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Code guide

Vapour Barrier Requirements in Ontario

Vapour barriers in the 2024 Ontario Building Code: required in insulated assemblies, warm side, 60 ng permeance, poly standards, basements, air barrier vs vapour barrier.
maximum permeance, dry cup (per Pa·s·m²)
60 ng
maximum permeance, dry cup (per Pa·s·m²)
where the vapour barrier goes
Warm side
where the vapour barrier goes
wet cup minimum for variable-permeance basement barriers
300 ng
wet cup minimum for variable-permeance basement barriers
lap for clamped poly air barrier joints
100 mm
lap for clamped poly air barrier joints

Last updated

Under the 2024 Ontario Building Code, every thermally insulated wall, ceiling and floor must include a vapour barrier (OBC Div. B 9.25.4.1.OBC Div. B 9.25.4.1. Required Barrier to Vapour Diffusion(1) Thermally insulated wall, ceiling and floor assemblies shall be constructed with a vapour barrier so as to provide a barrier to diffusion of water vapour from the interior into wall spaces, floor spaces or attic or roof spaces.) on the warm side of the insulation (9.25.4.3.OBC Div. B 9.25.4.3. Installation of Vapour Barriers(1) Products installed to function as the vapour barrier shall protect the warm side of wall, ceiling and floor assemblies. (2) Where different products are used for the vapour barrier and the insulation, the vapour barrier shall be installed sufficiently close to the warm side of the insulation to prevent…), made of a material with a permeance of not more than 60 ng/(Pa·s·m²) by the dry cup test (9.25.4.2.OBC Div. B 9.25.4.2. Vapour Barrier Materials(1) Except as provided in Sentence (2), vapour barriers shall have a permeance not greater than 60 ng/(Pa×s×m²) measured in accordance with ASTM E96 / E96 M, "Standard Test Methods for Water Vapor Transmission of Materials," using the desiccant method (dry cup). (2) Thermally insulated foundation wall assemblies are…). Insulated basement walls may use a variable-permeance membrane instead.

This page explains the vapour barrier rules in Section 9.25 for houses and other Part 9 buildings, and how they differ from the air barrier rules. Your building department decides how the code applies to your project.

Vapour barrier rules at a glance

Item Rule in short Where to look
Where required insulated walls, ceilings and floors 9.25.4.1.
Material not more than 60 ng/(Pa·s·m²), dry cup 9.25.4.2.(1)
Insulated foundation walls variable permeance allowed: 60 dry cup, over 300 wet cup 9.25.4.2.(2)
High-moisture spaces design under Part 5 9.25.4.2.(3)
Polyethylene CAN/CGSB-51.34-M clauses 4.4 and 5.7 9.25.4.2.(4)
Other membranes CAN/CGSB-51.33-M 9.25.4.2.(5)
Location warm side, close enough to prevent condensation 9.25.4.3.
Protection covered where it could be damaged 9.25.2.3.(7)
Air barrier continuous, sealed or lapped and clamped 9.25.3.1., 9.25.3.3.

This is our summary of the articles, not a copy of the code. Read the full text before you build.

Why a vapour barrier

Warm indoor air holds moisture. In winter, water vapour moves outward through the wall or ceiling toward the cold side, and if it reaches a surface below its dew point, it condenses inside the assembly, where it can rot framing and ruin insulation. The vapour barrier limits that diffusion from the interior into wall, floor, attic and roof spaces (9.25.4.1.OBC Div. B 9.25.4.1. Required Barrier to Vapour Diffusion(1) Thermally insulated wall, ceiling and floor assemblies shall be constructed with a vapour barrier so as to provide a barrier to diffusion of water vapour from the interior into wall spaces, floor spaces or attic or roof spaces.).

What material qualifies

A vapour barrier must have a water vapour permeance of not more than 60 ng/(Pa·s·m²), tested by the desiccant (dry cup) method in ASTM E96 (9.25.4.2.(1)OBC Div. B 9.25.4.2. Vapour Barrier Materials(1) Except as provided in Sentence (2), vapour barriers shall have a permeance not greater than 60 ng/(Pa×s×m²) measured in accordance with ASTM E96 / E96 M, "Standard Test Methods for Water Vapor Transmission of Materials," using the desiccant method (dry cup). (2) Thermally insulated foundation wall assemblies are…). Common choices:

  • Polyethylene. Where poly serves only as the vapour barrier, it must meet the thermal stability and oxidative induction clauses of CAN/CGSB-51.34-M (9.25.4.2.(4)OBC Div. B 9.25.4.2. Vapour Barrier Materials(1) Except as provided in Sentence (2), vapour barriers shall have a permeance not greater than 60 ng/(Pa×s×m²) measured in accordance with ASTM E96 / E96 M, "Standard Test Methods for Water Vapor Transmission of Materials," using the desiccant method (dry cup). (2) Thermally insulated foundation wall assemblies are…).
  • Other membranes, which must meet CAN/CGSB-51.33-M, and be covered or kept out of direct sunlight if UV can harm them (9.25.4.2.(5) and (6)OBC Div. B 9.25.4.2. Vapour Barrier Materials(1) Except as provided in Sentence (2), vapour barriers shall have a permeance not greater than 60 ng/(Pa×s×m²) measured in accordance with ASTM E96 / E96 M, "Standard Test Methods for Water Vapor Transmission of Materials," using the desiccant method (dry cup). (2) Thermally insulated foundation wall assemblies are…).
  • Coatings and some sheet or panel materials, such as vapour-retarder paint or foil-faced insulation, where they meet the permeance limit. The code also controls where low-permeance sheathing can go on the cold side, so the wall does not trap moisture between two barriers (9.25.5.1.OBC Div. B 9.25.5.1. General(1) Except as provided in Sentences (2) to (4), sheet and panel-type materials incorporated into assemblies described in Article 9.25.1.1. shall conform to Article 9.25.5.2. where (a) the material has (i) an air leakage characteristic less than 0.1L/(s × m^2) at 75 Pa, and (ii) a water vapour permeance less than 60…).

Spaces with high moisture generation, such as an indoor pool, need the assembly designed under Part 5 (9.25.4.2.(3)OBC Div. B 9.25.4.2. Vapour Barrier Materials(1) Except as provided in Sentence (2), vapour barriers shall have a permeance not greater than 60 ng/(Pa×s×m²) measured in accordance with ASTM E96 / E96 M, "Standard Test Methods for Water Vapor Transmission of Materials," using the desiccant method (dry cup). (2) Thermally insulated foundation wall assemblies are…).

Where it goes

The vapour barrier must protect the warm side of wall, ceiling and floor assemblies (9.25.4.3.(1)OBC Div. B 9.25.4.3. Installation of Vapour Barriers(1) Products installed to function as the vapour barrier shall protect the warm side of wall, ceiling and floor assemblies. (2) Where different products are used for the vapour barrier and the insulation, the vapour barrier shall be installed sufficiently close to the warm side of the insulation to prevent…). Where it is a separate product from the insulation, it must sit close enough to the warm side of the insulation to prevent condensation at design conditions (9.25.4.3.(2)OBC Div. B 9.25.4.3. Installation of Vapour Barriers(1) Products installed to function as the vapour barrier shall protect the warm side of wall, ceiling and floor assemblies. (2) Where different products are used for the vapour barrier and the insulation, the vapour barrier shall be installed sufficiently close to the warm side of the insulation to prevent…). In practice, in Ontario, that means just behind the drywall, on the room side of the studs.

Where the vapour barrier could be damaged, cover it with gypsum board, plywood, OSB or a similar finish (9.25.2.3.(7)OBC Div. B 9.25.2.3. Installation of Thermal Insulation(1) Insulation shall be installed so that there is a reasonably uniform insulating value over the entire face of the insulated area. (2) Insulation shall be applied to the full width and length of the space between furring or framing. (3) Except where the insulation provides the principal resistance to air leakage…).

Basement walls

An insulated basement wall needs a vapour barrier like any other insulated assembly. Below grade, though, moisture can also move inward from the concrete in summer. The code therefore lets thermally insulated foundation walls use a variable-permeance vapour barrier, with a permeance of not more than 60 ng/(Pa·s·m²) by the dry cup method and more than 300 by the wet cup method (9.25.4.2.(2)OBC Div. B 9.25.4.2. Vapour Barrier Materials(1) Except as provided in Sentence (2), vapour barriers shall have a permeance not greater than 60 ng/(Pa×s×m²) measured in accordance with ASTM E96 / E96 M, "Standard Test Methods for Water Vapor Transmission of Materials," using the desiccant method (dry cup). (2) Thermally insulated foundation wall assemblies are…). It stays tight in dry winter conditions and opens up when humid, so the wall can dry toward the room.

In unfinished basements, mineral fibre insulation covered with a membrane that meets Section 9.25.4. does not need a gypsum covering (9.25.2.3.(7.1)OBC Div. B 9.25.2.3. Installation of Thermal Insulation(1) Insulation shall be installed so that there is a reasonably uniform insulating value over the entire face of the insulated area. (2) Insulation shall be applied to the full width and length of the space between furring or framing. (3) Except where the insulation provides the principal resistance to air leakage…). See the basement insulation guide for R-values and the basement requirements guide for the rest.

Vapour barrier vs air barrier

The two are often the same sheet of poly, but they are separate requirements:

  • Air barrier. Assemblies separating heated space from unheated space or the ground need a continuous air barrier system that stops air leaking in or out, and it must extend throughout the basement (9.25.3.1.OBC Div. B 9.25.3.1. Required Barrier to Air Leakage(1) Wall, ceiling and floor assemblies that separate conditioned space from unconditioned space or from the ground shall be constructed so as to include an air barrier system that will provide a continuous barrier to air leakage (a) from the interior of the building into wall, floor, attic or roof spaces sufficient to…). Air leaks carry far more moisture than diffusion.
  • Sealing joints. Flexible sheet air barriers need joints sealed with tape or sealant, or lapped at least 100 mm and clamped between framing and rigid panels, and joints behind tubs, showers and fireplaces must be sealed (9.25.3.3.(2) and (2.1)OBC Div. B 9.25.3.3. Continuity of the Air Barrier System(1) Where the air barrier system consists of an air-impermeable panel-type material, all joints shall be sealed to prevent air leakage. (2) Where the air barrier system consists of flexible sheet material, all joints shall be (a) sealed with compatible material such as tape or flexible sealant, or (b) except as…).
  • Poly doing both. A vapour barrier used as the air barrier must meet the air barrier rules too (9.25.3.3A.OBC Div. B 9.25.3.3A. Vapour Barriers Used as Air Barriers(1) A vapour barrier used as an air barrier shall comply with the requirements of this Subsection.).

Common mistakes

  1. Vapour barrier on the cold side. It belongs on the warm, interior side.
  2. Torn or unsealed poly. Gaps at outlets, top plates and rim joists leak air and moisture.
  3. Two barriers trapping a wall. Low-permeance sheathing outside and poly inside can trap moisture unless the wall is designed for it.
  4. Plain poly on a basement wall where a variable-permeance membrane would let it dry.
  5. Exposed membranes in finished rooms. Cover them where they could be damaged.

How to look it up yourself

Open Division B, Part 9 of the 2024 Compendium. Section 9.25 covers heat transfer, air leakage and condensation control. Search terms that appear in the code text:

  • "Required Barrier to Vapour Diffusion" (9.25.4.1.)
  • "Vapour Barrier Materials" (9.25.4.2.)
  • "Installation of Vapour Barriers" (9.25.4.3.)
  • "Continuity of the Air Barrier System" (9.25.3.3.)

Read the article in the code viewer alongside the installation article.

BCIN exam angle

Vapour barrier permeance, the warm-side rule and the 100 mm lap for air barrier joints are common House exam questions. Study the House chapter on heat transfer, air leakage and energy efficiency, then try the free quiz on the House exam page.

Based on the official 2024 Building Code Compendium on ontario.ca.

© King's Printer for Ontario, 2024. Reproduced with permission.

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Frequently asked questions

Is a vapour barrier required in Ontario?
Yes. Thermally insulated wall, ceiling and floor assemblies must have a vapour barrier that limits the diffusion of water vapour from the inside into the wall, floor, attic or roof spaces (Division B, Article 9.25.4.1.).
Which side does the vapour barrier go on?
The warm side, which in Ontario's heating climate is the inside. It must be close enough to the warm side of the insulation to prevent condensation at design conditions (Division B, Article 9.25.4.3.).
What counts as a vapour barrier?
A material with a water vapour permeance of not more than 60 ng/(Pa·s·m²), measured by the dry cup method in ASTM E96. Polyethylene used only as the vapour barrier must meet the thermal stability and oxidative induction clauses of CAN/CGSB-51.34-M, and other membranes must meet CAN/CGSB-51.33-M (Division B, Article 9.25.4.2.).
Do basement walls need a vapour barrier in Ontario?
An insulated basement wall is an insulated assembly, so it needs a vapour barrier. Insulated foundation walls may use a variable-permeance vapour barrier: not more than 60 ng/(Pa·s·m²) dry cup and more than 300 wet cup, which lets the wall dry inward in summer (Division B, Sentence 9.25.4.2.(2)).
Is a vapour barrier the same as an air barrier?
No. A vapour barrier slows water vapour diffusing through materials; an air barrier stops air leaking through gaps. Air leakage carries far more moisture, so the air barrier must be continuous and sealed. Polyethylene can do both jobs if its joints are sealed or lapped and clamped as the air barrier rules require (Division B, Subsection 9.25.3.).