Air and Vapor Barriers for Beginners

2025-04-01

Understanding Air and Vapor Barriers

Source: Air Barriers vs Vapor Barriers | You NEED To Know The Difference

Air Barriers

• Keep outside air from leaking into your home.
• Help prevent warm, humid air from meeting cooler surfaces indoors (which can lead to condensation).

Vapor Barriers

• Slow down the movement of moisture through materials.
• Limit how much water vapor passes through walls.

Why It Matters

• Even a small gap in drywall can let a significant amount of moist air move behind your walls, leading to potential mold or rot issues.
• Air leakage is often a bigger threat than vapor diffusion, so sealing cracks or holes is essential.

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Experiment Results

• Vapor diffusing through a 4×8 sheet of drywall:
  315 mm of water = 0.3 liters
• If there is a 1-square-inch hole in the drywall:
  28390 mm of water = 28.3 liters

Note: A material’s vapor permeability (its ability to let vapor pass) does not automatically make it a good air barrier. Both air and vapor barriers work together to keep your home comfortable and protected from moisture damage.