Quick answer
In most Ontario attics, closed‑cell spray foam acts as its own vapour barrier, so an additional sheet is not required. Open‑cell spray foam is vapour‑permeable and does not replace a barrier; a separate vapour‑control layer may be needed depending on climate and roof design. Rigid foam boards also require a vapour barrier unless the board’s rating and installation method provide sufficient resistance.
Get a free written quote from Spray Foam Kings.
What is a vapour barrier and why it matters in an attic
A vapour barrier is a material that limits the movement of water vapour through building assemblies. In an attic it helps prevent moisture from the living space below from reaching cold roof sheathing, where it could condense and cause mould, rot, or reduced insulation performance. The effectiveness of a barrier depends on its permeability rating and how it is sealed at seams, penetrations, and edges.
Spray foam and vapour control
Open‑cell spray foam
- Permeability: Open‑cell foam is semi‑permeable, allowing moisture to pass through.
- Typical use: Ideal for attic floor cavities where the primary goal is air sealing and modest insulation (R‑3.7 per inch).
- Vapour barrier requirement: Because it does not stop vapour, a separate barrier (e.g., polyethylene sheeting) is often installed on the warm side of the foam when local climate conditions or roof assembly call for it.
Closed‑cell spray foam
- Permeability: Closed‑cell foam is a class‑II vapour retarder at 2 inches or more, effectively acting as a vapour barrier.
- Typical use: Preferred for attic slopes, cathedral ceilings, and situations where both high R‑value (R‑6 per inch) and vapour control are needed.
- Vapour barrier requirement: When installed at the recommended thickness, an additional vapour barrier is generally unnecessary.
| Foam Type | R‑Value per inch | Vapour Barrier Rating | Typical Attic Application |
|---|---|---|---|
| Open‑cell | R‑3.7 | Permeable (does not act as barrier) | Attic floor cavities, interior walls |
| Closed‑cell | R‑6 | Class‑II vapour retarder at ≥2 in | Attic slopes, cathedral ceilings, rim joist sealing |
Rigid foam boards and vapour barriers
Rigid foam board (e.g., EPS, XPS, polyiso) can be used on attic roofs or walls. Its vapour‑control properties depend on thickness and material type. In Ontario, a minimum of 2 inches of XPS or polyiso typically provides a vapour barrier, but many installers still add a separate polyethylene layer to ensure continuity at seams and penetrations.
Installation process used by Spray Foam Kings
PROCESS: Inspection & measurement > Removal of existing insulation (if required) > Air‑sealing & surface preparation > Application of spray foam (open‑ or closed‑cell) > Post‑install ventilation & clean‑up
Step‑by‑step details
- Inspection & measurement – A certified technician evaluates the attic layout, existing insulation, and moisture conditions, then provides a written, itemised quote.
- Removal of existing insulation – When needed, old fiberglass or cellulose is removed (cost ≈ $400–$800 for 1,200 sq ft).
- Air‑sealing & preparation – Gaps, cracks, and rim joists are sealed to maximise foam performance.
- Application of spray foam – Open‑cell or closed‑cell foam is sprayed to the specified thickness, creating a seamless air‑tight envelope.
- Ventilation & clean‑up – The space is ventilated for at least 24 hours before re‑occupancy; the crew leaves the site clean and ready for use.
Cost considerations for an attic vapour‑barrier solution
| Factor | Why it matters | Price (if listed) |
|---|---|---|
| Foam type (open vs closed) | Determines vapour‑control ability and R‑value | Open‑cell $1.50–$3.50 / sq ft; Closed‑cell $2.50–$5.00 / sq ft |
| Square footage | Larger areas require more material and labour | Example: 1,200 sq ft attic at R‑20 – Open‑cell $2,800–$4,200; Closed‑cell $6,000–$8,400 |
| Existing insulation removal | Adds labour and disposal costs | ~$400–$800 for 1,200 sq ft |
| Access difficulty (e.g., steep roof, limited crawl space) | Impacts labour time and equipment needed | No specific price listed; may affect overall project range |
| Rebates & incentives | Can reduce out‑of‑pocket cost | Up to $10,600 through Enbridge HER+ and Canada Greener Homes (handled by Spray Foam Kings) |
Typical project costs reported by Spray Foam Kings:
- Small areas – $1,500–$3,000 CAD
- Medium projects – $3,000–$5,000 CAD
- Full‑home installations – $10,000–$20,000+ CAD
Why choose Spray Foam Kings for your attic vapour‑barrier needs
- Certified installers – CUFCA trained, NFCA fire‑proofing certified, BASF and Caliber Spray Foam licensed.
- Insurance & safety – $5 million commercial general liability insurance and WSIB‑covered crew.
- Ontario Building Code compliance – All installs meet SB‑12 requirements; we advise confirming any local amendments with your municipality.
- Warranty – Workmanship warranty covers delamination, shrinkage, or adhesion failures at no charge.
- Long‑term performance – Spray foam material lifespan of 25–30 years under normal conditions.
- Province‑wide service – Toronto, Scarborough, North York, Etobicoke, Vaughan, Mississauga, Markham, Brampton, Hamilton, Oakville, Aurora, Newmarket, Richmond Hill, Ajax, Pickering, Oshawa, Burlington, Barrie, London, Waterloo, Kitchener, Cambridge, Guelph, St. Catharines, Kingston, Ottawa and the rest of Ontario.
For more detail on attic insulation options, see our page on Attic Spray Foam Insulation Services. Before any installation, we recommend reviewing What to Do Before Insulating Your Attic? for preparation tips.
Frequently asked questions
Do you need a vapour barrier with spray foam?
Closed‑cell spray foam at the recommended thickness (typically 2 inches) acts as a class‑II vapour retarder, so an extra barrier is usually unnecessary. Open‑cell foam is vapour‑permeable; a separate vapour‑control layer may be required depending on climate, roof assembly, and local building practices.
Do you need a vapour barrier with rigid foam?
Rigid foam can serve as a vapour barrier if the board’s material and thickness meet the required permeability rating (e.g., ≥2 inches of XPS or polyiso). Many installers still add a polyethylene sheet to ensure a continuous barrier at seams and penetrations, especially in colder parts of Ontario.
How long does it take to install spray foam in an attic?
A typical attic job takes 4–6 hours on‑site, depending on size and access conditions. Larger or more complex installations may require additional time for preparation and ventilation.
Will my home be safe to occupy right after the foam is applied?
Re‑occupancy is allowed after a minimum of 24 hours of ventilation for small jobs, and 24–48 hours for whole‑home projects. This ensures any off‑gassing has dissipated and the foam has cured.
Can I claim rebates for spray foam insulation?
Yes. Spray Foam Kings is a registered Enbridge HER+ partner and can help you apply for up to $10,600 in rebates, as well as eligibility for the Canada Greener Homes program. We handle all paperwork on your behalf.
What warranty protects my attic insulation?
All installations include a workmanship warranty that covers delamination, shrinkage, or adhesion failures at no charge. The foam material itself is expected to last 25–30 years under normal conditions.
Get a free written quote from Spray Foam Kings.
Last updated October 31, 2026.