Vapor Barriers & Moisture Control

Check your flooring's installation guide, since many floating floors over concrete call for one. Roberts Moisture Barricade is a 6 mil polyethylene Class 1 vapor barrier for vinyl, laminate, SPC and engineered wood over concrete subfloors, above, on or below grade, with an adhesive strip to join the seams; Roberts says it is recommended by the NWFA. For laminate without an attached pad, the Biyork EVA underlayment (3 mm) has a built-in vapor barrier.

It depends on the floor going on top. Sheets and rolls are laid down before the floor: poly film under floating floors, or GCP Kovara MBX, a three-layer membrane for slabs up to 99.5% relative humidity that needs no shotblasting. Under wood, tile and stone, Aquabar "B" (two layers of kraft paper laminated with asphalt) is a Class II vapor retarder, not a vapor barrier, per its data sheet. Liquid coatings are applied to the concrete itself when a self-leveler or glued-down flooring goes on top, such as Ardex VB 100, Mapei Planiseal MSP or TEC LiquiDam EZ. Compare the moisture rating and the flooring maker's requirements before you choose.

Test the slab, then compare each product's rating with your reading. Ardex VB 100 is for RH readings up to 100%. Mapei Planiseal MSP protects up to 15 lb MVER (ASTM F1869), 99% RH (ASTM F2170) and pH 12. Laticrete Vapor Ban Primer W is rated for 100% RH and MVER up to 25 lb, and can go over new concrete in as little as 5 days. TEC LiquiDam EZ goes directly onto green concrete up to 100% RH. CustomTech TechMVC is a single-coat epoxy for concrete up to 100% RH (professional use only). Find meters in moisture testing tools, and contact us if you're not sure which one fits.

Some can. Mapei Planiseal MSP is also a primer for self-leveling compounds, so no second primer is needed. Laticrete Vapor Ban Primer W is a moisture vapor coating and a primer in one for Laticrete self-leveling underlayments, with no sand broadcast needed. With Ardex VB 100, no priming is needed before Ardex underlayments, which can go on as little as 60 minutes after the second coat. Then see floor patch, levelers and primers.

Use a construction-grade polyethylene film. Ralston 5000 Series is a 6 mil (150 µm) film made as a long-lasting vapour barrier within wall cavities and designed to exceed CAN/CGSB-51.34-M86. Polytarp Super Six is used on exterior wall insulation in new homes and buildings to prevent moisture build-up, and complies with CAN/CGSB-51.34-M86. Follow your local building code for where the vapour barrier goes.

Use the joining method the product was made for. Roberts Moisture Barricade has an adhesive strip to join pieces and seal out moisture; Roberts says any seam the strip does not join gets its 50-040 Underlayment Seam Tape. GCP Kovara MBX seams are butted, never overlapped, and covered with the 2.5 in. Kovara MBX Seam Tape, which also secures the membrane where it is cut around columns and poles.