Vapor Barrier Calculator
A vapor barrier keeps ground moisture out of your crawl space, basement, or slab. This calculator figures out exactly how much material you need, including the overlap at seams and the run-up along perimeter walls. Enter your dimensions and roll size to get the total square footage and number of rolls.
Vapor barriers for crawl spaces should be at least 6-mil polyethylene, though 10-mil and 20-mil are increasingly standard for durability. The barrier covers the entire floor and runs up the foundation walls 6-12 inches, where it gets fastened and sealed. Seams must overlap by at least 6 inches and should be taped with vapor barrier tape.
For concrete slabs, a 10-mil or 15-mil vapor barrier goes under the slab before the pour. This prevents moisture from wicking up through the concrete, which protects flooring materials installed later. ASTM E1745 rates vapor barriers as Class A, B, or C based on puncture resistance and permeance.
Do not confuse vapor barriers with vapor retarders. A vapor barrier has a perm rating below 0.1 (like polyethylene sheeting). A vapor retarder has a perm rating between 0.1 and 1.0 (like kraft-faced insulation). Crawl spaces and slabs need a true barrier, not just a retarder.