Insulation Fundamentals: Vapor Retarders


Fiberglass insulation offers many great benefits to a home, including improving acoustics, increasing energy efficiency, and not posing a fire hazard. However, the advantages of fiberglass can also be optimized with the added protection of a vapor retarder. When added after insulation is installed, a vapor retarder can prevent water damage. 

The Value of a Vapor Retarder

According to the Building Science Corporation, moist water is attracted to the cooler side of building assemblies; so, the water tries to escape to the outside in the cooler months. As the water vapor migrates outdoors, it passes through the inner wall and insulation, settling on the wood sheathing of the home’s exterior. From there, the water can soak into the wood, penetrating the insulation and causing a decrease in R-value or cause deterioration of the plaster/drywall. 

Installing a vapor retarder will protect your home from accumulating water that can lead to mold, mildew, and rotting. A vapor retarder is any material that limits the transmission of water vapor. If you’re using unfaced fiberglass insulation, a suitable vapor retarder is a 2-mil nylon film called MemBrainTM. Or, you can buy faced fiberglass insulation, which has the vapor retarder already attached.

During the summer, the humidity of the wall rises. This causes an inward drive of moisture condensation. Without a vapor retarder in place, the wall is exposed to the trapped water vapor in the cavity, putting your home’s wall structure at risk of mold, mildew, and damage. 

Materials Impermeable to Water Vapor 

Vapor retarders should have a permeance of .01 or less. The lower the perm rating, the better the material will control the diffusion of water. Below are materials that are impermeable to water. 

Foil-Faced Insulation is typically installed in attics and walls. It acts as a durable seal against water vapor, preventing water migration through the batts.

Aluminum Foil is tear-resistant, and heat and radiant reflective, adding greater thermal efficiency to your home. The sun’s radiant energy causes your roof to get hot; that heat then travels through the roofing materials and makes its way to the attic. According to Energy Saver, once the heat reaches the aluminum foil vapor retarder, the temperature reduces, keeping your attic cool.

Polyethylene Film has a low permeability, stopping water vapor from traveling through your home. Our MemBrain Smart Vapor Retarder reacts to low and high humidity. It remains tight in the winter to prevent moisture from entering a wall cavity and has an increased permeability in the summer, allowing the water to escape. 

Where to Install a Vapor Retarder 

The use of an interior vapor retarder is dependent on certain climate zones. Generally, they are required in colder environments, specifically in climate zones 4 through 8

A vapor retarder should be installed over the fiberglass insulation and face your living space. This positioning will keep the moisture out. During application, be careful not to compress the fiberglass insulation. This will cause the insulation to lose its R-value. 

Ceilings. When considering where you want to install a vapor retarder, look up. Ceilings need to be protected against water vapor because they link your home from room to room. Adding protection to the ceiling will prevent moisture from traveling to your upstairs living space. Our multipurpose insulation, CertaPro®, is a light-density fiberglass batt with a kraft, foil, or a foil scrim kraft (FSK) fire-resistant vapor retarder facing. CertaPro is perfect for insulating your ceilings and floors while keeping the threat of moisture away. 

Walls. Areas that are exposed frequently to water vapor (i.e. kitchen and bathroom) must include a vapor retarder to prevent mold growth. Exterior building envelopes, including walls separating the conditioned space from the non-conditioned space, such as the garage, are required per building code to install an interior vapor retarder. 

If you have any questions, we’re always available to assist you. Don’t hesitate to reach out!

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