What is 6 Mil Vapor Barrier? A Simple Guide to Poly

6 Mil vapor barrier is one of the least appreciated, less recognized building components. A quick Google search will give you a dozen options from barrier thicknesses to materials.

While the most common vapor barrier thickness is 6 mil, it’s wise to know your options when it comes to vapor barriers and vapor retarders.

What is a Vapor Barrier?

A vapor barrier, or vapor retarder, is a plastic sheet of polyethylene. The purpose of vapor barriers is to stop vapor and moisture from penetrating through building envelopes.

It’s commonly found behind drywall in exterior steel stud framed walls, exterior wood-framed walls, concrete slabs, foundation walls, and even some interior walls.

6 mil clear poly 100 foot roll
100 foot roll of clear 6 Mil Poly

Vapor barriers are measured in mils referencing their thickness. For vapor barriers in walls, most specifications and building codes will call for 6 mil vapor barrier.

However, the thickness of the vapor barrier can range from 3 mils to 20 mils, each with its preferred uses.

For comparison, the typical plastic grocery bag is 1/2 mil thickness. So vapor barriers used in building construction are more than ten times thicker than the average plastic bag at the checkout line.

Vapor Barrier or Vapor Retarder?

A true vapor barrier has a permeance rating of 0.00 when tested using ASTM E96. Vapor retarders are categorized into three different classes.

  1. Class I vapor retarder has a 0.1 perm or less.
  2. Class II vapor retarder has a perm of 0.1 to 1.0.
  3. Class III vapor retarder has a perm of 1.0 to 10.

Any vapor barrier or retarder sheet with a rating above 10 perms is referred to as a vapor-permeable membrane.

How Vapor Barriers Work

The vapor barrier works by halting or slowing the progress of moisture from seeping inside the building.

A common problem in construction is water and moisture intrusion. Water wants to go from areas of high concentration to low concentration.

This is why dew from the highly moist air forms on the surface of less damp grass on humid nights.

The exact process occurs constantly outside and inside buildings. Water wants to go from humid air or earth into less moist air or ground.

Vapor barriers stop that process by blocking water vapor from condensation in walls or in concrete slabs and foundations.

condensation in construction
It’s vital to stop moisture from moving through the building envelope. A properly installed vapor barrier will stop moisture dead in its tracks.

Are Vapor Barriers Necessary?

The short answer is yes. While, in some mild climates, vapor barriers aren’t strictly necessary, in most cases and according to most building codes, vapor barriers are essential and required.

They function as insulation, flame retardants, and stopgaps for water damage of all sorts.

plastic sheeting vapor barrier
6 Mil. Poly Vapor Barrier

Foregoing poly sheeting and vapor barriers in construction, or incorrect installation, could damage the building envelope and cause wood rot and drywall mold issues.

It could also lead to buckling and warping problems with the concrete foundation.

Where to Correctly Install Vapor Barrier

The vapor barrier installation process should be followed according to the local building codes.

Depending on the climate, moisture migration is one of the most important aspects of construction.

You don’t need heavy rain and roof leaks for moisture to wreak havoc on buildings. Where humid air flows to dry air, water vapor can cause significant issues over time.

vapor barrier wall assembly
Here’s a vapor barrier on a furred CMU wall. Depending on the region of the country, it’s important to understand if the vapor barrier should be applied on the interior or exterior of the wall assembly.

For instance, if you’re building in high humidity, the moist air is outside the construction. Vapor barrier should be fastened on the wall’s outer edge for moisture protection.

In cold climates where the air outside is dry, the moisture barrier should be behind the drywall on the interior side of the studs.

For concrete foundations, moisture barriers should be installed under and extended to the foundation walls to prevent moisture from seeping up from the ground beneath.

The Standard 6 Mil Vapor Barrier

The bare minimum should be a 6 mil polyethylene vapor barrier at foundations and crawl space encapsulation.

Preferably, a 12 to 20 mil heavy-duty plastic vapor barrier can be a good product for under concrete slabs. It’s heavy-duty, thick, and very durable for the toughest construction areas.

6 mil vapor barrier

Some flooring methods recommend a vapor barrier. For example, a 6 mil vapor barrier is recommended on the subfloor, under luxury vinyl plank floating floors, certain wood floors, and laminate flooring, depending on dry climates.

When it comes to purchasing vapor barriers, understanding the different types of moisture barriers is essential.

string reinforced poly vapor barrier
For heavy-duty protection and covering, choose a string-reinforced poly.

Vapor Barrier Classes

All of the above materials are classified according to a metric called “perms,” or their permeability rating. This is how much water they allow to pass through. These materials are split into three classes:

  • Class I (0.1 perms or less): poly sheathing, glass or metal sheets, rubber membrane
  • Class II (0.1 – 1.0 perms): unfaced polystyrene, 30 lb. asphalt-covered paper, Bitumen-coated Kraft paper
  • Class III (more than 1.0 perms): gypsum board, fiberglass insulation, board lumber, concrete block, cellulose insulation, brick

Buying Mil Vapor Barrier and Plastic Sheathing

Common plastic barriers and construction film are measured in mils, ranging from 3 to 20 depending on the use. 

For walls and floors, the standard is 6 mil polyethylene or 6 mil plastic sheeting Visqueen (a common poly brand).

In humid climates, 3 mil Visqueen or a 4 mil poly vapor barrier will help cool the home, but the thinner the barrier, the easier it will tear.

black 6 mil poly

6 mil vapor barrier polyethylene is what you’re looking for in most cases. Even for use as a simple drop cloth, it would be wise to go with a 6 mil vapor barrier minimum.

For crawl spaces, look for specifically-labeled crawl space barriers. These will be heavy-duty and much thicker, such as a 12-mil barrier up to 20-mil thicker poly.

The same heavy-duty sheathing should be used for foundations and placement beneath concrete slabs. 3 to 6 mil vapor barrier is too thin and would eventually tear and allow vapor through.

6 Mil Vapor Barrier Options

When using a 6 mil vapor barrier for crawl space encapsulation, look for long-lasting products made with virgin resins.

6 mil vapor barrier engineered for crawl spaces is usually produced without recycled materials to prevent degradation.

StegoCrawl 6 mil vapor barrier is one example of poly featuring virgin resins.

You might have noticed black plastic sheeting vapor barriers and wondered about the difference. A black crawlspace vapor barrier underlayment is slightly more effective than clear plastic sheeting.

6 mil vapor barrier

The black tint helps stop evaporation and inhibits the growth of organic materials.

For all your DIY projects, most large outlets and local hardware stores carry a variety of vapor barriers.

At Home Depot, vapor barriers can be found in 100-200 sq ft rolls, at around $0.15-0.20 per sq. foot.

1 mil vapor barrier plastic sheeting
For your most lightweight jobs, the 1 mil clear poly will do the trick.

As with all purchases, check with your local municipality to make sure you buy vapor barrier per your local building code.

Most buildings require specific thicknesses, perms, and flame retardant grading.

READ more about Demising Walls –

What Is A Demising Wall?

Different Types of Vapor Barrier Material

We’ve covered plastic vapor barriers, poly sheathing, and polyethylene vapor barriers.

While these are the most common, there are numerous other vapor barrier and vapor retardant materials out there, such as:

  • Elastomeric coatings
  • Paper-backed aluminum
  • Aluminum foil
  • Metallic film
  • Vapor retardant paints
  • Asphalt-coated Kraft paper
  • Exterior-grade plywood
  • Glass or metal sheets
  • Sheet roofing membranes
  • Foil-faced foam board
  • Polyethylene sheeting
  • String reinforced vapor barrier

One of the most important aspects of vapor barrier use is to seal all the penetrations and seams. Around window openings, doors, plumbing stacks, vent fans, and electrical outlets.

If holes are found in the vapor barrier, repair them with foil tape, polyethylene, or caulk. Large holes should be cut away and replaced.

Here’s an excellent summary of how vapor barriers keep moisture out of buildings. Keeping wall cavities dry is vital for long-lasting and well-performing buildings.

READ more about the many options for your next project –

What You Need To Know About Ballistic Drywall

Green Board: The Basics Of Water-Resistant Drywall

Basics Of Sheetrock Screws And Drywall Screws

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