In coastal areas of the U.S., it’s common to have houses built over closed crawlspaces. This is partly because the water table is often so high that if you build a basement under a house, it’s like having an indoor swimming pool. About 30% of all new houses built in the southern part of the U.S. are built on crawlspaces. In South Carolina, the number is more like 70%.

The trouble with crawlspace construction is that it’s more likely to have mold and wood rot problems than other types of construction. Both of these problems are types of fungal growth.

If mold is growing on the wood in the crawlspace, even in spots, you have problems. If it’s a white mold, it’s usually a surface mold, but consider it a sign that humidity levels are too high. White mold usually doesn’t cause real damage to the wood; however, if you see black or brown mold, that’s rot fungus: Structural damage is occurring that could require replacement of wood members. Immediate action is required!

If you find something that looks like roots going from the ground into the wood, that’s Poria fungus. It’s very bad, and can completely destroy a house in just a few months. Cut the roots and immediately get expert attention.

Wood rot only occurs when the moisture content is above the fiber saturation point, about 27% to 28% wood moisture content. In order to keep this in perspective, consider the following: Fresh-cut wood is typically around 200% moisture; that is, twice as much water as wood. Kiln-dried wood has seven to 10% wood moisture content. Air-dried wood has a typical moisture content of 19%, but that varies with the relative humidity (RH). With an RH of 10% and normal temperatures, you can expect wood moisture content to be around 3%. With a relative humidity of 100%, you can expect a wood moisture content of 23 to 26%.

The 28% wood moisture content necessary for rot to occur doesn’t come about from just high humidity. Water has to make contact with the wood before the wood rots. Any time the relative humidity is over 70%, you can get spot condensate because temperatures vary throughout a crawlspace.

Bugs, mold, and rot affect wood in the crawlspace. All three have the same basic requirements: food, water or humidity above 70%, temperatures between 40F and 120F, air, and, in some cases, security or protection from being disturbed.

Mold spores and a wood moisture content above just 20% are required for mold to start.

Some types of decay fungi can cause a 60% strength loss in wood in just four to six weeks.

Termites also are active only in moist wood.

The Fix

The most common fix for a humid crawlspace in dry climate areas is to add about one square foot of net free crawlspace vent area for every 150 sq.ft. of crawlspace area if no plastic is laid on the ground in the crawlspace. If plastic is laid in the crawlspace, you only need one square foot of net free crawlspace vent area for every 1,500 sq.ft. of crawlspace area.

Note that most crawlspace vents, also called brick vents, have only 60 to 75 sq.in. of free area, or about .5 sq.ft net free area each. Building codes may dictate the number of crawlspace vents installed in a house and whether they may be permanently sealed. Check with your local building department about code requirements.

In areas where there’s very high outside humidity during the summer, ventilating the crawlspace can bring more moisture into the space than it removes.

Soil evaporation also can be a significant source of crawlspace moisture. In some areas, the water table is within three feet of ground level. With a 3-ft. water table, the ground can evaporate up to 12 gal. of water/day in a 1,000 sq.ft. crawlspace area. By adding 6 mil plastic overlapped at the seams, this is reduced to less than 1 gal./day in the same area.

Determining the Fix

Here’s how to determine how much moisture is introduced into tile crawlspace of a 1,500 sq.ft. house with a three-foot-high crawlspace having a 4,500 cu.ft. volume during a 24-hour period. The average temperature in the crawlspace is 72F at 85% RH. Consider the following formula:

1) Exhaust air: Assume one air change/hr. (1 ACH).

Vcs*24/8.33*ACH/SV*AH = gal./day, or

4,500*24/8.33*-1/13.7*.0142 = -13.4 gal./day

Where:

*Vcs is crawlspace volume

*(24/8.33) is the hours in a day
divided by lbs. of water/gal.

(This converts lbs./hr. to gals./day.)

*ACH is air changes/hr.

*SV is the specific volume of the air.

*AH is the absolute humidity in lbs. of water/lb. of dry air.

The specific volume of the air and the absolute humidity can be plotted on a psychrometric chart. In order to got these values, you must measure the wet bulb and dry bulb temperatures in the crawlspace, then plot those values on the psychrometric chart. You’ll have to take several readings and plot the averages. If your psychrometric chart shows absolute humidity in grains instead of decimal fractions of a pound, divide the grains of water by 7,000. This is because there are 7,000 grains of water/lb.

2) Makeup air: Assume outdoor air at 94F at 45% RH.

4,500*24/8.33*1/14.3*.0155 = 14.l gals./day

3) Soil evaporation: Assume 12 gals/1,000 sq.ft.

12*1.5 = 18 gals./day

So:

1) = -13.4 gallons per day

2) = + 14. 1 gallons per day

3) = + 18.0 gallons per day

Total change in crawlspace moisture is + 18.7 gals./day

As you can see, ventilation hasn’t reduced moisture levels in the crawlspace; it’s actually made it wetter. If fans are added, more air is moved into the crawlspace, and the added moisture increases the humidity.

If 6 mil. plastic is laid down in the crawlspace, ground evaporation is reduced to about .5 gals. or less/day/
1,000 sq.ft. of crawlspace. This assumes that the plastic has 100% coverage, with six to 12 inches of overlap at the seams. It’s also helpful, in very humid areas, to turn the plastic up 12 to 18 inches at the foundation walls. This will reduce the moisture infiltration from outside air through the foundation wall. Be careful to protect the plastic from damage.

Pest control personnel, the cable TV installer, and the plumber can have reasons to be in the crawlspace. If they disturb the seams or tear the plastic, soil evaporation can rise again.

If the same process is used to determine how much water is added to a crawlspace with plastic ground cover, consider:

The amount exhausted stays the same: 1) = -13.4 gals./day

The amount from makeup air stays the same: 2) = + 14. 1 gals./day

The amount from soil evaporation is reduced: 3) = + 0.75 gals./day

Total change in crawlspace moisture= + 1.45 gals./day

By adding a vapor retarder to the ground in the crawlspace, the moisture added to the crawlspace is reduced by 13.5 gals. in our sample house. The 1.45 gals. of water remaining is equivalent to 5. 8 quarts or 11.6 pints of water per day, which can be removed by mechanical means.

A Mechanical Solution

Here’s a better way to reduce damage to the wood in a damp crawlspace occurring in a high-humidity area.

Dig a hole in the lowest spot in your crawlspace and put a perforated 5 gal. bucket ( or sump) in it. Surround the bucket with gravel or septic drain field cloth.

Dig trenches in the crawlspace ground and lay slotted drainage pipes in them, pitched down to the sump pit. Cover the pipes with the soil you took out to make the trenches. Put a sump pump into the bucket and pipe the discharge line well away from the house. Do this step only if the ground in your crawlspace is damp or if there’s condensate forming on the ducts or floor joists.