Crawl Space Vapor Barrier Thickness Explained
Liner is sold by thickness in mils — thousandths of an inch — and the number on the box is only half the story. Reinforcement matters as much as thickness for how long the liner survives real use.
What thickness actually measures
A mil is one thousandth of an inch. A 6 mil liner is thin sheeting; 12 to 20 mil products are substantially heavier and usually reinforced with a woven scrim between layers.
Thickness alone does not equal durability. A reinforced 12 mil liner typically outperforms an unreinforced sheet of the same nominal thickness because the scrim resists tearing and puncture.
Choosing by how the space is used
- Nobody ever enters the space: a lighter ground-only barrier can be adequate
- HVAC, plumbing, or storage access: reinforced liner is worth it, because foot traffic destroys thin sheeting
- Sharp rock or debris on the soil: heavier liner, and ground preparation before installation
- Full encapsulation: reinforced liner run up the piers and walls with sealed seams and terminations
Seams, terminations, and why they matter more than mils
Most liner failures are not punctures in the middle of a sheet — they are open seams and terminations that were never properly sealed to the piers and walls.
When comparing quotes, ask how seams are sealed, how the liner is terminated at the foundation wall, and whether piers are wrapped. Those details determine whether the system controls moisture or just covers dirt.
Barrier versus encapsulation
A vapor barrier slows ground moisture. Encapsulation seals the whole space and controls the air in it. If your goal is a genuinely drier crawl space and lower humidity in the house above, thickness is the wrong variable to optimize — the system type is the decision that matters.
Pick reinforcement and sealed detailing over raw thickness. A well-sealed 12 mil reinforced system beats a thicker liner installed loose over dirt.