Best Vapor Barrier Products For Exterior Walls

If you’re building a new house or adding an addition to your home, you may look into vapor barriers. Do your exterior walls need a vapor barrier? If so, what product should you choose to deliver that barrier?

South Central Services has insulated hundreds of new and existing homes in and around Mechanicsburg, Pennsylvania. The question of vapor barriers is a complex one, but we are committed to understanding building science and applying these barriers appropriately.

By the end of this article, you will understand:

• Why your exterior walls might not need a vapor barrier
• What products are used to create vapor barriers for exterior walls

Don't have time to read right now? Check out everything you need to know at a glance.

Do Your Exterior Walls Even Need A Vapor Barrier?

Vapor transfer is a tricky topic in building science. Vapor barriers and air barriers are two different things. A vapor barrier is also not necessarily a waterproofing technique, though it can be in some situations.

The purpose of vapor barriers is to prevent vapor transfer through a building assembly. Since warm air holds more moisture than cold air, heat moving to cold can result in the movement of vapor. This movement of vapor can lead to condensation. Exterior walls with vapor barriers try to prevent moisture from condensing in the wall and causing mildew and mold.

Your exterior walls might not need a vapor barrier, depending on where you live. Climates with distinct seasons and significant temperature differences are the most likely to need a vapor barrier.

However, the need for vapor barriers also changes depending on the design of your home. Vapor barriers are one piece of a puzzle that makes up a building. Changing other building materials can influence what products should be used. You may even want to be open to vapor transfer, depending on the wall or roof assembly.

We recommend contacting a professional contractor to discuss the need for vapor barriers in your exterior walls. If you’re building a new home, consult your general or insulation contractor about your climate zone and design needs. For those building onto an existing home, discuss with your project manager or reach out to another professional.

To prepare you for your conversation with your contractor, here is a list of standard products used to create vapor barriers in exterior walls.

1. Fluid-Applied Vapor Barrier

Fluid-applied vapor barriers start as a liquid. This type of product can be applied in many different new construction assemblies. Manufacturers give clear installation instructions, but generally speaking, this product is applied similarly to mudding drywall.

This product from Prosoco can be used on seams, in corners, and around windows and plumbing penetrations.

Some benefits of fluid-applied vapor barriers include longevity and ease of installation. Many fluid-applied vapor barriers can withstand weather conditions during construction for an extended period of time. These products can also be installed by a professional or by a handy homeowner.

2. House Wrap

House wrap is one of the most recognizable vapor barriers in new builds. If you’ve driven by a house under construction recently, you’ve likely seen branded plastic wrapped around the building.

Not all house wraps are vapor barriers. For example, this Tyvek HomeWrap from DuPont specifies that it allows vapor to permeate but stops liquid water.

3. Foam Board

Rigid foam board is another product that can create a vapor barrier. Some foam boards are vapor closed and others are vapor open. The right foam board will not allow vapor to permeate through it. However, it is important to note that improper installation can compromise the vapor barrier foam board offers.

Inevitably, foam board insulation will have seams where the boards meet. No matter how tight that space is, it is not technically sealed without caulking or can foam. Foam boards can be used on exterior walls to create a vapor barrier, but the seams and joints of the foam boards must be sealed.

Foam board can be installed as a DIY. Be sure to purchase a foam board that qualifies as a vapor barrier and to seal any edges with either caulking or can foam.

4. Closed Cell Spray Foam

Closed cell spray foam insulation is the most versatile type of foam insulation. With as few as 2 inches of product, closed cell foam can create a seamless vapor barrier. When installed correctly and at the proper depths, closed cell spray foam creates a vapor barrier.

For an exterior wall, closed cell could be applied to the interior or exterior side of the wall. When closed cell is installed on the outside of a building, it does require additional coatings to protect it from UV rays it may be exposed to.

A potential downside to using closed cell spray foam is the need to hire a professional. Spray foam insulation is sold in DIY kits but should not be used for projects to this scale. The likelihood of a DIY kit delivering undesirable results is high. It is better to find a qualified contractor who can discuss the need for a vapor barrier and apply the foam correctly.

The Bottom Line About Vapor Barrier Products For Exterior Walls

Exterior walls often need vapor barriers, especially in our climate zone. As a homeowner, it can be incredibly difficult to determine the need for a vapor barrier on your own. Get in contact with a building professional to discuss whether or not your walls need vapor barriers.

When you have this conversation with your general contractor or insulation contractor, ask if they would recommend fluid-applied, foam board, or spray foam based on the construction of your home and your weather patterns.

Now that you understand what products are used to create vapor barriers for exterior walls, your next step is to:

• Discover where insulation belongs in a new construction home
• Weigh the pros and cons of hybrid insulation for your exterior walls
• Learn why homeowners trust South Central Service for new home insulation