The Perfect Duo: Fiberglass Insulation and Vapor Retarders


It’s impossible to prevent water vapor’s natural tendency to penetrate building exteriors. If moisture is left unattended, it can cause serious problems, compromising structural integrity and sustainability. Maintaining a dry building envelope is crucial for any successful sustainable design project. 

When trapped inside a wall cavity for an extended period of time, moisture can cause many building materials, such as wood, traditional paper-faced gypsum, and steel, to eventually deteriorate or corrode. In addition, wet insulation loses its R-value and greatly diminishes the energy efficiency of the building. 

The major problem that moisture causes is mold growth. It can lower indoor air quality and have a negative impact on the comfort and respiratory health of occupants. Mold occurs when airborne spores make contact with wood- or paper-based materials in the wall assembly. When exposed to moisture for an extended time, wood- and paper-based materials are a prime food source for mold. 

Fortunately, all of these potential problems can be controlled with proper moisture management strategies. Many of the best strategies for wall assemblies begin with two high-performance components: fiberglass insulation and a vapor retarder.

Fiberglass Insulation

Fiberglass is widely known for its high thermal resistance, fire resistance, and acoustical performance; it is also an effective weapon against moisture and mold damage. Fiberglass insulation neither absorbs nor holds water and will not deteriorate from extended moisture exposure. Being an inert material, it is a non-viable food source for mold and will not support mold growth.

Fiberglass insulation cannot fight the battle against moisture intrusion alone. It needs the added protection of a vapor retarder. 

Vapor Retarder

Usually composed of a thin film, such as polyethylene, vapor retarders prevent water vapor diffusion into a wall, ceiling, or floor during the colder months. The performance of these materials is measured by their water vapor transmission rate or permeance. Permeance is the speed at which a measured amount of water vapor transfers through a known surface area under standard atmospheric pressure conditions. Materials are separated into three general classes based on their permeance:

Class I – Vapor Impermeable is any material rated a .01 perm or less, such as rubber membranes, polyethylene film, glass, aluminum foil, sheet metal, foil-faced insulating sheathing, and foil-faced non-insulating sheathings. 

Class II – Vapor Semi-Impermeable is a material with the permeance of 1.0 or less or greater than .01, meaning it will allow some water vapor transfer. These materials include oil-based paints, most vinyl wall coverings, unfaced extruded polystyrene greater than 1” thick, and traditional hard-coat stucco applied over building paper and oriented strand board (OSB) sheathing.

Class III – Vapor Semi-Permeable is ideal for warmer climates so moisture can escape the building. These materials will have a rating of 10 perms or less and greater than 1.0. Such materials include plywood, bitumen-impregnated kraft paper, unfaced expanded polystyrene (EPS), unfaced extruded polystyrene (XPS)— 1” thick or less, fiber-faced isocyanurate, heavy asphalt-impregnated building papers (#30 building paper), and most latex-based paints.

In addition to fiberglass insulation and vapor retarders, the most successful moisture management strategies include:

  • A water-resistive retarder
  • An interior air barrier
  • An exterior wind barrier to reduce the potential for seasonal surface condensation and rainwater penetration

However, by incorporating smart vapor control products into the overall design, building and design professionals can simplify their moisture management strategy. CertainTeed’s MemBrain™ Smart Vapor Retarder & Air Barrier Film has adaptable permeance that allows the MemBrain to retard moisture under dry conditions. This allows building and design professionals to prevent the entrance of moisture and allow cavity moisture to escape while maintaining a building’s air barrier performance, all with one product.

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

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