digest
Short Summary
Main recommendation: For a basement subfloor, use the traditional system of a vapor barrier plus 5/8-inch tongue-and-groove OSB plywood screwed to the concrete with Tapcon screws, leaving an airspace underneath; the blue DMX product is better suited only to a single room where walls already exist. The speaker's main reason is that moisture from thaw-freeze cycles on a walkout must be able to move past the walls and be absorbed over time.
Key Points
- The blue DMX product is a 3-in-1 insulated, cushioned vapor barrier that the manufacturer has approved for laminate installed directly on top, but the speaker does not favor that approach for a new build.
- Building walls first and placing DMX underneath, or filling wall cavities with it, would trap moisture and cause a musty smell regardless of other measures.
- The two systems cost the same per square foot to install, so the choice comes down to convenience and the specific situation.
- New blue tuck tape, tested and approved for wet areas, seals the plastic seams into one continuous vapor barrier.
- A laser line between two steel posts is used as a single square reference for the whole project, instead of a standard square and tape measure.
Details Worth Noting
- Concrete instructions: Lay the plastic vapor barrier across the room with extra length, repeat on the other side, and tape all seams with blue tuck tape to form one continuous barrier (the covered area took about 10 minutes for almost 400 square feet).
- Concrete instructions: Set a laser line from one steel post to the other, offset from a marked point, and build all walls square from that line.
- Concrete instructions: Cut end-wall plywood to the door frame width plus 3 inches, and cut the adjacent piece with an extra 1/8 inch allowance, since the door's closed position sits over the laminate-to-tile transition.
- Concrete instructions: Secure each plywood sheet with about six to eight Tapcon screws, pulled tight but leaving a small airspace beneath; a screw within a foot of the tongue-and-groove joint also compresses the adjacent sheet, so extra screws are unnecessary.
- Concrete instructions: Eliminate spongy spots by adding screws where needed. If a hole does not grip, drill straight through without cleaning it, since the dirt helps bind the screw.
- Use an SDS hammer drill for effortless drilling; a hammer-only mode works but takes about five times as long, and renting one for about 25 dollars per day is recommended for large spaces.
- Only a small airspace is needed; the speaker notes moving air can carry away moisture before it spreads through the house.
Bottom Line
Choose the traditional plywood-on-Tapcon system with an airspace and vapor barrier for any new build with walls, reserving the blue DMX product for single-room jobs where walls already exist.
