What are vapor barriers? Types and when to use (IMPORTANT to Know!)

Most places that have a cold climate in the USA need a moisture control strategy that works. And a vapor barrier does just that. But you might be curious to know more about what vapor barriers exactly are and how they work. We have discussed how to install a vapor barrier here.


A vapor barrier is what is more commonly now known as a ‘vapor retarder’. And it cuts down the speed of water vapor moving through a material. Permeability or ‘perms’ are the units that are used to measure how much a material can hold back water vapor diffusion. This allows us to control the humidity in the room.

Vapor barriers are extremely effective in controlling moisture in:

  1. Ceilings
  2. Walls
  3. Floors
  4. Basements
  5. Crawlspaces
  6. Slab-on-grade foundations

Types of vapor barriers

There are three classes of Vapor Barriers. Have a look at what they are:

CLASS I (Permeability 0.1 or less)

  • Glass
  • Polyethylene sheet
  • Non-perforated aluminum foil
  • Sheet metal
  • Rubber membrane
  • Foil-faced insulated sheathing

CLASS II (Permeability greater than 0.1 but less than or equal to 1) 

  • Plywood
  • Polystyrene extrusion or unexposed distended polystyrene
  • Kraft paper coated with bitumen
  • 30-pound asphalt coated paper
  • Fiber faced polyisocyanurate

CLASS III (Permeability greater than 1 but less than or equal to 10)

  • Brick
  • Gypsum board
  • House wrap
  • Cellulose insulation
  • Concrete block
  • Unfaced fiberglass insulation
  • Board lumber

Any other material with a permeance rating of more than 10 is a vapor-permeable membrane. These materials are still evolving. There are some discussions about the best practices as builders keep learning how materials react to the weather.

What is the need for vapor barriers? When to use in the USA

Vapor barriers have become an important part of the construction of a building. They help to stop the growth of molds and rotting of building materials. Otherwise, water vapor can reach places like attics, roofs, ceilings, walls, or crawl spaces of a building. This condensation and intrusion of water vapor is a menace for even a strongly built structure. It affects the efficacy of insulation too. You can save yourself from huge recurring repair costs. Install a vapor barrier while building your house in an area that needs one. 

Installation of Vapor Barrier

It is not very easy to install an insulation vapor barrier. Therefore, it will be a smart move to install it while constructing a building. If you think you will install it later during a remodeling, that is going to be a lot more challenging. But if you are living in a house that you want to remodel, you should consider installing a vapor retarder. It will be extra protection for your home. You have to choose the best vapor barrier type and class based on your location. 

You should select materials with a higher permeable rate if you live in an exceptionally humid and moist climate. If you live amidst a moderate climate, you can go for semi-permeable materials. Furthermore, if you live in a colder climate, you should ideally install the vapor barrier facing the interior of the wall. In case you reside in a hotter climate, install it towards the exterior. 

During installation, you have to be careful about sealing every opening and gap. This is crucial if you do not want to let your insulation material absorb moisture in the future. Otherwise, your vapor barrier’s insulation performance will be affected. There are some cheaper alternatives if you do not have the budget for a proper vapor barrier. You can, for instance, apply acrylic or glossy paint on your walls.

Image of a vapor barrier. Image by PlumbingIntantFix, all rights reserved

There are two correct ways of installing a vapor barrier. The first is to buy an installation material that already has a vapor barrier affixed to it. There is an increasing need for homeowners to protect their homes from moisture. Owing to the demand, sellers now offer vapor-resistant insulation materials. This method cuts off the expense of buying these two things separately and installing them.

The second method is better if you do not have vapor-resistant insulation material. This is to add a vapor barrier on top of the insulation. You can apply unfaced insulation into cavity walls and then install plastic sheeting as covering. In this case, you need to install a vapor barrier on top of the insulation material. 

Difference between a Vapor Barrier & a Vapor Retarder

In current times, vapor barrier and vapor retarder are considered to be synonyms of each other. But this is not completely true. There is, albeit tiny, a difference between the two. A layer that has 0.1 perms or less can be considered to be a vapor barrier. In fact, an ideal vapor barrier should have a permeance rating of 0.00. 

In earlier days, even the construction industry referred to Polyethylene as a vapor barrier. But it is actually a water retarder. This is because any polyethylene-based material installed below concrete is unable to put a stop to the spreading of water vapor completely. These materials are helpful in the retardation or reduction of water vapor transmission. Therefore, the more appropriate term for such materials is ‘vapor retarders’ and not ‘vapor barriers’. Thus, in the truest sense, a product cannot be considered to be a vapor barrier if it does not have a permeance rate of 0.1 or less. 

Therefore, out of all the classes that have been mentioned at the beginning of this article, only Class I materials are vapor barrier products. Any material that has a permeance rating of 0.1 or less has an impermeable membrane. The rest are vapor retarders. 

However, if you are still confused, remember the terms do not matter. You have to assess the climate and evaluate which material will work in controlling vapor diffusion, which is the ingress of water vapor into the building


Do not get confused and think that air barriers are vapor barriers too. Because air contains water vapor, that doesn’t mean air barriers can stop the diffusion of water vapor. A vapor barrier does not have the job of preventing air movement.

Its purpose is to stop the transmission of water vapor through the air. Also, a vapor retarder allows some transportation of vapor through it. But a vapor barrier does not allow any back and forth passing of water vapor through it. In many cases, all your home might need is an energy assessment and then sealing all the leaks that the assessment shows. In addition, you can use a ‘vapor diffusion retarder’ or a ‘vapor barrier’ paint on the walls.