Step 21 - Subfloor
How to Install a Moisture Barrier
Moisture barriers prevent ground moisture and concrete vapor from damaging flooring. Required for basements, concrete slabs, and anywhere moisture testing showed problems.
Time needed
2-4 hours
Cost
$50-150 (materials)
Difficulty
Easy
Installation Steps
1. Choose the Right Barrier Type
6-mil Polyethylene Sheeting:
Most common. Cheap and effective. Use over concrete slabs and in basements.
Foam Underlayment with Barrier:
Combined moisture barrier and cushion. Good for laminate and LVP.
Cork Underlayment:
Premium option with natural moisture resistance and sound dampening.
2. Roll Out and Overlap
Install barrier across entire floor with proper overlap at seams.
- 1. Clean subfloor completely - no debris
- 2. Roll out barrier perpendicular to flooring direction
- 3. Overlap seams by 6-8 inches minimum
- 4. Run barrier up walls 2-3 inches
- 5. Tape all seams with moisture barrier tape
- 6. Cut around obstacles leaving no gaps
3. Seal Seams Completely
Use moisture barrier tape (not regular duct tape) to seal all seams and edges.
Critical: Unsealed seams defeat the entire purpose. Moisture finds gaps. Tape every seam thoroughly.
When Moisture Barrier is Required
• All basement installations regardless of testing
• Concrete slabs less than 2 years old
• Any space with failed moisture tests
• Installing wood or laminate over concrete
Next Step
With moisture barrier in place, install underlayment for cushion and sound dampening.