How do I keep a crawl space vapor barrier in good shape?

Keep a crawl space vapor barrier in good shape by looking at it on a schedule, patching it as soon as it tears, keeping traffic and storage off it, and dealing with any water that collects on top. A liner rarely fails all at once. It fails a little at a time: a torn corner where the plumber knelt, a seam that lifted, an edge that slid down the wall.

It is worth the attention. The National Park Service notes that heavy plastic sheeting laid over damp crawl spaces, can have remarkable success in stopping vapor diffusion from damp ground into buildings,^[3] and the Department of Energy's crawl space guide warns the reverse: Improper installation of the vapor barrier will allow ground moisture to infiltrate the crawlspace and raise the relative humidity.^[1]

Inspect on a schedule

Twice a year, in spring and fall, is a sensible rhythm, along with any time after a storm that put water in the yard. The Park Service's maintenance list for crawl spaces is short: Check crawl space for animal infestation, termites, ponding moisture, or high moisture content.^[3] Add the liner itself to that list.

Look again after anyone has worked under the house. The DOE guide makes this the last step of any crawl space job: Any tears or rips in the vapor barrier that might have occurred during other repairs must be sealed^[1] before the work is done. Ask each tradesperson to check before they leave, and look yourself.

What to look for

• Seams. How they should look depends on the system. For a simple ground cover in a vented crawl space, the Virginia Energy Savers Handbook says: "Lap the seams 12 to 18 inches, but don’t bother to seal them."^[4] For a sealed crawl space, the DOE's method is "Lap the vapor barrier a minimum of 12 in. at sheet joints, and tape seal."^[1] In a sealed system, an open seam is a leak.
• Edges. In a sealed system the liner runs up the walls and up interior columns from a minimum of 4 in. to a maximum of 12 in. above the crawlspace floor.^[1] Look for edges that have slid down or pulled away. Closed crawl space guidance explains why they fail: "Mechanically fasten the liner material to the perimeter wall in addition to sealing it to the wall. Adhesives, sealants or tapes alone may be insufficient to support the material."^[2]
• Tears and punctures. Most are along the path from the access door to the furnace, water heater, sump or plumbing cleanouts.
• Water on top. Puddles on the plastic come from above or from outside, not from the soil. See why a crawl space gets wet to find the source.
• Around the sump and drains. The liner should be sealed to the pit, and the DOE says to make sure crock pits have well-sealed lids that fit tightly.^[1]

Patch it the right way

Small tears are a homeowner job. Clean and dry the area first so tape will stick. For seams and patches, the closed crawl space reference recommends: "Use duct mastic with embedded fiberglass mesh tape to seal seams in crawl space vapor retarder materials."^[2] If you use a tape or sealant instead, ensure that the product is accepted for use by the manufacturer of the material and will provide long-term performance.^[2] For a larger hole, cut a patch of the same liner material that overlaps well past the damage on every side and seal its edges.

If an installer put in your system, ask whether they left one of the items quality installers often include: A repair kit for fixing small amounts of damage to the vapor retarder after the house is occupied.^[2]

Protect it from traffic and storage

The thinnest liners are the easiest to damage. The guidance is plain: "Protect 6-mil un-reinforced polyethylene ground vapor retarders to avoid damage in areas that will be accessed for servicing of mechanical equipment. Thicker, reinforced polyethylene materials are more durable."^[2] For a sealed crawl space, the DOE specifies a minimum 10-mil or cross-laminated puncture-resistant 6-mil poly vapor barrier.^[1]

Protect the paths with indoor/outdoor carpet runners, carpet protection mats^[1] from the door to each piece of equipment. Keep the crawl space empty of stored boxes, lumber and sharp debris, which puncture the plastic and hold moisture against it. A sign at the door helps too: A posted sign informing anyone entering the crawl space of the system components maintaining the integrity of the system^[2] reminds every technician what not to disturb.

Keep pest control in mind

Pest inspectors need to see the top of the foundation wall. In a sealed crawl space the installer should leave a minimum 3” termite inspection gap between the top of the wall vapor retarder and the top of the masonry wall.^[2] Do not cover that strip when patching. If a pest control company treats the crawl space, ask it to reseal anything it lifts or cuts.

Keep the alarms working

A water alarm at the low spot is cheap insurance. These alarms alert the homeowners of potential pipe leaks or other unwanted water on the crawlspace floor that otherwise may go unrecognized for long periods of time.^[1] Change the batteries in the alarm and any humidity monitor at each inspection.

Why the details matter

Every gap lets the soil back in. Moisture in damp soil, such as in a crawl space, can evaporate into the air, raise the relative humidity in that space^[3] and the rooms above. Plastic is used because membranes are less expensive and easier to install^[5] than a concrete slab, but it is also easier to damage, which is why the upkeep matters. If the liner does get wet from a leak, dry it out quickly; the EPA says that if wet materials are dried 24-48 hours after a leak or spill happens, in most cases mold will not grow.^[6]

Stop and call a pro if

• Water pools on or under the liner after every rain.
• Large sections are torn, brittle or missing, or the liner needs replacing wall to wall.
• The edges keep pulling off the walls even after resealing.
• You find mold on the framing, soft wood or signs of termites or carpenter ants.
• A sealed crawl space's humidity is climbing even though the liner looks intact, which points to the dehumidifier or the air supply.

For the bigger picture, see how to keep a crawl space dry and how to lower crawl space humidity. More is under pest control answers, and you can browse crawlspace and moisture services or pest control businesses near Blacksburg.

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