Vapor Retarders, Air Barriers, and Moisture Control

Vapour Retarders, Air Barriers, and Moisture Control

Introduction: Why This Chapter Is Critical

In thermal insulation, moisture management is just as important as the insulating value itself. Wet insulation loses up to 90% of its thermal effectiveness. The vapour retarder and the air barrier are two distinct systems that work together to protect the building envelope. The Red Seal exam requires you to understand not only their function, but also their placement, materials, permeance calculations, and the requirements of the National Building Code (NBC).

This chapter covers the physical principles, material types, installation rules, vapour diffusion resistance calculations, and classic exam traps.

1. Fundamental Principles of Moisture Migration

1.1 The Three Transport Mechanisms

Moisture moves through a building by three distinct mechanisms:

Mechanism	Driving Force	Typical Speed	Controlled By
Vapour diffusion	Vapour pressure difference (Fick's law)	Slow (a few grams/m²/day)	Vapour retarder (low permeance)
Air convection	Air pressure difference (stack effect, wind, fans)	Fast (several hundred grams/m²/day)	Air barrier (airtightness)
Capillarity	Surface tension in material pores	Medium	Capillary barrier (flashing, membranes)

Key point for the exam: Air convection transports 100 to 1000 times more water vapour than diffusion. A defective air barrier is therefore more damaging than a missing vapour retarder.

1.2 Vapour Pressure and Dew Point

Water vapour exerts a partial pressure in the air. Warm, humid air contains more vapour than cold, dry air. In winter, warm indoor air has a higher vapour pressure than cold outdoor air — vapour therefore migrates from the inside to the outside.

The dew point is the temperature at which air reaches 100% relative humidity and vapour condenses into liquid water. In a wall, condensation occurs when the temperature inside the wall drops below the dew point of the air within it.

Saturated vapour pressure formula (approximation):

P_sat ≈ 610.5 × e^(17.269 × T / (237.3 + T))

Where T is in °C and P_sat is in Pa.

Example: Indoor air at 21 °C, 40% RH → Vapour pressure = 0.40 × 2487 Pa ≈ 995 Pa. Outdoor air at −15 °C, 80% RH → Vapour pressure = 0.80 × 165 Pa ≈ 132 Pa. Difference = 863 Pa → migration toward the outside.

1.3 Permeance and Permeability

Permeance (designated by the letter M or sometimes W) measures how easily water vapour passes through a material. It is expressed in ng/(s·m²·Pa) or in perms (grains/(h·ft²·inHg)).

Unit	Conversion
1 perm	≈ 57.2 ng/(s·m²·Pa)
1 ng/(s·m²·Pa)	≈ 0.0175 perm

Classifications according to the NBC:

Class	Permeance (ng/(s·m²·Pa))	Permeance (perms)	Role
Class I vapour retarder	≤ 15	≤ 0.26	Blocks almost all diffusion
Class II vapour retarder	15 to 60	0.26 to 1.0	Strongly slows diffusion
Class III vapour retarder	60 to 170	1.0 to 3.0	Allows limited drying
Vapour barrier (obsolete term)	—	—	Replaced by "vapour retarder" in the NBC

Exam trap: The 2015 and 2020 NBC uses the term vapour retarder, not "vapour barrier." Vapour barrier was the old term for a Class I material. Do not confuse it with air barrier.

2. The Vapour Retarder

2.1 Definition and Function

The vapour retarder is a material or assembly that limits water vapour diffusion through a wall. It does NOT control air leakage — that is the role of the air barrier.

Primary function: To prevent water vapour diffusing from the warm, humid interior from reaching the dew point in the insulated cavity and condensing there.

2.2 Placement in the Wall Assembly

General rule: The vapour retarder is placed on the warm side (interior in cold Canadian climates) of the insulation, within the inner ⅓ of the total thermal resistance.

The ⅓-⅔ rule: At least ⅓ of the total thermal resistance (RSI value) must be on the exterior side of the vapour retarder. This keeps the temperature at the vapour retarder above the dew point.

Calculation example: Wall with a total RSI of 4.0. The vapour retarder must be placed so that at least RSI 1.33 is on the exterior side of it.

2.3 Vapour Retarder Materials

Material	Permeance (ng/(s·m²·Pa))	Class	Typical Use
6 mil polyethylene (0.15 mm)	0.3 to 1.0	I	Walls, ceilings, floors
4 mil polyethylene (0.10 mm)	1.0 to 3.0	I	Less critical areas
Asphalt-coated kraft paper (two-sided)	10 to 40	II	Batt insulation with integral vapour retarder
Kraft paper (one-sided)	100 to 300	III	Drying permitted
Aluminum foil (on insulation)	< 1.0	I	Reflector + vapour retarder
Latex paint (2 coats)	170 to 340	III	Renovation, existing wall
Oil-based primer paint (2 coats)	30 to 60	II	Renovation
Self-adhering membrane (Ice & Water Shield)	< 1.0	I	Roofing, foundations

Exam trap: 6 mil polyethylene is the reference material for Class I vapour retarders, but its use is not always mandatory. The NBC allows Class II or III vapour retarders depending on the climate zone and type of construction.

2.4 Vapour Retarder Installation

Installation rules per the NBC (Article 9.25.4.2):

38.Warm side: Always on the interior (heated) side in Canadian climates.

39.Continuity: All joints must be sealed with a compatible sealant or approved tape.

40.Penetrations: All penetrations (pipes, wires, electrical outlets) must be hermetically sealed.

41.Support: The vapour retarder must be supported to prevent it from tearing under its own weight or during insulation installation.

42.Overlap: Sheets must overlap by at least 100 mm (4 in) at joints.

43.No through-stapling: Staples must not pass completely through the vapour retarder — this creates perforations. Use furring strips or staples with washers.

Classic exam error: You NEVER install two vapour retarders in the same wall (one on each side). This traps moisture and prevents drying. Only one vapour retarder, on the warm side.

2.5 Vapour Retarder in Roofs

In cathedral ceilings (no attic), the vapour retarder is placed below the insulation, on the interior side. In roofs with a ventilated attic, the vapour retarder is placed at the ceiling level of the top floor, above the interior walls.

Attic ventilation requirement: The NBC requires attic ventilation of at least 1/300 of the ceiling area (distributed between soffits and ridge) when a vapour retarder is installed at the ceiling. If the vapour retarder is absent or Class III, ventilation must be 1/150.

3. The Air Barrier

3.1 Definition and Function

The air barrier is a complete system (materials + assemblies + joints) that controls air movement between the interior and exterior. It prevents humid air convection into the insulated cavity.

Crucial difference from the vapour retarder:

Characteristic	Vapour Retarder	Air Barrier
Controls	Vapour diffusion	Air convection
Mechanism	Low permeance	Airtightness
Position	Warm side	Can be anywhere in the wall
Typical material	Polyethylene	Membrane, panel, assembly
Test	Permeance (ng/(s·m²·Pa))	Air leakage (L/(s·m²) at 75 Pa)

Essential point: 6 mil polyethylene can serve as both an air barrier AND a vapour retarder if installed continuously and sealed. But an air barrier can be a vapour-permeable material (e.g., a permeable air barrier membrane) — it blocks air, not diffusion.

3.2 NBC Requirements (Article 9.25.3.1)

The air barrier must:

57.Be continuous throughout the entire building (walls, roof, floor).

58.Withstand wind pressures and pressure differences (at least 750 Pa without failure).

59.Be durable (not degrade over time).

60.Be supported and fastened to resist wind loads.

61.Be sealed at all penetrations and intersections.

3.3 Types of Air Barriers

Type	Material	Air Permeance	Advantages	Disadvantages
Membrane	Polyethylene, permeable air barrier membrane (Membrain, Siga Majpell)	Very low	Flexible, easy to install	Must be protected
Rigid panel	OSB, plywood, gypsum board	Low	Structural, easy to seal	Joints must be sealed
Assembly	Concrete, masonry, coated concrete block	Low	Durable	Difficult to make continuous
Sprayed	Spray polyurethane foam (SPF)	Low	Insulates + air barrier	High cost, expertise required
Foam panel	XPS, polyisocyanurate with sealed joints	Low	Insulates + air barrier	Cost, joints

Exam trap: Gypsum board is an excellent air barrier if joints are treated (tape + compound). But it is NOT a vapour retarder — its permeance is very high (approximately 600 ng/(s·m²·Pa)).

3.4 Exterior Air Barrier

In modern construction, the air barrier is often placed on the exterior of the structure (over the sheathing). This allows:

Easier continuity at wall-roof intersections.

Protection of the structure from air infiltration.

Drying toward the exterior possible if the air barrier is vapour-permeable.

Permeable air barrier membrane: These membranes (e.g., Tyvek, Blueskin VP100) allow water vapour to pass through (permeance > 170 ng/(s·m²·Pa)) but block air. They allow the cavity to dry toward the exterior.

Golden rule: Never use a vapour-impermeable membrane on both sides of the wall. At least one side must be permeable to allow drying.

4. Calculations and Applications

4.1 Calculating Vapour Diffusion Resistance

The vapour diffusion resistance (Z) of an assembly is:

Z = Σ (thickness / permeability)

Or more simply, for layers in series:

Z_total = Z₁ + Z₂ + Z₃ + ...

Where each Z = 1 / permeance of the layer (in s·m²·Pa/ng).

Example: Wall with 6 mil polyethylene (permeance = 1 ng/(s·m²·Pa)) and interior gypsum (permeance = 600 ng/(s·m²·Pa)).

Z_poly = 1 / 1 = 1.0 s·m²·Pa/ng

Z_gypsum = 1 / 600 = 0.0017 s·m²·Pa/ng

Z_total = 1.0017 s·m²·Pa/ng

The polyethylene completely dominates the diffusion resistance.

4.2 Calculating Condensation in a Wall

Graphical method (Glaser): Plot the temperature curve and the saturated vapour pressure curve through the wall. If the actual vapour pressure exceeds the saturated pressure, condensation occurs.

Simplified version for the exam:

88.Calculate the temperature drop through each layer (proportional to the RSI thermal resistance).

89.Calculate the saturated vapour pressure at each interface (table or formula).

90.Calculate the actual vapour pressure (proportional to the diffusion resistance Z).

91.If P_vapour > P_sat at any interface → condensation.

Numerical example:

Wall: interior 21 °C, 40% RH → exterior −20 °C, 80% RH.

Layer	RSI	Interface temperature (°C)	P_sat (Pa)
Interior air	—	21	2487
Gypsum 13 mm	0.08	20.5	2405
Polyethylene	0	20.5	2405
Insulation RSI 3.5	3.5	−19.5	132
Exterior sheathing	0.1	−19.8	125
Exterior air	—	−20	125

Actual vapour pressure: interior = 0.40 × 2487 = 995 Pa. Exterior = 0.80 × 125 = 100 Pa.

The vapour pressure drops mainly through the polyethylene (high Z resistance). On the interior side of the polyethylene, P_vapour ≈ 995 Pa. On the exterior side, P_vapour ≈ 100 Pa. Since 995 Pa < 2405 Pa (P_sat at 20.5 °C), no condensation at the polyethylene. In the insulation, the temperature drops rapidly — check at mid-insulation: T ≈ 0 °C, P_sat ≈ 611 Pa. P_vapour in the insulation ≈ 100 Pa (because the polyethylene has already blocked diffusion). 100 < 611 → no condensation. ✅

Without a vapour retarder: P_vapour in the insulation would be ≈ 500 Pa at mid-wall. At 0 °C, P_sat = 611 Pa → no condensation. But at ⅔ of the insulation depth, T ≈ −10 °C, P_sat ≈ 260 Pa. P_vapour ≈ 300 Pa → condensation! ❌

4.3 Calculating Vapour Flow Rate (Fick's Law)

Q = M × A × ΔP

Where:

Q = vapour flow rate (ng/s)

M = permeance (ng/(s·m²·Pa))

A = area (m²)

ΔP = vapour pressure difference (Pa)

Example: Area of 100 m², permeance of 1 ng/(s·m²·Pa), ΔP = 800 Pa.

Q = 1 × 100 × 800 = 80,000 ng/s = 0.08 g/s ≈ 6.9 kg/day

With a Class I vapour retarder, the flow rate is negligible. Without a vapour retarder (gypsum only, M = 600), Q = 600 × 100 × 800 = 48,000,000 ng/s ≈ 4.1 tonnes/day — but this calculation ignores the resistance of the other layers.

5. National Building Code (NBC) Requirements

5.1 Article 9.25.4.2 — Vapour Retarder

The NBC requires a vapour retarder in low-rise residential construction (Part 9) when:

The design outdoor temperature is below 0 °C.

Insulation is installed in walls, floors, or ceilings separating the heated space from the exterior or from an unheated space.

Exceptions: Concrete or masonry foundation walls may not require a vapour retarder if the insulation is placed on the exterior.

5.2 Article 9.25.3.1 — Air Barrier

The air barrier is mandatory in all buildings. It must be:

Continuous.

Resistant to wind pressures (750 Pa minimum).

Sealed at all penetrations.

5.3 Climate Zones and Vapour Retarder Classes

The NBC divides Canada into climate zones (1 to 7). For zones 5, 6, and 7 (most inhabited regions), a Class I or II vapour retarder is required in walls. For zones 4 and lower, a Class III vapour retarder may suffice.

Climate Zone	Example City	Required Vapour Retarder
4	Vancouver	Class III or II
5	Toronto, Montréal	Class II or I
6	Ottawa, Québec City	Class I
7	Winnipeg, Edmonton	Class I

Exam trap: The 2020 NBC introduced stricter requirements for zones 6 and 7, notably the requirement for Class I vapour retarders in cathedral ceilings.

5.4 Insulation of Pipes and Ducts

For plumbing pipes and HVAC ducts:

Cold water pipes must be insulated with an integral vapour retarder (vapour seal) to prevent condensation in summer.

Hot water and steam pipes must have a vapour retarder on the exterior side of the insulation.

Air conditioning ducts must be insulated with an air barrier/vapour retarder to prevent condensation.

Typical materials: Nitrile rubber foam (Armaflex), mineral wool with aluminum facing (ASJ), polyethylene foam.

6. Special Applications

6.1 Cathedral Ceilings and Flat Roofs

In cathedral ceilings:

The vapour retarder is placed below the insulation, on the interior side.

A minimum 25 mm (1 in) ventilation space must be provided between the insulation and the roof deck if the insulation is not a rigid panel with an integral air barrier.

For flat roofs with a waterproofing membrane, the vapour retarder is placed below the insulation, above the deck.

Rule: In a flat roof, the vapour retarder is ALWAYS on the warm side of the insulation (i.e., below the insulation, above the metal or concrete deck).

6.2 Foundation Walls

Concrete foundation walls are vapour-permeable. Insulation can be placed:

On the exterior: A vapour retarder is not required, but a capillary barrier (membrane) is necessary.

On the interior: A Class I or II vapour retarder is required on the interior (warm) side of the insulation.

Trap: Never install a vapour retarder on both sides of a foundation wall — this traps moisture in the concrete and causes deterioration.

6.3 Indoor Pools and Spas

Indoor pools have very high relative humidity (60 to 80%). The vapour retarder must be:

Class I (polyethylene or aluminum membrane).

Installed on the interior side of the insulation.

Perfectly continuous, with all joints sealed.

Special rule: In indoor pools, the interior vapour pressure is so high that the vapour retarder must be reinforced and all penetrations sealed with silicone sealant.

7. Traps to Avoid

151.Confusing air barrier and vapour retarder — These are two distinct systems. The air barrier controls air; the vapour retarder controls vapour.

152.Using the term "vapour barrier" — The NBC uses "vapour retarder." "Vapour barrier" is an obsolete term.

153.Placing the vapour retarder on the wrong side — Always on the warm side (interior) in Canadian climates. Except for foundation walls with exterior insulation.

154.Installing two vapour retarders — Never install one on each side of the wall. This prevents drying.

155.Forgetting to seal penetrations — Electrical outlets, pipes, and wires must be sealed. A 5 mm hole in the vapour retarder negates its effectiveness over 1 m².

156.Using polyethylene as an air barrier without sealing it — Polyethylene is only an air barrier if all joints are sealed and penetrations are airtight.

157.Ignoring attic ventilation — If the vapour retarder is at the ceiling, ventilation must be 1/300. Without a vapour retarder, 1/150.

158.Calculating permeance while forgetting the units — The NBC uses ng/(s·m²·Pa). Manufacturers sometimes use perms. Conversion: 1 perm = 57.2 ng/(s·m²·Pa).

159.Believing gypsum is a vapour retarder — Gypsum is a good air barrier (if joints are treated) but a very poor vapour retarder (permeance ≈ 600).

160.Neglecting drying — A wall must be able to dry in at least one direction. Do not block both sides with impermeable materials.

8. Summary

Three moisture transport mechanisms: diffusion (slow), convection (fast), capillarity (medium). Convection dominates by far.

Vapour retarder: Controls diffusion. Class I (≤ 15 ng/(s·m²·Pa)), Class II (15-60), Class III (60-170). Always on the warm side.

Air barrier: Controls convection. Must be continuous, durable, resistant to 750 Pa. Can be vapour-permeable.

⅓-⅔ rule: At least ⅓ of the thermal resistance on the exterior side of the vapour retarder.

Calculations: Fick's law (Q = M × A × ΔP), Glaser method for condensation, perm ↔ ng/(s·m²·Pa) conversion.

NBC: Article 9.25.3.1 (air barrier), 9.25.4.2 (vapour retarder). Climate zones 5-7 → Class I or II.

Golden rule: One vapour retarder per wall, on the warm side. Allow drying in at least one direction.

9. Self-Assessment Questions

172.What is the fundamental difference between an air barrier and a vapour retarder?

173.A material with a permeance of 30 ng/(s·m²·Pa) is which class?

174.Why should you never install two vapour retarders in the same wall?

175.What is the minimum ventilation for an attic with a vapour retarder at the ceiling?

176.What is the vapour flow rate through a Class I vapour retarder (M = 15) over 200 m² with ΔP = 900 Pa?

177.Where is the vapour retarder placed in a flat roof?

178.Is gypsum board a good vapour retarder? Why?

179.What is the conversion between perms and ng/(s·m²·Pa)?

Answers: 1) Air vs vapour. 2) Class II. 3) Prevents drying. 4) 1/300. 5) Q = 15 × 200 × 900 = 2,700,000 ng/s = 2.7 g/s ≈ 233 kg/day (too high — a true Class I would be M ≤ 1). 6) Below the insulation, above the deck. 7) No, permeance ≈ 600. 8) 1 perm = 57.2 ng/(s·m²·Pa).

This chapter covers the full range of knowledge required for the "Vapour Retarders, Air Barriers, and Moisture Control" section of the Red Seal exam. Review the calculations and classifications until they become automatic.