Both spray foam and blown-in insulation are used in Ontario attic retrofits, and both can deliver strong results. The right choice depends on your attic's specific conditions, your goals for air sealing versus R-value improvement, and your budget.
Blown-in insulation (loose-fill cellulose or fibreglass)
Blown-in insulation — typically either cellulose (recycled paper) or fibreglass — is the standard and most cost-effective method for topping up attic insulation in Ontario homes. It's installed by blowing loose fibre material into the attic space on top of existing insulation, increasing the total R-value.
Cellulose blown-in is the more common choice for retrofit attics. It achieves roughly R-3.7 per inch, so getting from R-20 to R-60 requires approximately 11 inches of new material. It settles slightly over time but remains effective for decades. Fibreglass blown-in provides similar R-values with slightly less settling.
The limitation of blown-in alone: it adds insulation value but does relatively little for air sealing. Air leakage through penetrations (pot lights, attic hatches, plumbing stacks, electrical boxes) needs to be addressed separately before or alongside blown-in installation for the best results.
Spray polyurethane foam (SPF)
Spray foam comes in two types: open-cell (lower density, ~R-3.7 per inch) and closed-cell (higher density, ~R-6 per inch). Closed-cell spray foam also acts as a vapour barrier and provides structural reinforcement — characteristics that make it useful in specific applications like basement rim joists, cathedral ceilings, and rooflines.
The major advantage of spray foam is that it seals and insulates simultaneously. A spray foam application on the underside of roof sheathing (converting the attic to a conditioned, unvented space) eliminates attic air leakage paths entirely. This is called an "unvented" or "hot roof" assembly.
The hot roof consideration
Spray foam applied to the underside of roof sheathing creates an unvented attic — a different building science approach from a vented attic with blown-in insulation on the floor. Unvented assemblies work well but require correct application and vapour management. They're not universally appropriate for all roof types and climates. We assess attic conditions as part of every audit before recommending an approach.
Cost comparison
Blown-in cellulose is substantially less expensive per R-value achieved. A typical attic top-up to R-60 in a 1,500 sq ft bungalow runs $2,000–$4,000 installed with blown-in. Spray foam applications covering the same area can run $6,000–$12,000 or more depending on foam type and depth required.
The premium for spray foam is often justified when the application is a rim joist (where air sealing is critical), a cathedral ceiling with limited depth, or a situation where eliminating attic ventilation complexity is the primary goal. It's rarely the most cost-effective choice for a straightforward attic floor insulation top-up.
What the rebate programs cover
Both blown-in and spray foam attic insulation are eligible measures under the HRS Program. Rebates are based on R-value improvement achieved — the material type doesn't affect eligibility. The post-retrofit audit verifies the achieved R-value before rebate payment is processed.