Study Notes for ETCC 243: Air & Water Vapor Envelope

Chapter 6: Air & Water Vapor Envelope

Course Details

  • Course Title: ETCC 243

  • Topic: Building Material & Systems

Introduction to Air

  • Components of Outside Air:

    • Dust

    • Pollen

    • Ozone

    • Other pollutants

  • **Important Concepts: **

    • Entry/Exit dynamics related to infiltration and exfiltration.

Water and Water Vapor

  • Definitions:

    • Water: A liquid form of H₂O.

    • Water Vapor: A gaseous form of H₂O; generally occurs in combination with air.

  • Key Interaction:

    • Leakage of air through the building envelope allows for the leakage of water vapor.

Leakage and Diffusion

  • Leakage:

    • The migration of a gas (like air and water vapor) through gaps, cracks, and voids in the building envelope (container).

  • Diffusion:

    • The migration of air and water vapor even when the envelope is fully sealed without gaps and voids, meaning there’s no leakage.

Visual Representation

  • Concepts of Leakage and Diffusion

    • Repeatedly illustrated to emphasize understanding of their differences and implications for building design.

Factors Affecting Air Leakage

  1. Leakage Area:

    • Size of gaps, cracks, and openings in the building envelope.

  2. Pressure Difference:

    • Caused by external factors:

      • Wind

      • Temperature

Air Pressure Differences
Wind-induced Pressure Differences

  • Results in air leakage (infiltration and exfiltration) through the building envelope driven by wind pressure.

Temperature-induced Pressure Differences

  • Results in air leakage due to the temperature difference between the inside and outside air, affecting infiltration and exfiltration.

Air Barrier Mechanics

  • Function of Air Barriers:

    • Reduce air diffusion by utilizing a barrier that allows for water vapor passage, but is not entirely impermeable to air.

    • More appropriately termed as a retarder rather than an actual barrier.

  • Terminology:

    • Commonly referred to as an air barrier or weather barrier.

Regulatory Standards

  • International Energy Conservation Code (IECC):

    • Mandates the inclusion of air barriers in building design.

Characteristics of Air Barriers

  • Essential Properties:

    1. Air Impermeability: Should allow minimal or no air passage.

    2. Water Resistivity: Functions as an air-weather barrier; must be water-repellant (hydrophobic).

    3. Vapor Permeability: Needs to be vapor permeable to prevent condensation.

Air-Weather Barrier Configurations

  • Location in Wall Assemblies:

    • Typically positioned within assemblies framed with wood or cold-formed steel (CFS).

Membrane Air-Weather Barriers

  • Example:

    • Membrane air-weather barrier placed over the exterior wall sheathing of a five-story wood frame apartment building.

Sealing Techniques for Windows and Doors

  • Implementation:

    • Use of self-adhering flashing tape over air-weather barriers at window sills, jambs, and heads to prevent air and water ingress.

Relationship Between Air and Vapor Diffusion

  • Independence of Diffusion Types:

    • Diffusion of air and vapor occur independently and are controlled by individual pressure differences across the envelope.

    • Air diffusion is influenced by air pressure differences, while vapor diffusion responds to vapor pressure differences.

Dalton’s Law of Partial Pressures

  • Concept Overview:

    • Although air and water vapor are mixed, they exert pressure on boundaries independently.

Migration of Water Vapor

  • Two Mechanisms:

    1. Vapor Leakage:

    • Through holes and cracks within the assembly.

    1. Vapor Diffusion:

    • Through the material of the assembly itself.

Dew Point Temperature

  • Definition:

    • The temperature at which the air's relative humidity (RH) reaches 100%.

  • Condensation Phenomenon:

    • If the air temperature decreases below the dew point, the water vapor condenses into liquid water.

Types of Condensation

  • Concealed Condensation:

    • Occurs inside the envelope assembly.

  • Surface Condensation:

    • Occurs on the exterior surface of the envelope, such as glass on windows.

Prevention of Surface Condensation

  • Method:

    • Increase the R-value of the assembly to ensure that the dew point is located within the insulation, rather than at the surface.

Vapor Retarders

  • Functionality:

    • Enclose an interior space completely to prevent interior vapor from permeating into the assembly.

  • Design Requirements:

    • The assembly beyond the vapor retarder must be vapor permeable to prevent condensation from any small, unintended vapor permeation.

Code Requirements for Vapor Retarders

  • Specifications:

    • Class I or II vapor retarders are required on all interior surfaces of exterior walls framed with wood or cold-formed steel in climate zones 5, 6, 7, 8, and marine 4.

    • These codes aim to limit the migration of interior vapor into building assemblies to prevent condensation.

Understanding Condensation in Relation to Relative Humidity

  • Key Insight:

    • Condensation occurs when relative humidity reaches 100%.

  • Influence of Temperature:

    • Relative humidity can change with temperature variations while the moisture content remains constant.

  • Pressure Dynamics:

    • Water vapor tends to move from high pressure (inside) to low pressure (outside).

Vapor Retarder Classification

  • Table of Vapor Retarders (Approximately):

    • Class I: perm rating ≤ 0.1 perm

    • Class II: perm rating between 0.1 and 1.0 perm

    • Class III: perm rating between 1.0 and 10.0 perm

Problems Related to Ice Dams

  • Consequences:

    • Significant increase in load on eave overhangs and gutters.

    • Restricts proper drainage leading to potential roof leaks.

  • Preventative Measures:

    • Adequate ceiling insulation and attic ventilation to mitigate the formation of ice dams and water vapor buildup.

Attic Ventilation: Importance and Approaches

  • Ventilation Requirement:

    • Adequate attic ventilation necessitates cross ventilation by providing both intake and exhaust ventilation.

  • Ventilation Mechanics:

    • Warm air naturally rises, thus exhaust ventilation should be positioned at a higher elevation within the attic space.

  • Exhaust Ventilation Alternatives:

    • Gable ventilators

    • Ridge ventilators

    • Turbine ventilators

    • Gable fans

Summary of Ventilation Techniques

  • Visual Representations:

    • Various configurations for attic ventilation that include combinations of soffit and gable vents, ridge vents, and turbine vents for effective airflow management.