Vapor Retarders & Air Barriers

Introduction

Water can exist in three different states; solid, liquid, and gas. Solid and liquid are the two most common when thinking about preventing water intrusion with a building, however, water as a gas (or vapor) is just as much of a concern. Water vapor can enter a building by air movement as well as diffusion through materials and assemblies.

• Note: ‘vapor’/’gas’ for this article purpose are going to be used interchangeably and as the same definition.

Relative Humidity & Moisture In Air

Water vapor is always present in air everywhere across the globe; the concept of 0% humidity is an impossibility. The amount of water that can be ‘held’ (exist) in air depends on the air's temperature.

• Warm air is able to hold more water than cold air.

Relative humidity is the ratio (percentage) of the water vapor of air to its saturation vapor pressure. Represented as a percentage, it's the amount of water vapor in the air compared to how much it can hold at that temperature.

• Water vapor can enter a building by air movement and through materials.

• 25-30% humidity is generally considered the best for humans and buildings. It provides a little bit of moisture, but not too much.

• 30-50% is generally considered comfortable.

• 60%+ will feel uncomfortable

• 20-70% is generally considered the tolerable range. More or less and it will be uncomfortable over time.

• 100% Humidity means that it has reached the saturation point/dew point. The air is no longer able to hold any more water.

• 70% humidity means that the air holds 70% of the water it can hold at that temperature.

• Amount of moisture in the air by weight per given weight of air.

• Cold air holds less water than warm air. Therefore, if it's cold outside, 90% relative humidity = less water vapor in the air than if it's warm outside with 90% relative humidity.