Plumber cut the slab and the vapor barrier under it. Does a basement patch need one?
A Missouri homeowner finishing the basement of a 2018 house for vinyl plank says the plumber cut the slab for a wet bar, tore the vapor retarder underneath, never restored it and left a lumpy patch, while electricians slit the plastic on the walls. The code, membrane makers and flooring guides answer most of it.
The patch needs one. The International Residential Code calls for a vapor retarder between a slab and its base course, with joints lapped at least 6 inches. The 2021 edition raised the minimum from 6 mil to 10 mil and requires material that meets ASTM E1745 Class A, Stego Industries notes; which edition applies depends on the local jurisdiction. Flooring makers count on that layer too. Shaw's installation guide for vinyl plank and tile says slabs on or below grade must have an effective vapor retarder directly beneath them.
Staples are not a repair. W. R. Meadows, whose instructions follow the ASTM E1643 practice, tells crews to cover damage with a piece of membrane that overlaps it by at least 6 inches in every direction, wipe the bonding area free of dust, dirt and moisture, and tape every edge. Around a pipe it calls for a collar of membrane at least 12 inches wide and one and a half times the pipe's circumference, slit into fingers and taped to both the pipe and the sheet below. For a trench cut through an old slab, that means new retarder in the trench, taped to the old film, before the concrete goes back in.
The lumpy patch is a flooring problem. Shaw wants concrete flat within 1/8 inch over 6 feet or 3/16 inch over 10 feet, repaired with cement-based patching or leveling compound that meets its moisture and pH limits, and tested for internal relative humidity under ASTM F2170 before the floor goes down. Shaw warns that a test only describes the slab on the day it was run.
The wall plastic raises a separate question. If those walls are framed and filled with fiberglass or another fibrous insulation behind the sheet, the Energy Department's Building America Solution Center says polyethylene should not be there at all. It traps moisture coming through the concrete, which feeds mold and rot. Energy Star new homes prohibit it. The center's retrofit guidance puts foam board or closed-cell spray foam against the concrete and no plastic behind the drywall.
Why it matters: Trades cut basement slabs on almost every finish job. Write the vapor retarder repair, a patch lapped and taped 6 inches past the damage, into the plumbing scope, and get the humidity test that flooring makers such as Shaw require before LVP goes down.
Sources
- PERMINATOR TAPE: Vapor Barrier Tape (data sheet 723-A and application instructions)W. R. Meadows, Jul 23, 2014
- No Vapor Retarders on Interior Side of Air-Permeable Foundation InsulationBuilding America Solution Center (U.S. Department of Energy), Jun 30, 2017
- Installation Guidelines for Luxury Vinyl Plank and TileShaw Floors, Dec 3, 2019
- Your Guide to Navigating the IRC Building Code Changes to Vapor RetardersStego Industries, Apr 24, 2024