Vapor permeance, permeability, vapor barriers, vapor retarders and perm ratings. It’s all pretty confusing, isn’t it? In this article, I’m going to explain the science of vapor movement, the perm ratings system and rating of specific building materials.
Key Differences in Building Environments:
The exterior walls of a building play a crucial role in controlling four main differences between the conditioned indoor and unconditioned outdoor environments:
- Water Control: Managed through cladding, siding, windows, doors, flashing, and water resistive barriers like Tyvek and ZIP systems.
- Air Control: Achieved with plywood or OSB sheathing, certain insulation types, plastic sheets, caulk, tape, and fluid-applied weather resistive barriers.
- Temperature Control: Controlled using insulation materials such as fiberglass, Rockwool, spray foam, denim, hemp wool, and cellulose.
- Water Vapor Control: Regulated by the arrangement of material layers within the wall assembly.
The Significance of Vapor Control:
As construction methods evolve towards more air-tight homes with continuous spray foam insulation and inoperable windows, managing water vapor becomes critical. These changes result in higher relative humidity levels inside homes, which can lead to mold and mildew growth. Thus, inventing new methods and materials to prevent condensation is essential.
Understanding Key Terms:
- Relative Humidity: The percentage of water in the air relative to its maximum capacity at a given temperature. Ideal indoor relative humidity should range between 30% to 50% to avoid issues like rot, mold, and structural damage.
- Dew Point: The temperature at which water vapor condenses into liquid. Preventing condensation inside walls requires placing vapor barriers or retarders at specific locations before warm air cools down and condenses.
- Vapor Diffusion: The movement of water vapor molecules through porous materials. Vapor typically moves from areas of high vapor pressure (warm air) to low vapor pressure (cold air), depending on the climate.
Perm Ratings and Material Classification:
Perm ratings measure a material’s ability to allow water vapor to pass through. Lower perm ratings indicate less vapor permeability, while higher ratings suggest greater permeability. Materials are classified as follows:
- Vapor Impermeable (0.1 perms or less): Includes glass, 1 mil aluminum foil, cork, rubber, vinyl flooring, 6mil polyethylene, sheet metal, and foil-faced polyiso.
- Semi-Impermeable (0.1 to 1.0 perms): Comprises vapor retarder latex paint, oil-based paint, asphalt-coated kraft paper, foil-faced fiberglass insulation, 2” closed cell spray foam, XPS, and plain kraft facing on insulation.
- Semi-Permeable (1.0 to 10 perms): Includes 3” EPS, OSB, tar paper, 2” MDF wood fiber board, fluid applied vapor retarders, typical latex paint, and plywood sheathing.
- Vapor Permeable (Over 10 perms): Encompasses wood siding, Zip coating, plaster on metal lath, mineral wool, cellulose insulation, brick veneer, Tyvek, and Rockwool.
Optimal Vapor Control and Material Selection:
Understanding perm ratings helps in selecting the right materials for specific climate zones and building requirements. It is vital to ensure effective vapor control to prevent issues like mold growth and damage to the building envelope.
In summary, mastering the science of vapor control empowers builders and architects to create well-insulated, energy-efficient, and moisture-resistant structures. Properly chosen vapor retarders and barriers contribute significantly to maintaining a healthy indoor environment and enhancing the overall longevity of the building. For further information and a helpful graphic comparing the vapor permeance of various exterior insulations, refer to the provided link in the video description.
By staying informed and implementing the right vapor control strategies, we can build smarter and more sustainable structures that stand the test of time.
