4 Mil Vs 6 Mil Vapor Barrier (Here Are The Differences)

Patricia Oelze
by Patricia Oelze

A vapor barrier is an essential part of construction in any home and can prevent damage from moisture like mold and mildew. Even if you live in a warm climate, the potential for moisture to get into your home is high. While there are different kinds of vapor barriers, the most common is poly and it comes in different thicknesses.

In most states, a 6 mil vapor barrier is required. However, there are some states where you can use a 4 mil vapor barrier if you want to. In some cases, both a 6 mil and 4 mil barrier is needed.

Do You Need Insulation Installers?

Get free, zero-commitment quotes from pro contractors near you.

Thicknesses Can Vary

The most common vapor barrier material is polyethylene (poly). It is measured by mils, which are the thickness of the poly. Each mil is 0.001 inch and most codes call for 6 mil, which is 0.006 inches thick. However, you should not use a poly vapor barrier in your basement.

Depending on what the home is made of, and where you are working, the thickness can vary. For example, crawl spaces in many homes need a 6 mil material as well as a 12 mil material to extend to the foundation walls. Some homeowners go so far as to use a 20 mil vapor barrier because it is puncture resistant.

Why to Use a Vapor Barrier

Vapor is like a poison to your home. And it can get in many different ways because most building materials such as insulation and drywall allow the passing of vapor. A lot of it comes up through the ground as well, so you need to be sure that there is a vapor barrier on all of your floors as well as the walls and ceilings. Below is a table explaining which vapor barriers go where.

ClimateWhat to UseWhere to Use Them
Hot humid climate3 mil or 6 milAt the exterior outside of the insulation
Mixed dry climate3 mil or 6 milInterior when codes require it
Mixed humid climateBetter to use a kraft or smart vapor retarding paint or material

· For warmer climates place at the interior

· In colder climates, the barrier should go on the exterior

Cold climate6 mil or higher

On the interior

Does Thickness Really Matter?

When it comes to vapor barriers, exceeding the code is better than just meeting it. Poly is a nice choice because it is easy, inexpensive, and comes in many different thicknesses. You want to have a thicker poly, so it is durable and less permeable. For example, a piece of paper is about one mil, a plastic grocery bag is 2.5 mils, and a credit card is 30 mils.

The most common rule for vapor barriers in cold climates is to install it in the interior with at least two-thirds of your insulation on the outside of the vapor barrier. The connections have to overlap from six to 12 inches and be sealed with vapor barrier tape. Butyl tape is even better because it does not stretch, and it is self-healing.

Cladding is Important Too

Another thing you need to know about is your home’s cladding type. More than half of all homes in the United States have homes with absorbent materials. These include stone, fiber cement, wood, stucco, and brick. These are the ones that will release moisture into your home. Those that will not do absorb moisture include vinyl siding, aluminum, and scyon.

Different Types of Vapor Barrier Material

A true vapor barrier will completely prevent any moisture through. Most vapor barrier material is actually vapor retardant because they do not completely block the vapor. There are many types of vapor barriers besides poly on the market including:

  • Aluminum backed with paper
  • Aluminum foil
  • Asphalt Kraft paper
  • Elastomeric coating
  • Exterior plywood
  • Extruded poly
  • Foil-faced foam board
  • House wrap
  • Metal or glass sheets
  • Metallic film
  • Sheet roofing membranes
  • Vapor retardant paint

What About Perms?

There is also something called a permeability (perm) rating. This is how easy it is for water vapor to pass through a material. The less permeable the material is, the better because that means less vapor can get through. Poly is one of the best choices for perms.

Class one0.1 perm or less

· Poly sheet

· Rubber membrane

· Sheet metal

· Glass

· Vapor retardant paint

· Exterior plywood

· Foil-faced insulation board

Class twoBetween 0.1 and 1.0 perms

· Kraft paper coated with bitumen

· Interior plywood

· 30 lb. asphalt coated paper

· Extruded poly

Class threeBetween 1.0 and 10 perms

· House wrap

· 15 lb. asphalt coated paper

· Brick

· Concrete

· Board lumber

· Cellulose insulation

· Fiberglass insulation

· Drywall (unpainted gypsum board)

Can You Use 3 Mil in Warm Areas?

Using 6 mil poly is best for all outside walls if you live in a cold climate. But in warm, dry climates or mixed humid climates, you can use 3 mil or 6 mil. Once again, it is still best to use 6 mil, but 3 mil is acceptable. The main problem being the thinner barrier can be damaged easier.

There Is a Catch

No matter how much vapor barrier you use in whatever thickness and perm if you do not have an air barrier as well. The vapor barrier can only prevent the moisture from getting in, but an air barrier helps to regulate the indoor climate by keeping out the air.

This is important since air can also allow moisture in. The air barrier will minimize the heat loss through radiation, convection, and conduction. The requirements of an air barrier include:

  • It has to be strong and stiff enough to resist the forces during and after construction
  • It must be impermeable to airflow
  • It should be one continuous piece around the whole home
  • It has to be expected to last the lifetime of the home

Poly Takes Care of Both Requirements

According to the International Energy Conservation Code (IECC), an air barrier has to regulate the indoor climate by stopping the flow of air and the moisture that comes with it. Poly is actually good at doing both, which is why using a 6 mil vapor barrier or higher is best. It can cover both requirements.

Do You Need Insulation Installers?

Get free, zero-commitment quotes from pro contractors near you.

Related Questions

Why Does Placement Matter?

All homes should be sealed to protect the insulation and inside walls from getting moisture to prevent mold and mildew. Although in certain areas it is best to use semi-permeable material because there is no way to completely prevent moisture from getting in.The problem is that keeping moisture out is important, but it also has to let out any moisture that comes from inside the home. That is why vapor barriers should be installed on the hotter and moister side of the wall. In colder climates, this would be the inside and in warmer climates, you would put them on the outside.

Should I Use a Vapor and Air Barrier in My Ceiling?

Although many will claim that a house has to breathe so you should not use an air barrier, it is important to still have both. There are other ways for a house to breathe besides having a leaky attic or roof. The vapor barrier is important in the ceiling since much of the moisture comes from the condensation that accumulates.

Patricia Oelze
Patricia Oelze

I am a DIYer who loves writing about anything home-related. When I am not writing, you can find me studying for my PhD in Psychology, photographing nature, and swimming at the lake with my grandkids.

More by Patricia Oelze