Crawl space vapor barriers in Little Rock

A crawl space vapor barrier is the plastic sheet on the dirt floor under the house. It's the cheapest piece of crawl space moisture work and the one most often done badly: torn by the last person who crawled through, never taped, stopping a foot short of the walls, or missing entirely under older Little Rock houses. Done right, it shuts off the one moisture source that never takes a day off.

Why bare dirt matters

Soil under a house is damp even when it looks dry, and it gives off water vapor all year. The Department of Energy's Building America Solution Center says every crawl space, vented or not, should have a polyethylene ground cover that acts as a capillary break and vapor barrier, keeping ground moisture and soil gases from being drawn up into the house through gaps around the subfloor. Left uncovered, that vapor ends up as condensation on floor joists and rim joists, which is where crawl space mold usually starts.

How a vapor barrier should be installed

The DOE guide spells out the details, and they make a good checklist to hold an installer to:

DOE also suggests a vinyl runner or an extra layer of plastic along any path that gets foot traffic, such as the route to a furnace or air handler. If your HVAC equipment sits under the floor, that's the stretch that tears first.

What Arkansas code says about vented crawl spaces with a barrier

Most older Central Arkansas houses have vented crawl spaces. Section R408.2 of the Arkansas residential code (the 2021 IRC as adopted by the state) requires at least 1 square foot of vent opening for every 150 square feet of crawl space, with a vent within 3 feet of each outside corner. Covering the ground with a Class I vapor retarder, which sheet polyethylene is, changes both rules:

• Total vent area may drop to 1/1,500 of the floor area, a tenth of the usual amount. For a 1,500 sq ft crawl space that's 1 square foot of vents instead of 10.
• Vents no longer have to sit near every corner, as long as they're placed to give cross ventilation.

That tells you how much weight the code gives the ground cover in a vented crawl space. Sealing the vents altogether is a different project: Arkansas's unvented crawl space section adds a mechanical exhaust and supply air requirement, covered on the crawl space encapsulation page.

Vapor barrier or full encapsulation?

The weak spot in a Little Rock summer is the vents themselves. DOE's guide notes that in humid climates, water vapor in warm outdoor air comes in through crawl space vents and condenses on cooler framing, and that building scientists recommend insulated, unvented crawl spaces for those climates. A barrier handles the ground; it does nothing about July air with an average dew point around 71°F (Little Rock humidity by month). Put a hygrometer under the house once the plastic is down. If it still reads high through the summer, the next step is sealing the space and controlling humidity; see crawl space dehumidifiers.

What drives crawl space vapor barrier cost

We don't set prices, and quotes vary with the house. These are the things that change the number:

• Square footage, plus how far up the walls the plastic runs.
• Sheeting. Plain 6-mil poly costs less than thicker reinforced liners, which hold up better where people crawl.
• Piers and penetrations. Every pier, post and pipe has to be wrapped and sealed, so a pier-and-beam house takes longer than one with a perimeter wall and a few columns.
• Clearance. A low crawl space with ductwork hanging in the way slows everything down.
• Prep. Old torn plastic, scrap lumber and debris come out first, and standing water after storms has to be dealt with before new plastic goes down.
• Existing mold. Joists that already show growth need cleaning first; see what drives mold removal cost.

Call and we'll connect you with a local pro who does crawl space work around Little Rock and Central Arkansas.