Ontario Insulation Requirements: OBC Minimums for Basements, Walls & Attics
Ontario’s Building Code sets some of the strictest residential insulation minimums in North America. Whether you’re finishing a basement, building an addition, or insulating a new home, these are the R-values your project needs to pass inspection — and why they matter for your long-term heating bill.
Quick Reference — OBC SB-12 Minimums
Basement walls: R-20 (batts) or R-12 continuous (spray foam / rigid foam)
Above-grade walls: R-22 effective (most common packages use R-20 + R-5 continuous)
Attic: R-60
Exposed floor: R-31
Vapour barrier: Required on the warm (interior) side in all assemblies
Why Ontario’s Code Is Different from the US
Most online insulation advice is written for American homeowners following the US International Residential Code (IRC) or the IECC. Ontario uses the Ontario Building Code (OBC), and specifically Supplementary Standard SB-12 for energy efficiency.
Ontario’s requirements are stricter. A basement wall that meets code in Zone 5 of the US (R-15 minimum) will fail an Ontario inspection for the same wall (R-20 minimum). The attic requirement is even more telling: Ontario requires R-60, versus R-49 in equivalent US climate zones.
This gap exists because Ontario’s code has been progressively tightened since 2012, and the most recent updates in the OBC 2024 edition push residential insulation standards further. The practical result is that insulation that “feels like a lot” to a contractor who learned on US code standards is often still below OBC minimums.
OBC Minimum R-Values by Assembly Type
The following table reflects the prescriptive minimums in OBC 2024 Supplementary Standard SB-12 for Part 9 buildings (houses and small buildings) in most of Ontario. Values may vary slightly for northern Ontario locations with higher heating degree days.
| Assembly | OBC SB-12 Minimum | Alternate Compliance | Notes |
|---|---|---|---|
| Basement wall | R-20 | R-12 ci | R-20 = batts in stud wall. R-12 ci = rigid foam or spray foam with no thermal bridging |
| Above-grade wall | R-22 effective | R-20 + R-5 ci | Most common package is 2×6 with R-19 batts + 1″ XPS exterior |
| Attic / ceiling | R-60 | — | Raised from R-50 under OBC 2024. Blown-in cellulose or fibreglass most practical |
| Exposed floor | R-31 | — | Floors over garages, crawl spaces, or unheated space |
| Slab-on-grade | R-10 | — | Under slab in heated slabs. Required at perimeter minimum 24″ down |
| Crawl space wall | R-17 | — | If conditioned crawl space; unheated crawl spaces insulate the floor above instead |
Always Verify With Your AHJ
These values reflect common OBC SB-12 prescriptive minimums. Your local Authority Having Jurisdiction (building department) is the final word on requirements for your specific project, municipality, and construction type. Northern Ontario municipalities and some local amendments may have different requirements.
Basement Wall Insulation in Ontario: The Two Paths
The OBC gives contractors two main paths to meet basement wall code. Understanding both helps you choose the right system for your project:
Path 1: R-20 with a Stud Wall and Batts
The traditional approach: build a 2×4 or 2×6 stud wall in front of the foundation, fill the cavity with batt insulation (fibreglass or mineral wool), add a 6-mil poly vapour barrier on the interior face of the studs, then drywall.
To reach R-20 with a 2×4 stud wall (3½″ cavity), you need to supplement the batt with rigid foam against the concrete: 1½″ XPS (R-7.5) + R-14 batts in the stud = R-21.5. Without the rigid foam layer, a 2×4 cavity filled with R-13 batts only gets you R-13 — well below code.
A 2×6 stud wall gives you 5½″ of cavity. R-19 or R-21 batts plus any rigid foam against the concrete gets you to R-20 easily. This is the most common permitted basement assembly in Ontario right now.
Path 2: R-12 Continuous Insulation (No Stud Wall Required)
Spray foam or rigid foam applied directly to the foundation wall — with no stud wall — can meet code at a lower nominal R-value because there’s no thermal bridging. R-12 continuous insulation means the entire wall surface has at least R-12 with no gaps, no framing breaks, and no thermal short-circuits.
Closed-cell spray foam at 2″ achieves approximately R-13, exceeds the R-12 ci minimum, and simultaneously serves as the vapour barrier. This is the most compact basement insulation solution — no stud wall, no poly, just foam and drywall.
XPS rigid foam at 2½″ (R-12.5) achieves R-12 ci as a board system. You’d still need to build a stud wall in front for the drywall attachment, but the stud wall doesn’t need to carry insulation — the XPS on the concrete handles the thermal load.
Continuous Insulation vs Effective R-Value
R-12 continuous (ci) is not the same as R-12 in a stud cavity. “Continuous” means the insulation has no interruptions from framing, fasteners, or structure. A stud wall with R-14 batts has an effective R-value around R-11 because the wood studs (at R-1.25/in) create thermal bridges every 16″. Continuous insulation like rigid foam or spray foam against the foundation eliminates this loss — which is why R-12 ci is treated as equivalent to R-20 in a studded assembly.
Common Ontario Basement Insulation Systems & Their R-Values
| System | Approximate R-Value | Vapour Barrier | Meets OBC? |
|---|---|---|---|
| 2″ closed-cell spray foam on concrete | R-13 | Foam acts as barrier | ✓ Yes (R-12 ci path) |
| 2½″ XPS on concrete + 2×4 stud | R-12.5 ci + framing | 6-mil poly required | ✓ Yes (R-12 ci path) |
| 1″ XPS + 2×6 stud + R-19 batts | R-24 | 6-mil poly required | ✓ Yes |
| 2×4 stud + R-14 batts only | R-14 (effective ~R-11) | 6-mil poly required | ✗ Fails — below R-20 |
| 2×6 stud + R-19 batts only | R-19 (effective ~R-16) | 6-mil poly required | ✗ Fails — below R-20 |
| R-7.5 XPS + 2×4 stud + R-14 batts | R-21.5 | 6-mil poly required | ✓ Yes |
| 2″ mineral wool board + 2×4 + R-14 batts | R-22.4 | 6-mil poly required | ✓ Yes |
Above-Grade Wall Requirements
For above-grade walls, the OBC SB-12 requires an effective R-value of approximately R-22 in most of Ontario, accounting for thermal bridging through studs. In practice, the most common compliant packages are:
- 2×6 stud wall + R-19 batts + 1″ XPS exterior sheathing — R-24 total, very common in Ontario new construction
- 2×6 stud wall + R-22 mineral wool + no exterior CI — Check with your building department; effective value may fall just under minimum
- 2×6 stud wall + open-cell spray foam (full cavity) + 1″ rigid foam — R-25+, exceeds code comfortably
The 2×4 stud wall is rarely code-compliant for above-grade walls in Ontario without significant exterior continuous insulation. A 2×4 with R-14 batts has an effective value around R-11 — only half the minimum.
Attic Insulation: Ontario Now Requires R-60
The attic R-60 requirement is one of the most significant increases in the OBC in recent years (raised from R-50). For new construction this is straightforward — blown-in insulation is installed to the required depth during building. For existing homes and additions, it often means topping up existing insulation.
To reach R-60 with blown-in fibreglass: approximately 18″ of depth. For blown-in cellulose: approximately 17″ at R-3.5/inch. A common and cost-effective approach for existing homes is to maintain existing batt layers and blow cellulose over top to bring the total to R-60.
Vapour Barrier Requirements in Ontario
Ontario Building Code Section 9.25.3 requires a vapour barrier on the warm (interior) side of all insulated assemblies in Ontario’s cold climate. The correct installation sequence for a typical basement is:
- Foundation wall (concrete, ICF, CMU)
- Rigid foam or spray foam insulation (if applicable, against concrete)
- Stud wall with cavity insulation (if applicable)
- 6-mil polyethylene vapour barrier (stapled to interior face of studs)
- Drywall
The vapour barrier must be on the interior side of the insulation assembly — between the insulation and the living space. Installing it on the wrong side (between insulation and concrete) is a common mistake that can trap moisture in the wall cavity and lead to mould.
Exception: Closed-cell spray foam at a thickness of 2″ or more qualifies as a Class II vapour retarder under the OBC. If you’re using closed-cell spray foam as your primary insulation, a separate 6-mil poly is not required.
Check Your Assembly Against OBC Requirements
Use our free R-value calculator to build your wall assembly layer by layer and get an instant pass/fail result against Ontario code minimums.
Open R-Value Calculator →Frequently Asked Questions
What is the minimum R-value for basement walls in Ontario?
Under OBC SB-12, basement walls require a minimum of R-20 when insulating with batt insulation in a stud wall, or R-12 continuous when using rigid foam or spray foam applied directly to the foundation wall. Closed-cell spray foam at 2″ (approximately R-13) satisfies the R-12 continuous path and also serves as the vapour barrier.
What R-value is required for attic insulation in Ontario?
The OBC currently requires R-60 for attic insulation. This was raised from R-50 in recent code updates. Blown-in cellulose or fibreglass are the most practical ways to achieve R-60, requiring approximately 17″–18″ of depth.
Does Ontario follow the same code as the US IRC?
No. Ontario uses the Ontario Building Code (OBC) and SB-12, not the US International Residential Code. Ontario’s requirements are generally stricter, particularly for attics (R-60 vs R-49) and basements (R-20 vs R-15 in equivalent US zones). Always use OBC values for Ontario projects.
Can I use spray foam to meet Ontario basement insulation code?
Yes. Closed-cell spray foam at 2″ (approximately R-13) meets the R-12 continuous insulation path under OBC SB-12. It also acts as a vapour barrier, so no separate 6-mil poly is required. Open-cell spray foam is vapour-permeable and still requires an additional vapour barrier, and you’d need more thickness to reach R-12.
Is a vapour barrier required in Ontario basements?
Yes. OBC Section 9.25.3 requires a vapour barrier on the warm (interior) side of the insulation. For most assemblies this means a 6-mil polyethylene sheet stapled to the interior faces of the studs, with drywall installed over top. The exception is closed-cell spray foam at sufficient thickness, which qualifies as the vapour barrier itself.
What is SB-12 in the Ontario Building Code?
SB-12 (Supplementary Standard SB-12) is the energy efficiency appendix to the Ontario Building Code. It prescribes minimum thermal resistance values for the building envelope — basement walls, above-grade walls, attics, floors, and slabs. It also allows “compliance packages” that trade off between components, so exceeding the minimum in one area can allow flexibility in another.
Do I need a permit to insulate my basement in Ontario?
Finishing a previously unfinished basement (framing, insulation, vapour barrier, drywall) typically requires a building permit in Ontario. Requirements vary by municipality, so confirm with your local building department before starting. Permitted work must meet OBC insulation minimums, which is why understanding these requirements matters before you buy materials.