Quick answer
In Ontario homes, spray foam insulation acts as an effective air barrier and, when installed at the recommended thickness, also serves as a vapour barrier. An additional vapour barrier is only needed in specific situations, such as when open‑cell foam is used in a cold‑climate wall cavity without sufficient thickness.
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Understanding air barriers and vapour barriers
What is an air barrier?
An air barrier stops uncontrolled airflow through the building envelope. It prevents drafts, reduces heat loss, and limits the movement of moisture‑laden air that can condense inside wall or roof assemblies. Materials such as closed‑cell spray foam, rigid foam board, or specialised membranes are commonly used as air barriers.
What is a vapour barrier?
A vapour barrier resists the diffusion of water vapour. In cold climates, it is placed on the warm side of the insulation to keep moisture from entering the colder cavity where it could condense. Vapour barriers are typically thin plastic sheeting (6 mil polyethylene) or a material that has a low permeance rating.
How spray foam provides both functions
Spray foam comes in two main types:
- Open‑cell spray foam – R‑3.7 per inch, ideal for attic floors and interior wall cavities. It is breathable, so it does not act as a class‑II vapour retarder at typical thicknesses.
- Closed‑cell spray foam – R‑6 per inch, dense and water‑resistant. At 2 inches or more it meets the class‑II vapour retarder requirement in Ontario, while also sealing all air leaks.
When closed‑cell foam is installed to the depth required for the desired R‑value, it creates a continuous air‑tight seal and, at the appropriate thickness, stops vapour diffusion. Open‑cell foam creates an excellent air barrier but may still allow vapour to pass, so a separate vapour barrier could be required in colder wall assemblies.
When an additional vapour barrier is required
| Situation | Recommended spray foam type | Need for extra vapour barrier? |
|---|---|---|
| Exterior walls in cold‑climate zones, 2‑inches or more of closed‑cell foam | Closed‑cell (≥2 in) | No |
| Interior walls where only 1‑inch of closed‑cell foam is used | Closed‑cell (≤1 in) | Yes, add a vapour‑retarder on the warm side |
| Attic floors with open‑cell foam (typical 3‑4 in) | Open‑cell | No, because the attic is ventilated; vapour barrier not required |
| Basement or crawl‑space walls with open‑cell foam | Open‑cell | Yes, especially if the space is unconditioned; a vapour barrier on the interior side helps control moisture |
Spray Foam Kings installs closed‑cell foam to meet the Ontario Building Code SB‑12 requirements on every project, ensuring the foam thickness aligns with the local climate zone. For projects where open‑cell foam is chosen, we assess whether a separate vapour barrier is advisable and install one if needed.
Comparison of air barrier and vapour barrier functions
| Characteristic | Air Barrier | Vapour Barrier |
|---|---|---|
| Primary purpose | Stops uncontrolled airflow | Stops water‑vapour diffusion |
| Typical material | Closed‑cell spray foam, rigid foam board, membrane | Polyethylene sheeting, foil‑faced board, thick closed‑cell foam |
| Effect on energy use | Reduces heat loss/gain by eliminating drafts | Prevents condensation that can degrade insulation performance |
| Installation location | Throughout the building envelope (walls, roof, floor) | Warm side of insulation in cold‑climate assemblies |
| Interaction with spray foam | Closed‑cell spray foam *is* an air barrier | Closed‑cell foam ≥2 in acts as a vapour barrier; otherwise a separate layer may be added |
Cost considerations
Spray Foam Kings offers transparent, itemised pricing for all foam installations. The cost of the foam itself and the labour to achieve a continuous air‑tight seal are the main drivers of the overall price.
Price / cost factors
| Factor | Why it matters | Price (as listed) |
|---|---|---|
| Foam type (open‑cell vs closed‑cell) | Determines R‑value per inch and vapour‑retardancy | Open‑cell $1.50–$3.50 / sq ft installed; Closed‑cell $2.50–$5.00 / sq ft installed |
| Thickness required for vapour control | More inches increase material cost but may eliminate the need for a separate vapour barrier | Example: 2 in closed‑cell for R‑12 (≈$5 / sq ft) |
| Project size | Larger areas benefit from economies of scale | Typical 1,200 sq ft attic at R‑20: Open‑cell $2,800–$4,200; Closed‑cell $6,000–$8,400 |
| Access and removal of existing insulation | Removal adds labour and disposal fees | Attic removal $400–$800 for 1,200 sq ft |
| Rebates and incentives | Enbridge HER+ and Canada Greener Homes programmes can offset up to $10,600 | Handled by Spray Foam Kings, paperwork included |
Typical installation process
Diagram: illustrative overview, not a photo of a job.
Key factors to consider
Diagram: illustrative overview, not a photo of a job.
Frequently asked questions
Do you need a vapour barrier with spray foam?
When closed‑cell spray foam is installed at 2 inches or more, it meets the class‑II vapour‑retarder standard for Ontario, so an additional vapour barrier is not required. If open‑cell foam is used, or closed‑cell foam is less than 2 inches in a cold‑climate wall, a separate vapour barrier on the warm side of the assembly is advisable.
Do you need a vapour barrier with rigid foam?
Rigid foam board can act as a vapour barrier if it has a low permeance rating (typically <1 perm). In Ontario, when rigid foam is installed to the thickness required for the climate zone, it often eliminates the need for a separate vapour barrier. However, if the board’s permeance is higher or the installation does not achieve a continuous seal, a vapour barrier may still be needed.
How does closed‑cell spray foam compare to a traditional vapour barrier?
Closed‑cell spray foam provides both air‑tightness and vapour resistance in a single material when applied at the proper thickness. It also adds structural strength and has a lifespan of 25–30 years under normal conditions. A traditional vapour barrier only stops vapour diffusion and must be paired with an air barrier for full envelope performance.
Can I rely on open‑cell foam alone for air sealing in my attic?
Yes. Open‑cell foam expands to fill gaps and creates an effective air barrier in attic floors and slopes. Because attics are usually ventilated, a separate vapour barrier is not required in most Ontario attic applications. For conditioned attics, a vapour barrier may be considered.
What rebates are available for spray foam projects in Ontario?
Spray Foam Kings is a registered Enbridge HER+ partner and can help you claim up to $10,600 in rebates. The company also assists with the Canada Greener Homes program paperwork. These incentives can be applied to both open‑cell and closed‑cell installations.
Where can I learn more about thermal barriers for spray foam in Ontario?
For details on the need for a thermal barrier over spray foam, see the page “Does Spray Foam Need a Thermal Barrier in Ontario?” at https://sprayfoamkings.ca/does-spray-foam-need-thermal-barrier-ontario/. Additional information about vapour barriers with spray foam is available at https://sprayfoamkings.ca/do-you-need-vapor-barrier-spray-foam-ontario/.
By understanding the roles of air barriers and vapour barriers, and how Spray Foam Kings’ products meet both functions, homeowners across the Greater Toronto Area can make informed decisions that protect comfort, durability, and energy efficiency.
Get a free written quote from Spray Foam Kings.
Last updated November 6, 2026.