What thickness vapor barrier do I need?

Reinforced 12–20 mil meeting ASTM E1745 for a crawl space that will be entered. Thickness alone is not the specification — reinforcement matters more. Reinforced 12 mil will outlast unreinforced 20 mil in a space anyone crawls through.

How much does vapor barrier installation cost?

Typically $1,500–$4,000 for the liner component, driven by square footage, perimeter length and how much old material has to come out. That is the liner alone; vent sealing and dehumidification are separate line items.

Does a vapor barrier alone fix crawl space humidity?

Partly. It stops moisture rising from the soil, which is usually the largest single source. It does nothing about humid outside air entering through open vents, which on this coast is the other half of the problem. That is the gap full encapsulation closes.

Should the vapor barrier go up the walls?

In a sealed crawl space, yes — floor, piers and perimeter walls, terminated near the top. In a vented crawl space the walls are outside the thermal boundary and floor-only is normal practice.

Is a vapor barrier the same as a termite barrier?

No. A termite barrier is pest-control work — a chemical soil treatment or physical shield installed by a licensed pest company — and we do not do it. A vapor barrier is a moisture liner. The two coexist under coastal homes: if you have a termite bond, tell both contractors, because the inspection strip your pest company needs at the top of the foundation wall affects where the liner terminates.

Crawl Space Vapor Barrier Installation

What the work involves in Wilmington and the Cape Fear region, what it costs, and the situations where it is the wrong thing to buy.

Last updated: 2 August 2026

910 Crawl Space Pros provides crawl space vapor barrier installation across Wilmington and the Cape Fear region. Vapor barrier installation is the placement of a sheet material across the crawl space floor — and, in a sealed crawl space, up the piers and perimeter walls — to stop moisture moving from the soil into the space as vapour. The specification of that sheet is where most quotes differ.

What we do on the first visit

Measure the footprint and perimeter, check clearance for working, assess whether existing liner can be removed cleanly, and confirm the space is dry enough to line at all.

What ASTM E1745 actually requires

ASTM E1745 is the standard specification for plastic water vapor retarders used in contact with soil under concrete slabs and in crawl spaces. It sets minimum requirements across three properties: water vapor permeance, tensile strength, and puncture resistance. A product either meets it or it does not.

A Class I vapor retarder is one with a permeance of 0.1 perms or less. That number is the entire point of the material — it is what stops soil moisture entering the space as vapour.

Why 6 mil builder sheet is not a crawl space liner

The 6 mil polyethylene sold as a general-purpose construction sheet is unreinforced. It meets neither the puncture resistance nor the tensile strength that a working crawl space needs, and it tears the first time someone crawls across it to service a duct or run a cable.

A torn vapor retarder is not a vapor retarder. Reinforced liner — a scrim layer sandwiched between plies — is what allows the material to survive being walked on for a decade rather than a season. That is the difference the mil rating alone does not tell you: reinforced 12 mil outperforms unreinforced 20 mil in service.

Termination and seams are where liners fail

The floor is the easy part. The failures we are called to look at are almost always at the edges: liner that stops short of the wall, liner taped to a dusty block surface that released within a year, or piers left unwrapped so moisture tracks up them.

Proper termination means the surface is cleaned first, the liner is mechanically fastened as well as sealed, and it runs high enough up the wall to sit above the level water has historically reached. Seams are overlapped generously and taped with a tape rated for the liner, not general-purpose duct tape.

What's included

Removal and disposal of failed existing liner
Debris clearance so nothing sharp sits under the new material
Reinforced 12–20 mil liner meeting ASTM E1745
Generous seam overlap, taped with a seam tape rated for the material
Piers individually wrapped and sealed
Perimeter termination mechanically fastened to a cleaned surface
Liner run above the historic high-water line where one is present
Photographs of seams and terminations before the space is closed

	DIY	Cheapest quote	Done properly
Liner	6 mil builder sheet, laid loose	6–8 mil, taped seams, not fastened to walls	Reinforced 12–20 mil to ASTM E1745, sealed to piers and walls
Groundwater	Not addressed	Sealed over — water ends up under the plastic	Drainage and sump first, liner second
Humidity control	None, or a household dehumidifier on an extension cord	Undersized unit, condensate draining to the soil	Unit sized to cubic footage, pumped to a proper discharge
Vents	Left open, or foam blocks pushed in	Covered, not sealed	Sealed and detailed so the space is genuinely closed
Wood already affected	Bleach from a spray bottle	Surface treated, moisture source left in place	Growth removed, framing treated, dried, then sealed
What it costs	$300 – $900 in materials	$1,500 – $3,000	$4,200 – $14,000
How long it holds	A season or two	Often fails within a few years	Liner life measured in decades when water is handled first

How vapor barrier work runs

01
Assess and measure

Humidity, moisture content in the framing, standing water, clearance and access — photographed throughout.
02
Written scope

Line by line, in the order the work has to happen, with a price against each component.
03
The work

Water handled first where present, then the fix itself, then verification.
04
Verify and hand over

Post-work readings recorded and left with you, along with the photographs.