Restoration Guide: Exterior Wall Vapor Retarders

Editor's Note: This is article 5 of 18 in the Exterior Walls Chapter of the Old House Web Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.

5. AIR INFILTRATION BARRIERS AND VAPOR RETARDERS

Section 1--The Basics

Vapor diffusion is critical in any wall system to prevent excessive moisture buildup. Vapor diffusion describes the process of water vapor spreading or moving through permeable materials. It is caused by a difference in water vapor pressure and can result in moisture infiltrating your walls.

Installing a vapor retarder in a home minimizes the amount of vapor that can spread into a wall or roof, where it can build up condensation and decay interior wall coverings. Moisture buildup also breeds mildew and mold, which can cause serious health hazards.

1.1: Where to Install Vapor Retarders

Where vapor barriers are installed depends upon the region of the country. In climates with average temperatures above 65 degrees, vapor retarders are best placed on the interior side of the insulation. However, in humid climates, such as those found along the Gulf Coast where moisture is more likely to travel inward, vapor retarders should be placed closer to the exterior. In many regions throughout the U.S., vapor retarders are placed against framing members prior to installation of exterior wall claddings.

Don't install vapor barriers on both sides of a wall--it defeats the purpose of the vapor retarder since the opposite wall must be a permeable surface to allow moisture to dry.

Section 2--Types of Vapor Retarders

There are two types of vapor retarders: flexible or coatings.

Flexible vapor retarders:

• Metal foil
• Plastic film
• Treated paper--primarily asphalt saturated felt
• Laminated foils

Coatings:

• Paint
• Mastic
• Hot melt

In residential construction, exterior- or interior-applied plastic film is one of the most widely used vapor retarders. However, many interior vapor retarders also include foil-faced insulation or treated paper-faced products.

Section 3--Increasing Vapor Diffusion if You Can't Disturb Exterior or Interior Surfaces

In some home restoration projects, especially on historic homes, it's either impossible or cost-prohibitive to remove wall cladding materials. It's possible to increase vapor diffusion by laying down a coating of latex vapor retarder paint. Manufacturers such as Sherwin-Williams and Glidden both make this product, which can be applied over gypsum or previously painted surfaces.

Section 4--Treated/Foil Paper Vapor Retarders

Asphalt-treated felt paper or foil-faced insulation are typically used for most home restoration work. Both are cost-effective ways to increase moisture evaporation, but neither product is the best on the market. In climates with average temperatures above 65 degrees, a more continuous vapor retarder should be used. If walls are stripped to bare framing, consider using a plastic-film retarder.

Section 5--Polyethylene Vapor Retarders

Plastic polyethylene barrier films are usually clear or black, the latter providing additional ultraviolet protection. Clear plastic film is a cheap vapor retarder, but it's also easy to tear or puncture. It's most often applied with staples over exposed wall studs. Keep joints to a minimum, and tape all seams and penetrations, such as electrical boxes. This material is best used during home restoration when interior surfaces are removed.

Black polyethylene is best for exterior surfaces in hot, humid climates. As with clear poly, all joints, seams and penetrations should be taped for maximum effectiveness.

Fiber-reinforced polyethylene is best for home renovation projects that have rough and irregular surfaces, such as exposed nail heads on exterior sheathing or splintered/sharp-edged sheathing.

Section 6--Air Infiltration Barriers

Air infiltration barriers are commonly called housewraps, and products such as Tyvek by DuPont have gained tremendous market share since their inception in the 1970s. Housewrap plays a critical role in energy efficiency by providing a barrier to prevent hot and cold air from entering your home. Housewrap prevents air from infiltrating wall surfaces, but it doesn't form an impenetrable vapor barrier so moisture still can escape. Housewraps are either perforated or non-perforated.

Certain types of housewraps, such as polyethylene-based products, are perforated to allow for vapor migration. Many housewraps are code-approved to substitute for any stipulated vapor protection barrier.

One of the main considerations for using housewrap is its strength. The material can withstand a great deal of pulling and stretching, and it won't tear when nailed or stapled.

6.1: Installing Housewrap

In the majority of home restoration work, housewrap is placed over exterior sheathing. Housewrap is typically sold in rolls that are 9 feet long and can be cut to fit with a circular saw if necessary. Start at an outside corner, stapling heavily to keep the wrap in place, and continue around the house making sure the roll remains plumb. Use a 2x4 inch stud to make sure interior corners are properly sealed, or the wall cladding could tear the housewrap.

Extend the bottom of the wrap 2 inches over the foundation. Vertical laps should be at least 6 inches.

Housewrap and vapor barriers can help protect your walls and keep weather from penetrating your home. The next chapter, Insulation, delves further into ways to keep the elements out of your home.