Air Barriers vs. Vapour Barriers: What Homeowners Should Know
Air barriers and vapour barriers are important parts of a home’s building envelope, but they perform different jobs. Understanding the difference can help homeowners recognize why gaps, damaged materials and poorly completed renovations may contribute to drafts, condensation and moisture damage.
An air barrier controls the movement of air through walls, ceilings and other parts of the building envelope. A vapour barrier, also called a vapour retarder, limits the movement of water vapour through building materials.
What Is an Air Barrier?
An air barrier is a continuous system intended to reduce uncontrolled air leakage through the building envelope.
Air leakage can carry heat and moisture through openings around:
- Windows and exterior doors.
- Electrical boxes and light fixtures.
- Plumbing and wiring penetrations.
- Attic hatches.
- Wall-to-floor and wall-to-ceiling connections.
- Mechanical ducts and exhaust vents.
An effective air barrier must be continuous and properly sealed at joints, transitions and penetrations. Materials that can form part of an air-barrier system include house wrap, exterior sheathing, drywall, polyethylene and other airtight materials when their seams and connections are properly sealed.
What Is a Vapour Barrier?
A vapour barrier limits the diffusion of water vapour through walls, ceilings and insulated assemblies.
In cold Canadian climates, the vapour barrier is generally located on or near the warm interior side of the insulation. Its purpose is to reduce the amount of indoor moisture that can move into colder portions of the wall or ceiling and condense.
Common vapour-control materials include:
- Polyethylene sheeting.
- Smart vapour-retarder membranes.
- Foil-faced materials.
- Approved vapour-barrier paints.
- Certain types of rigid insulation and sheathing.
The correct material and location depend on the wall assembly, climate, insulation configuration and intended drying potential.
Air Leakage and Vapour Diffusion Are Different
Air leakage occurs when air moves through openings in the building envelope. Vapour diffusion occurs when water vapour moves through a material because of differences in vapour pressure.
A small opening can allow a significant amount of warm, moisture-laden air to enter a cold wall or attic space. When that air cools, moisture may condense on colder materials.
For this reason, maintaining a continuous air barrier is an important part of controlling heat loss and moisture movement.
Can One Material Perform Both Functions?
Yes. Certain materials can act as both an air barrier and a vapour barrier when they meet the required performance characteristics and are installed continuously.
Polyethylene sheeting is a common example. It can resist vapour diffusion and control airflow when all seams, edges, penetrations and connections are properly sealed.
However, simply installing polyethylene does not automatically create an effective air-barrier system. Open seams, damaged areas and unsealed electrical or plumbing penetrations can allow air leakage.
What Is a Weather Barrier?
A weather barrier or water-resistive barrier is normally installed behind exterior cladding. It helps manage rainwater that passes behind siding, brick veneer or other exterior finishes.
The exterior wall assembly should provide a drainage path that allows incidental water to move downward and exit safely. Depending on its design and installation, exterior sheathing or house wrap may also form part of the air-barrier system.
Common Air- and Vapour-Barrier Problems
Conditions commonly observed during home inspections include:
- Torn or missing polyethylene sheeting.
- Unsealed joints and overlapping seams.
- Openings around electrical boxes.
- Gaps around plumbing and wiring penetrations.
- Poorly sealed attic hatches.
- Discontinuities where additions meet the original building.
- Missing air sealing around windows and doors.
- Bathroom or kitchen exhaust ducts terminating in the attic.
- Insulation installed without proper air or vapour control.
- Multiple low-permeance layers that restrict drying.
Why Condensation Develops
Warm interior air can contain considerable moisture from cooking, bathing, laundry, occupants and humidifiers. If this air reaches a cold surface within a wall, ceiling or attic, the moisture may condense.
Repeated condensation can contribute to:
- Wet or compressed insulation.
- Wood deterioration.
- Corrosion of metal fasteners.
- Staining on ceilings or walls.
- Mould-like growth.
- Reduced insulation performance.
Indoor humidity, ventilation, insulation levels, exterior temperatures and air leakage can all influence condensation.
Renovations Require Careful Planning
Adding insulation or finishing a basement can change how an existing wall manages heat and moisture. Installing impermeable materials in the wrong location may trap moisture between layers and reduce the assembly’s ability to dry.
Before modifying an exterior wall, basement wall or attic assembly, the complete construction should be considered. Complex or unusual assemblies should be reviewed by a qualified building-envelope professional.
What Can a Home Inspection Identify?
A home inspection can identify visible and accessible concerns involving insulation, air leakage, vapour-control materials, attic ventilation, condensation and moisture staining.
The inspection is visual and non-invasive. It cannot confirm concealed materials or the continuity of barriers located behind finished walls and ceilings.
Where significant moisture, condensation or questionable renovations are observed, further investigation by a qualified contractor or building-envelope specialist may be recommended.
Learn More About Air and Moisture Control
Natural Resources Canada provides additional guidance on air leakage, vapour control and insulation in its Keeping the Heat In homeowner guide.
Protect Your Home’s Building Envelope
Inspec-Pro Home Inspections provides professional residential inspections throughout Ottawa, Gatineau, Outaouais and surrounding communities.
Call 819-962-5880, email info@inspec-pro.ca, or request a home inspection quotation.