If you’re planning to insulate your Toronto, Mississauga, or Ottawa home with spray foam, you’ve probably heard conflicting advice about thermal barriers. Some contractors say you can leave spray foam exposed in attics or basements. Others insist it must be covered. So what does the Ontario Building Code actually require?

The short answer: yes, spray foam insulation needs a thermal barrier in most residential applications across Ontario. Understanding when, where, and why this matters can save you from failed inspections, insurance complications, and potential fire safety issues.

What Is a Thermal Barrier and Why Does Ontario Require It?

A thermal barrier is a protective layer—typically drywall, plywood, or an approved spray-on coating—that separates foam plastic insulation from living spaces. According to Section 9.10.17.10 of the Ontario Building Code, foamed plastic insulation like spray foam must be protected by a thermal barrier that can withstand fire for at least 15 minutes.

This requirement exists because spray foam, while an excellent insulator, is a combustible material. In the event of a fire, an approved thermal barrier slows ignition and flame spread, giving occupants critical extra minutes to evacuate safely. Even closed-cell spray foam—which is denser and more fire-resistant than open-cell—still requires protection in finished living areas.

For Toronto and GTA homeowners, this isn’t just about code compliance. Many insurance companies in Ontario require homes to meet current building standards. Exposed spray foam in basements or attics could complicate claims or coverage.

Where Must You Install a Thermal Barrier in Ontario Homes?

Finished basements: If you’re converting your basement into a living space—rec room, bedroom, or home office—spray foam on basement walls must be covered with drywall (minimum 12.7 mm or 1/2″) or another approved barrier. This applies throughout the Greater Toronto Area, including Hamilton, Oakville, and Burlington.

Attics with living space access: Walk-up attics or finished attic spaces require thermal barriers. However, unfinished attics that are not habitable and only accessed occasionally for maintenance may have different rules—consult with a licensed insulator to confirm your specific situation.

Crawl spaces: Ontario’s code allows more flexibility for crawl space insulation. If the area is not intended for storage or occupancy and access is limited, exposed spray foam may be acceptable under certain conditions. Always verify with your local building inspector.

Rim joists and enclosed cavities: Spray foam applied to rim joists or within wall cavities—where it’s sandwiched between structural elements—is typically covered by framing and finish materials, satisfying thermal barrier requirements naturally.

What Materials Qualify as an Approved Thermal Barrier?

The Ontario Building Code specifies several acceptable thermal barriers for spray foam installations:

Gypsum drywall (12.7 mm / 1/2″ minimum): The most common solution for basements and finished spaces across Toronto, Scarborough, and Markham.
Plywood or OSB (11 mm minimum): Often used in garages or utility areas.
Cement board: Suitable for high-moisture areas.
Intumescent coatings: Specialized spray-on thermal barriers that expand when exposed to heat, providing a 15-minute fire resistance rating. These are used in areas where drywall is impractical, such as irregular surfaces in older Toronto homes.

Standard paint or thin coatings do not qualify as thermal barriers. The material must provide meaningful fire resistance and meet the performance criteria outlined in Ontario Building Code sections 9.29.4 through 9.29.9.

Can You Leave Spray Foam Exposed Anywhere in an Ontario Home?

There are limited exceptions where spray foam may be left exposed, but they’re narrow and situation-specific:

Unfinished, unoccupied attics: In some cases, if the attic is truly uninhabitable and used only for mechanical access, exposed spray foam may be permitted. However, this varies by municipality. Cities like Ottawa and Toronto have strict interpretations.
Exterior applications: Spray foam applied to the exterior of a foundation (below grade) does not require a thermal barrier since it’s not in contact with living spaces.

Garages are a gray area. While not technically “living space,” Ontario’s code often requires thermal barriers in attached garages to prevent fire from spreading to the main house. If your garage shares a wall with your home, plan to cover any spray foam insulation with drywall.

What Happens If You Don’t Install a Thermal Barrier?

Skipping the thermal barrier can create several problems for Ontario homeowners:

Failed building inspections: If you’re doing a basement renovation or new build in the GTA, inspectors will flag exposed spray foam that doesn’t meet code. You’ll need to correct it before receiving occupancy approval.
Insurance complications: Some insurers require homes to meet current building standards. Exposed spray foam could be grounds for denying coverage or claims.
Safety risk: Without a thermal barrier, foam insulation can ignite faster in a fire, reducing evacuation time.
Resale issues: When selling your Hamilton, Mississauga, or Brampton home, buyers’ inspectors will note code violations, potentially lowering your property value.

Does Closed-Cell Spray Foam Still Need a Thermal Barrier?

Yes. While closed-cell spray foam is denser and more fire-resistant than open-cell, it still requires a thermal barrier in Ontario. Closed-cell foam offers superior R-value per inch (around R-6 to R-7) and acts as both insulation and vapor barrier—but that doesn’t exempt it from thermal protection requirements.

The one advantage of closed-cell foam is that it’s less prone to off-gassing and degradation, so the thermal barrier protects the foam as much as the foam protects your home.

How Much Does Adding a Thermal Barrier Cost in Ontario?

If you’re already finishing a basement or attic, the thermal barrier (drywall) is part of your standard finishing costs—typically $1.50 to $3.00 per square foot for materials and installation in the Toronto area. For spaces you want to leave unfinished, you have two options:

Drywall installation: Around $2 to $4 per square foot, depending on accessibility and complexity.
Intumescent spray coating: Approximately $1.50 to $2.50 per square foot for professional application. This is popular in mechanical rooms or areas with exposed beams.

Given that spray foam itself costs $3 to $5 per square foot for closed-cell in Ontario, adding the required thermal barrier is a relatively small percentage of your total project cost—and it’s not optional.

Can You DIY a Thermal Barrier or Should You Hire a Pro?

If you’re comfortable with drywall installation, adding a thermal barrier is a manageable DIY project—provided you use the correct thickness (12.7 mm minimum) and follow proper fastening guidelines. However, most Toronto-area homeowners choose to have their insulation contractor coordinate the thermal barrier as part of the spray foam installation.

Professional installers understand local code nuances and can ensure your project passes inspection the first time. If you’re applying for energy efficiency rebates through programs like the Canada Greener Homes Grant, having a licensed contractor handle both insulation and thermal barriers simplifies documentation.

What About Spray Foam in Rim Joists and Wall Cavities?

Spray foam applied to rim joists—the framing at the top of your foundation where the floor system meets the walls—is one of the most effective energy efficiency upgrades you can make in an Ontario home. The good news: if the rim joist cavity is fully enclosed by framing and drywall (as is typical in finished basements), that drywall serves as your thermal barrier.

Similarly, spray foam injected into wall cavities during retrofits or new construction is protected by the wall’s gypsum board or exterior sheathing. The key is ensuring there are no exposed foam surfaces in habitable areas.

Do You Need a Thermal Barrier in Unfinished Attics Across Ontario?

This is where Ontario’s code gets nuanced. Section 9.10.17.10 requires thermal barriers in areas “where occupants may be present.” An unfinished attic used only for mechanical access—no storage, no flooring, infrequent entry—may qualify for an exception.

However, many municipalities in the GTA interpret this conservatively. If your attic has a permanent staircase (not a pull-down ladder), if you store items there regularly, or if it’s floored, inspectors will likely require a thermal barrier. Consult with your local building department before assuming you can leave attic spray foam exposed.

In practice, most homeowners in Toronto, Vaughan, and Pickering opt to install drywall or spray-on coatings even in unfinished attics—it’s a small investment that eliminates any gray area during inspections.

Final Thoughts: Thermal Barriers Are About Safety, Not Just Compliance

Ontario’s thermal barrier requirement isn’t bureaucratic red tape—it’s a critical fire safety measure that protects your family and your investment. Whether you’re insulating a basement, attic, or crawl space in Toronto, Hamilton, or anywhere in the province, budget for both the spray foam and the required thermal barrier from the start.

Working with an experienced insulation contractor ensures your project meets code, passes inspection, and qualifies for available rebates. Don’t cut corners on thermal barriers—the few hundred dollars you might save aren’t worth the risk.

Need expert spray foam insulation with proper thermal barriers for your Ontario home? Spray Foam Kings provides code-compliant installations across the Greater Toronto Area, including Toronto, Ottawa, Hamilton, and surrounding regions. We handle everything from spray foam application to approved thermal barriers, ensuring your project passes inspection the first time.

📞 Call us at 647-641-6881 or visit our Google Business profile to schedule a free consultation. Get a detailed quote at sprayfoamkings.ca and learn about available energy efficiency rebates that can offset your insulation investment.