?  First, keep the foundation vents closed in the summer if you live in the humid south.  Go under the house during a hot humid spell and feel the joists and ducts and insulation.  If they’re wet, you may need to install a dehumidifier in the crawlspace with a pipe to drain the water to the outside. 

The second moisture source is infiltration through the foundation walls.

Gutters are your first line of defense – often all you must do to control foundation wall moisture is just keep the gutters clean and functional.   Make certain that water from the downspouts is directed away from the house. Second is grading – the slopes around the house perimeter that allow water to run off, away from the crawlspace. If the soil touching the foundation wall becomes saturated, the moisture quickly seeps through the foundation wall.

Many foundation walls are “waterproofed”, and waterproofing certainly helps, but the term is misleading.  Many events can compromise foundation waterproofing – it’s best to just keep the water away from the foundation. On the best foundations there are underground drainage systems installed, so that water that does enter the ground near the house is drained away before it can reach the foundation.  These can be added after the house is built, but it is not an easy job.

Moisture that comes through the foundation walls finds its way into the soil under the crawlspace.  Sometimes water sources under the house can contribute to moisture here as well.   And that’s why all new houses are required to have a plastic vapor barrier covering the soil.  I highly recommend taking this inexpensive step to lower moisture levels in the crawlspace.  You may say, “But the soil in my crawlspace is dry!”  If you put down the vapor barrier you’re likely to see that it is dry because all the moisture is able to evaporate – into the air of the crawlspace. 

My own house began as a three-room cottage over a hundred years ago, two thirteen foot square rooms with a hallway between them.  Over the years there were several additions, and since the house is on a slope, earth had to be removed to keep the new floors on the same level as the original floors.  The unfortunate part of this story – the hillside is solid rock.  So the builder removed only as much rock as absolutely necessary to place the floor joists.  They did almost nothing to drain the foundation and the joists are on the ground. 

One side of the house had a roof that drained so that the water just ran under the floor.   It had termites to the top of the roof framing.  We had to completely remove that part of the house and rebuild.  But most of the house survived its poor design.  We dug trenches around the house with a jack hammer into the rock and made sure every side of the house has good drainage.  For eight years now we’ve had no crawlspace moisture problems. 

The third source of moisture is plumbing leaks.  

I recommend that at least once a year you check your toilets for movement.  No, I don’t mean that kind of movement.  Straddle the toilet and push left to right and back to front with your legs.  If there is any movement of the toilet it is poorly attached to the plumbing drain and is likely to be leaking.  Step firmly on the floor in front of the toilet and beside the bathtub or shower – are the floors soft in these areas?  You want to find bathroom leaks before someone falls through the rotted floor.

In the crawlspace you can temporarily pull away the floor insulation to look at the plumbing pipes where they penetrate the floor and determine if there are leaks that are saturating the sub-floor.  If you find loose toilets or plumbing leaks, have them repaired by a licensed plumbing contractor.   If the leaks have damaged the sub floor or joists, call a licensed general contractor for repair.

So, to sum up, Moisture in crawlspaces is bad.  It causes decay of the wood structure.  The three main sources of moisture: Condensation, Foundation wall transfer, and leaks.  Stopping the moisture is much cheaper and easier than repairing the structure.