RLC Engineering, LLC.
What are soil covers?
Soil covers are sometimes referred to as vapor retarders, vapor barriers, moisture barriers or ground covers. The most common type is 6-mil polyethylene sheets; however roll roofing and EPDM rubber can sometimes be used.
Why are soil covers needed?
The combination of significantly reduced ventilation, lower foundations and cooled surfaces results in ideal conditions for mold growth and wood decay.
As much as twelve gallons of moisture vapor can be released from the soil into the crawl space of a 1000 square foot house in a single day. A soil cover reduces the amount of moisture added to the crawl space, thereby reducing the potential for mold and decay. The first and simplest line of defense is a complete soil cover in the crawl space.
Before installing a soil cover:
Prepare the crawl space soil. Soil under the house should be crowned
to prevent standing water in the crawl space. Remove wood scraps and other
debris. Smooth the soil to prevent hills and valleys.
How to install a soil cover:
2. Unroll the soil cover. Spread the soil cover over the entire surface of the crawl space. Trim around piers. Lap the soil cover up foundation walls if the outside ground level is above the crawl space soil level. Leave at least 8 inches between the soil cover and any wood framing or flooring members to prevent hidden termite pathways.
3. Overlap joints approximately 12 inches. Anchor the soil cover in place with bricks, sand or long nails.
Condensation and liquid water are often present under a soil cover. This is normal, and indicates that the soil cover is performing its job. Do not fold back the soil cover to dry out this water.
If water puddles on top of the soil cover, determine and solve the source of the water. Correct plumbing leaks, site drainage and duct condensation problems. If necessary use a knife or nail to poke holes or small slits in the soil cover to let the water seep down through the soil cover. (These small holes will let gravity push water down through the holes, but not allow a significant amount of water vapor from moving up through the holes.)
Special considerations for older homes:
Note: Other changes to a house such as new windows, doors, and heating and cooling systems can cause moisture levels to rise. If this type of change is made to a house, complete soil coverage is warranted.
CAUTION: If signs of decay are evident
in the crawl space, complete soil coverage is necessary even if excessive
drying is noted inside the house. Once started, less moisture is necessary
to continue the decay process. Stop the decay, then deal with low indoor
humidity levels some other way.
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