Quick answer
A vapor retarder is a material or coating built into a wall, ceiling or floor to slow the movement of water vapor through it by diffusion, so moisture from indoor air does not condense inside the assembly.
Codes sort vapor retarders into three classes by how many perms they let through. The strongest class, under 0.1 perm, is what most people call a vapor barrier, such as polyethylene sheet.
Quick facts
The key facts about Vapor Retarder, with sources:
- Class I
- 0.1 perm or less: polyethylene sheet, unperforated aluminum foil 1
- Class II
- Over 0.1 to 1.0 perm: kraft-faced batts, vapor retarder paint 1
- Class III
- Over 1.0 to 10 perms: latex or enamel paint 1
- Where required (2021 IRC)
- Interior side of frame walls in climate zones 4 (Marine), 5 to 8; not required in zones 1 to 3 1
- Not the same as
- An air barrier, which stops air movement 3
- Main risk if misused
- Trapping moisture so the wall cannot dry 3
Key takeaways
- A vapor retarder slows water vapor; it does not stop liquid water or air leaks.
- The 2021 IRC sorts vapor retarders into Class I, II and III by perm rating, and "vapor barrier" usually means Class I.
- Where you build sets the rule: the IRC does not require one in climate zones 1 to 3 and limits plastic sheeting in zones 1 to 4.
- Too much vapor control can trap moisture, so walls need a way to dry in at least one direction.
- Paint and kraft facing are vapor retarders too, so check the whole assembly, not one layer.
Vapor retarder, in plain English
Indoor air carries water vapor from showers, cooking and breathing. In winter, that warm, damp air tries to move out through walls and ceilings toward the cold outside.
If it reaches a cold surface inside the wall, such as the back of the sheathing, it can condense into liquid water. A vapor retarder is a layer that slows that movement.
Building Science Corporation, a building science consultancy, defines a vapor retarder as the element designed and installed in an assembly to retard the movement of water by vapor diffusion 3. Diffusion means vapor moving through a material itself, the way perfume scent spreads through a closed room.
The amount a material lets through is measured in perms. The fewer the perms, the stronger the retarder. The 2021 International Residential Code (IRC) uses perm ratings to sort materials into three classes 1.
The word "barrier" causes confusion. Most materials slow vapor rather than stop it. BSC proposes that "vapor barrier" mean a Class I retarder only, the strongest class 3.
How vapor moves, and where the retarder steps in
Vapor moves from warm and humid to cool and dry. The retarder sits on the side of the insulation that is warm and humid for most of the heating season.
- Indoor air holds vapor. In winter the inside is warmer and often more humid than outside.
- Vapor pushes outward. It diffuses through drywall and into the insulated cavity.
- The temperature drops across the wall. Somewhere inside, a surface may be cold enough for vapor to condense.
- A retarder on the warm side slows the flow. Less vapor reaches the cold surfaces, so less condenses. That is why the IRC places it on the interior side of frame walls 1.
- The wall still needs to dry. BSC warns that interior vapor barriers can stop a wall from drying inward, which is a problem in air-conditioned buildings 3.
- Interior paintLatex paint can count as Class III
- DrywallOften the air barrier when sealed
- Vapor retarder (warm side)Class I or II where the code calls for it
- Insulated stud cavityBatts, blown-in or spray foam
- SheathingWood structural panel or fiberboard
- Water-resistive barrierHouse wrap or felt
- CladdingSiding, with a vent gap in some designs
The retarder goes on the side that is warm in winter. In hot, humid climates the logic flips, and an interior barrier can do harm. Source: [1] [3]
Air leaks carry far more moisture than diffusion. BSC notes that air barriers, not vapor barriers, control moisture-laden air 3. A sealed building envelope does more for wall moisture than plastic sheeting does.
The three vapor retarder classes
| Class | Perm rating | Example materials |
|---|---|---|
| Class I | 0.1 perm or less 1 | Sheet polyethylene, nonperforated aluminum foil 1 |
| Class II | More than 0.1, up to 1.0 perm 1 | Kraft-faced fiberglass batts, vapor retarder paint 1 |
| Class III | More than 1.0, up to 10 perms 1 | Latex or enamel paint 1 |
| Vapor permeable (not a class) | More than 10 perms, in BSC’s terms 3 | Most house wraps and unfaced fiberglass |
| "Smart" or variable-perm retarder | Class I or II when dry, opens above 1 perm when humid | The IRC allows these in all zones when they test above 1 perm by the ASTM E96 water method 1 |
Vapor retarder vs air barrier
A vapor retarder slows vapor moving through a material. An air barrier stops air flowing through gaps. One layer, such as sealed polyethylene, can do both, but a perfect vapor retarder full of holes still lets moist air pour through.
Example: picking a vapor retarder for a zone 5 basement-to-bedroom remodel
This walks an example project through the 2021 IRC rules. The project details are ours; the rules are quoted from IRC Section R702.7 and Tables R702.7(1) to R702.7(3) 1. Your jurisdiction may have adopted a different edition.
| Wall or question | What the IRC says | Result for this example |
|---|---|---|
| Climate zone | Determined under Section N1101.7 1 | Zone 5 |
| Above-grade exterior frame wall | Class I or II permitted in zone 5; Class III only with listed cladding or exterior insulation 1 | Kraft-faced batts (Class II) meet the rule |
| Same wall with vented siding over wood structural panels | Class III is permitted in zone 5 1 | Latex paint alone would also be allowed |
| Basement concrete wall | Basement walls and below-grade portions are exceptions 1 | No vapor retarder required by this section |
| Adding exterior foam board later | Class II with foam must follow Table R702.7(4) and test above 1 perm 1 | Use a variable-perm product or ask a designer |
| Interior plastic on the basement wall | Not required; BSC warns it can stop below-grade walls drying 3 | Leave it out unless a designer specifies it |
- If Climate zones 1 and 2
- Then Class III only; Class I and II not permitted (unless above 1 perm by the water method)
- If Zones 3 and 4 (except Marine 4)
- Then Class II or III; Class I not permitted
- If Marine 4 and zones 5 to 8
- Then Class I or II permitted; Class III only per Table R702.7(3)
- If Any zone, below grade or basement wall
- Then Exempt from this rule
Source: [1]
The takeaway: in one remodel the answer can differ wall by wall. Above-grade walls need a retarder in zone 5, basement walls do not, and the best choice depends on the siding and sheathing outside.
Where it shows up: insulation facings, paint cans, plans and inspections
On insulation. Kraft paper on fiberglass batts is a Class II retarder 1. Vermont’s energy office notes batts come with or without vapor retarder facings such as kraft, foil-kraft or vinyl 5. Unfaced batts have none.
On paint. Ordinary latex or enamel paint counts as Class III. Some paints are sold as "vapor retarder primer" and are rated Class II when applied as directed 1.
On plans and quotes. A basement or addition plan should show what goes on the interior side of each exterior wall. If a quote says "6-mil poly," ask whether your climate zone and wall design call for it.
At the insulation inspection. Where a permit applies, the inspector may check the retarder before drywall. Our permits and approvals guide explains the inspection sequence.
Read the facing
Faced batts print the facing type on the paper. Install the facing toward the side that is warm in winter, as the maker directs, and never add plastic over a kraft face unless a designer asks for it.
Benefits and limitations
What a vapor retarder does well
- Slows winter vapor drive into cold walls in cold climates.
- Meets code where the IRC requires it on the interior of frame walls 1.
- Kraft facing or vapor retarder paint can do the job without plastic 1.
- In zones 6 to 8 a Class I or II ceiling retarder can, with balanced venting, halve the required attic vent area 2.
Where a vapor retarder is the wrong fix
A vapor retarder cannot fix a leak. Rain getting past siding, plumbing leaks and roof leaks are liquid water problems. EPA notes that the key to mold control is moisture control, and that means fixing the water source 7.
It also cannot stop moist air leaking through holes. BSC stresses that vapor barriers are not intended to stop air migration, which is the job of air barriers 3.
Finally, a retarder in the wrong place can cause harm. BSC links polyethylene, foil-faced batts and vinyl wallpaper on the inside of air-conditioned walls with moldy buildings 3. In a hot, humid climate the vapor drive runs from outside to inside much of the year.
What drives the cost, and how long it lasts
We do not publish material or labor prices. The cost drivers are simple. Kraft-faced batts build the retarder into the insulation. Vapor retarder paint adds a primer coat.
Polyethylene or variable-perm membranes add a separate material and the labor to seal seams and openings. Exterior foam may change which interior class you need, which changes the material list 1.
Mistakes are the hidden cost. Removing wet drywall and insulation from a wall that could not dry costs far more than choosing the right class at the start. See how hidden conditions fit a budget in our remodeling cost guide.
Once covered by drywall, a vapor retarder lasts as long as the wall if it is not punctured. Paint-based retarders change when you repaint, so note the primer type in your house records.
How a vapor retarder is chosen and installed in a remodel
A contractor or designer handles this, usually as part of insulation work under a permit.
- Confirm the climate zone with the building department; the IRC ties the rule to Section N1101.7 1.
- Identify each wall type: above grade, below grade, with or without exterior foam, vented or unvented cladding.
- Pick the class from IRC Table R702.7(2), and Table R702.7(3) or R702.7(4) where they apply 1.
- Seal air leaks first, since air carries more moisture than diffusion 3.
- Install the retarder on the interior side of the insulation, with seams lapped or taped per the maker.
- Seal around boxes and openings if the layer also serves as the air barrier.
- Pass the insulation inspection before drywall goes up.
Keeping the vapor control working
A vapor retarder hidden in a wall needs no service, but later work can damage it.
- Patch holes made for new outlets, fixtures or pipes on exterior walls.
- Keep indoor humidity reasonable in winter with bath and kitchen exhaust fans.
- Note the primer if you used vapor retarder paint, so a repaint keeps the rating.
- Avoid vinyl wallpaper on exterior walls of air-conditioned rooms, which BSC links with mold 3.
- Check for condensation on windows and cold corners as an early sign of high humidity.
Warning signs of a moisture problem in a wall
- Musty smell near exterior walls or in a finished basement.
- Peeling paint, stains or bubbling on exterior walls or below windows.
- Mold on drywall or behind furniture against outside walls.
- Soft or swollen trim and baseboards.
- Wet insulation found during any repair.
Mold and old paint
Opening a damp wall can release mold spores, and in homes built before 1978 the paint may contain lead. Follow EPA mold cleanup guidance, and use an EPA RRP-certified firm for paid work that disturbs paint in a pre-1978 home 6.
EPA advises drying water-damaged areas within 24 to 48 hours to prevent mold growth and fixing the source of the water 7. If walls are already wet or moldy, call a building envelope contractor or energy auditor to find the cause before closing anything up again.
What the 2021 IRC requires
The rule. 2021 IRC Section R702.7 says vapor retarder materials are classified by Table R702.7(1), and a vapor retarder of the class shown in Table R702.7(2) shall be provided on the interior side of frame walls, including compliance with Tables R702.7(3) or R702.7(4) where they apply.
An approved design using hygrothermal analysis is allowed as an alternative 1.
Exceptions. No vapor retarder is required for basement walls, the below-grade part of any wall, construction where moisture will not damage the materials, or in climate zones 1, 2 and 3 1.
Zone limits. Table R702.7(2) does not permit Class I or II on the interior in zones 1 and 2, and does not permit Class I in zones 3 and 4 (except Marine 4).
A footnote allows Class I and II products that test above 1 perm by the ASTM E96 water method in any zone 1.
Attic link. 2021 IRC R806.2 allows attic vent area to drop from 1/150 to 1/300 of the attic area if, among other conditions, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling in zones 6, 7 and 8 2.
Adoption. States and cities adopt code editions and amendments on their own schedule. Check yours with the building department or our permit checker.
Vapor retarder vs. related layers
| Layer | What it controls | Main difference |
|---|---|---|
| Vapor retarder | Water vapor by diffusion 3 | Rated in perms; usually on the warm side |
| Vapor barrier | A Class I vapor retarder, 0.1 perm or less 31 | The strongest class, not a separate product type |
| Air barrier | Air flow through gaps 3 | Stops moist air leaks; drywall or sheathing can serve |
| Water-resistive barrier | Liquid water behind cladding | On the outside, and usually vapor open so walls can dry |
| Shower waterproofing membrane | Liquid water in wet areas | Many are also very low perm, so they act as vapor barriers |
Myths about vapor retarders
- Myth Every wall needs plastic sheeting.
- Reality The 2021 IRC does not require any vapor retarder in climate zones 1 to 3, and does not permit Class I on the interior in zones 1 to 4 except Marine 4 1.
- Myth Vapor barrier and vapor retarder are different products.
- Reality A vapor barrier is just the strongest class, Class I 3.
- Myth A vapor retarder keeps walls dry from leaks.
- Reality It slows vapor only; liquid leaks must be fixed at the source 7.
- Myth More vapor control is always safer.
- Reality BSC warns vapor barriers often prevent walls from drying, and double barriers trap moisture 3.
- Myth Paint does not count.
- Reality Latex and enamel paint are Class III retarders under the IRC 1.
How humid vs cold climates change the rule
Climate zone is the deciding variable. In the cold zones (Marine 4 and 5 to 8), the 2021 IRC permits Class I or II interior vapor retarders and limits Class III to walls with vented cladding or enough exterior insulation, per Table R702.7(3) 1.
In the hot, humid south (zones 1 and 2), Class I and II interior retarders are not permitted, and the exceptions say no retarder is required in zones 1 to 3 at all 1.
BSC explains why: in air-conditioned buildings an interior vapor barrier blocks the wall from drying inward 3. Confirm your zone and the adopted edition locally.
When a vapor retarder decision matters in your project
It matters whenever insulation goes into an exterior wall or ceiling: basement finishing, additions, attic conversions and any gut remodel of an outside wall. It also matters when you add exterior foam during re-siding, since that can change the interior class you need.
It matters less for interior partitions, which do not separate inside from outside.
Your next step: ask your contractor which class goes on each exterior wall, why, and which code table they used. If the answer is "always poly," ask again with your climate zone in hand.
Questions about Vapor Retarder
What is the difference between a vapor barrier and a vapor retarder?
A vapor barrier is a type of vapor retarder. Codes group vapor retarders into three classes by perm rating, and "vapor barrier" usually means the strongest, Class I, at 0.1 perm or less, such as polyethylene sheet. Class II and III materials, such as kraft facing and latex paint, slow vapor less.
Do I need a vapor barrier in my walls?
It depends on your climate zone. The 2021 IRC requires an interior vapor retarder on frame walls in Marine zone 4 and zones 5 to 8, and does not require one in zones 1, 2 and 3. Basement walls and below-grade walls are exempt. Ask your building department which edition it enforces.
Which side of the insulation does the vapor retarder go on?
In heating climates it goes on the interior side, the side that is warm in winter. The 2021 IRC requires it on the interior side of frame walls where it applies. Kraft-faced batts are installed with the facing toward the living space, following the maker’s instructions.
Should I put plastic on basement walls?
Usually not. The 2021 IRC exempts basement walls and below-grade portions of walls from the vapor retarder rule. Building Science Corporation warns that vapor barriers can stop below-grade spaces from drying. If a designer specifies one for a particular assembly, follow that design.
Is kraft paper a vapor retarder?
Yes. Kraft-faced fiberglass batts are listed as a Class II vapor retarder in the 2021 IRC, meaning more than 0.1 and up to 1.0 perm. That is often enough for cold-climate walls without adding plastic. Do not add polyethylene over a kraft face unless a designer calls for it.
Does paint count as a vapor retarder?
Yes. The 2021 IRC lists latex and enamel paint as Class III vapor retarders, at more than 1.0 and up to 10 perms. Some primers are sold as vapor retarder paint and rate as Class II when applied as the maker directs. Check the product data before relying on paint alone.
What is a perm?
A perm is a unit of how easily water vapor passes through a material. The lower the perm rating, the more the material slows vapor. Perm ratings are found by ASTM E96 tests. The IRC sets Class I at 0.1 perm or less, Class II up to 1.0, and Class III up to 10.
Can a vapor barrier cause mold?
It can if it is in the wrong place. Building Science Corporation links polyethylene, foil-faced batts and vinyl wallpaper on the inside of air-conditioned walls with moldy buildings, because they stop walls from drying inward. Two vapor barriers, one on each side, can also trap moisture.
Is a vapor retarder the same as house wrap?
No. House wrap is a water-resistive barrier on the outside of the wall that sheds liquid water. Most house wraps are vapor open so the wall can dry outward. A vapor retarder sits on the inside, in heating climates, and slows vapor coming from indoor air.
Sources
- ICC, 2021 International Residential Code, Chapter 7 Wall Covering (R702.7; Tables R702.7(1)–(3)), retrieved .
- ICC, 2021 International Residential Code, Chapter 8 Roof-Ceiling Construction (R806.2), retrieved .
- Building Science Corporation, BSD-106: Understanding Vapor Barriers, retrieved .
- ASTM International, E96/E96M Standard Test Methods for Gravimetric Determination of Water Vapor Transmission Rate of Materials, retrieved .
- Vermont Department of Public Service, Vermont Energy Saver: Insulation, retrieved .
- U.S. EPA, Lead Renovation, Repair and Painting Program, retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Classes, exceptions and climate-zone limits were read from 2021 IRC Section R702.7 and Tables R702.7(1) to R702.7(3), and the attic venting exception from R806.2, on ICC Digital Codes.
Definitions and failure modes come from Building Science Corporation BSD-106. The test method title was checked on astm.org. The basement-to-bedroom example applies the code text to an example project, labelled as such.
DOE Energy Saver vapor barrier page returned "page not found" on the retrieval date and is not used. No prices are given.
Suggest a correction. We fix errors and say what changed.