Vapor Barrier Guide: Perm Ratings, Classes and Where Each One Belongs

Sep 14, 2026|Read time: 4min|Building Materials

By Dan Dormani · 10 September 2026

A vapor barrier is one of the cheapest layers in a building. It is also one of the easiest to put in the wrong place.

Spelled "vapour barrier" outside North America, the product is the same roll of film. What changes from market to market is the rule about where it may go. The physics does not move.

This guide covers the number that grades a vapor barrier, the classes built on it, and the job each class does.

What a Vapor Barrier Actually Does

The name promises more than the product delivers.

Diffusion, Not Air Leakage

A vapor barrier slows water vapour moving through a solid material. That movement is called diffusion. It is driven by a difference in vapour pressure across the layer.

Air leakage is a different problem. A gap around a pipe moves far more moisture than diffusion ever will.

Diffusion pushes vapour through the material itself. - Slow, steady, and driven by vapour pressure.
Air leakage carries vapour through holes in the assembly. - Fast, and usually the bigger number by far.
A vapor barrier addresses the first, not the second.
An air barrier addresses the second, and the two roles often fall to different products.

Perm Is the Only Number That Matters

Every vapor barrier is graded by permeance, measured in perms. A low perm number means the material passes very little vapour. A high one means it breathes.

Thickness alone tells you nothing useful.

Barrier, Retarder, Or Both

Building codes use the term vapor retarder. The trade uses vapor barrier for the tightest grades and vapor retarder for the rest.

How Perm Ratings Are Measured

One test method sits behind almost every perm figure on a datasheet.

ASTM E96 Dry Cup and Wet Cup

ASTM E96 seals a sample over a cup and weighs the cup as vapour moves through. Procedure A is the desiccant method, often called dry cup. Procedure B is the water method, often called wet cup.

In the dry cup test, desiccant sits inside the cup and the room air is held near 50 percent humidity. In the wet cup test, water replaces the desiccant, so the inside sits near 100 percent.

Why the Same Material Reports Two Numbers

Some materials open up as they get wet. Kraft paper passes far more vapour in the wet cup test than in the dry cup test.

The 2021 International Residential Code uses the ASTM E96 water method for one specific allowance. Class I and II materials that measure more than 1 perm on that test may go on the interior side of any frame wall in every climate zone.

The Three Vapor Retarder Classes

The International Code Council sorts every vapor barrier into three bands by perm rating.

Class	Perm rating	Materials named by the code
Class I	0.1 or less	Sheet polyethylene, nonperforated aluminium foil
Class II	Greater than 0.1, up to 1.0	Kraft-faced fibreglass batts, vapor retarder paint
Class III	Greater than 1.0, up to 10.0	Latex paint, enamel paint

Diagram: Perm rating scale showing Class I at a tenth of a perm or less, Class II from a tenth of a perm up to 1 perm, and Class III from 1 perm up to 10 perms, with the materials the code names placed on each band

Class I Is a True Vapor Barrier

Class I is the only band that earns the word barrier. Polyethylene sheet and unperforated foil sit here.

These materials stop drying as well as they stop wetting, which is why the code limits where they may go.

Class II Is the Middle Ground

Class II covers kraft facing on batt insulation and paints sold as vapor retarder coatings. The band is narrow, running from just above a tenth of a perm up to 1 perm.

Kraft-faced batts deliver a Class II layer as part of the insulation. - The facing has to stay intact and face the right way.
Vapor retarder paint delivers it as part of the finish. - The coverage rate on the tin is a performance figure, not a suggestion.
Both need continuous coverage to hit the rated number.
Neither replaces an air barrier.

Class III Is Ordinary Paint

Class III runs from 1 perm up to 10 perms. Standard latex and enamel paint on drywall land here.

Where Each Class Is Allowed

Placement is regulated because the wrong vapor barrier traps water instead of excluding it.

The Climate Zone Table

The code sets out which class may be used on the interior side of frame walls by climate zone.

Climate zone	Class I	Class II	Class III
1 and 2	Not permitted	Not permitted	Permitted
3, and 4 except Marine 4	Not permitted	Permitted	Permitted
Marine 4, 5, 6, 7 and 8	Permitted	Permitted	Conditions apply

Hot and Mixed Zones

In zones 1 and 2 a Class I vapor barrier is not permitted on the interior side of a frame wall. Neither is Class II.

The reason is direction. Warm humid air outside drives vapour inward for most of the year. An interior film would hold that moisture against the sheathing.

Cold and Marine Zones

Marine 4 and zones 5 through 8 allow a Class I vapor barrier. They also allow Class II.

Class III is still available there, but only under the conditions the code sets out. In Marine 4 it is permitted with vented cladding over wood structural panels.

The Exceptions Worth Knowing

Several exceptions sit directly under the rule, and they cover a lot of real work.

Basement walls are exempt from the frame wall requirement.
The below-grade portion of any wall is exempt.
Assemblies where trapped moisture will not damage the materials are exempt.
Climate zones 1, 2 and 3 need no vapor retarder at all under this section.
An engineered design using accepted hygrothermal analysis is allowed as an alternative.

There is one combination the code singles out. A Class I vapor barrier on the interior, where a Class I layer already sits outside, requires an approved design. The wall then has no way to dry in either direction.

Choosing a Vapor Barrier by Application

The class tables govern frame walls. Slabs and crawl spaces are graded against different standards.

Under a Concrete Slab

Under-slab sheeting is specified to ASTM E1745 rather than to the wall classes. That standard requires a rated permeance of a tenth of a perm or lower. Anything above that figure is not a qualifying vapor barrier.

The standard then sorts qualifying vapor barrier material into Class A, B and C. The split is by tensile strength and puncture resistance, with Class A the toughest.

Permeance decides whether the material qualifies at all.
Class A, B or C decides whether it survives the crew and the aggregate. - Rebar traffic and sharp stone are what tear the light grades. - A torn sheet is worth nothing, whatever its perm rating.
Heavier classes earn their price on difficult sites.

In a Crawl Space

A ground cover changes the ventilation the code demands. Without one, under-floor openings must total at least 1 square foot for every 150 square feet of under-floor area.

Cover the ground with an approved Class I vapor barrier and that ratio drops to 1 in 1,500. The same allowance relaxes the rule that an opening must sit within 3 feet of every external corner. The openings still have to give cross ventilation.

Inside a Framed Wall

Start with the climate zone, then work back to the class before any vapor barrier is chosen.

Diagram: Five step sequence for selecting a vapor barrier, moving from application through climate zone, permitted class, perm rating check and finally installation detailing

Confirm the application, since slabs and crawl spaces follow separate standards.
Read the climate zone before shortlisting any product.
Take the permitted class from the code table for that zone.
Check the datasheet perm rating against the class band.
Check the drying path, and avoid a second low-perm layer on the far side.

Installation Details That Decide Performance

A compliant material installed carelessly performs like no material at all.

Laps and Seams

Every seam is a hole until it is sealed. Laps are taped with the product the membrane maker names. Adhesion is specific to the film.

Seam tape is not interchangeable, and a tape that grips polyethylene may peel off a coated woven sheet.

Use the named tape, because the film and the adhesive are matched. - Substituting a general purpose tape voids most warranties.
Roll the seam rather than pressing it down by hand.
Keep the surface dry at the moment the tape goes on.

Penetrations and Terminations

Pipes, conduits and columns are where the vapor barrier usually fails. Each one needs a collar or a detailed patch, and a sheet that stops short of the wall leaves an open edge for vapour to walk around.

The Double-Barrier Trap

The most common serious mistake is two low-perm layers on opposite faces. Water that gets in between them has nowhere to go.

One low-perm layer per assembly is the safe default.
The drying direction should be decided before ordering.
A vapor barrier on both faces needs engineering, not enthusiasm.

Browse roofing underlayment for vapor barrier and membrane products, adhesives, sealants and tape for the seam tapes, and drywall for the interior finish that carries a Class III rating.

Frequently Asked Questions

What perm rating counts as a vapor barrier?

A rating of a tenth of a perm or less puts a material in Class I, the band the code treats as a true vapor barrier. Anything from a tenth of a perm up to 10 perms is a vapor retarder in Class II or Class III.

Is a vapor barrier required in a hot climate?

No. Under the 2021 IRC, no vapor retarder is required on frame walls in climate zones 1, 2 and 3, and a Class I vapor barrier is not permitted on the interior side in zones 1 and 2.

Can you put a vapor barrier on both sides of a wall?

Not without an approved design, because a Class I layer on the interior combined with a Class I layer on the exterior removes every drying path.

What standard applies to under-slab sheeting?

ASTM E1745. It sets a maximum permeance of a tenth of a perm, then sorts qualifying material into Class A, B and C by tensile strength and puncture resistance.

Does a crawl space vapor barrier reduce the vents needed?

Yes. Covering the ground with an approved Class I vapor barrier cuts the required ventilation opening area from 1 in 150 of the under-floor area to 1 in 1,500.

Conclusion

A vapor barrier is a simple product governed by an unforgiving rule. Get the perm rating and the placement right and it is invisible for the life of the building. Get them wrong and it becomes the reason the sheathing rots.

The sequence never changes. Identify the application, read the climate zone, take the permitted class from the code, then check the vapor barrier datasheet against that band.

Perms decide the class, and thickness on its own decides nothing.
The climate zone decides placement, not habit or regional custom.
Slabs and crawl spaces follow their own standards, not the wall table.
Seams and penetrations decide whether the vapor barrier works at all.