Attic Insulation: Materials, R-Values, and Installation in Canada
The attic is typically where the greatest thermal losses occur in Canadian homes. Understanding material choices, required R-values by climate zone, and correct installation sequence prevents common mistakes that compromise performance for decades.
Why the attic loses the most heat
Heat rises. In a heated living space, warm air accumulates near the ceiling and pushes against the attic floor assembly. Without sufficient insulation and airtight sealing, this thermal energy escapes through conduction across the insulation and convection through gaps around light fixtures, plumbing penetrations, and hatch covers.
In most Canadian houses built before 1980, the attic floor has less than R-20 of insulation — well below the R-40 to R-60 range recommended for zones 5 through 8 under current National Building Code of Canada guidelines. The heat loss difference between R-20 and R-60 at −25°C is significant in practical terms.
Blown-in cellulose
Cellulose is made from recycled paper fibre treated with borate compounds for fire and pest resistance. It is blown in loose-fill form using specialized equipment, allowing it to conform around obstructions such as joists, blocking, and electrical boxes.
Performance characteristics
- R-value per inch: approximately 3.2 to 3.8 depending on density
- Air resistance: dense-pack cellulose (at 3.5 lb/ft³ or higher) resists air movement effectively
- Settling: loose-fill cellulose settles 15 to 20% over time; installers should account for this by adding depth
- Moisture: cellulose absorbs moisture but dries readily if the assembly dries toward the exterior
Installation on attic floors
The standard approach is to blow cellulose over the attic floor joists after air-sealing all penetrations. The required depth for R-60 in Zone 7 is approximately 16 to 18 inches of settled depth. Before blowing, every penetration — electrical boxes, plumbing stacks, bathroom exhaust fans — must be sealed with fire-rated caulk or spray foam, because no amount of insulation compensates for convective bypass through gaps.
Fibreglass batts
Fibreglass batts are the most common insulation type installed in Canadian residential construction. They are available in standard widths sized to fit between 16-inch and 24-inch on-centre framing. Their R-value ranges from R-2.9 to R-3.8 per inch depending on density and product line.
Limitations in attic applications
Batts perform poorly when improperly fitted. Gaps, compression, and voids at the edges of joists create thermal bridges that reduce effective R-value substantially. Studies by Building Science Corporation have shown that a 4% void in batt installation can reduce whole-assembly R-value by up to 50% in cold climate conditions due to convective loops within the cavities.
For attic floor applications, batts are commonly installed in two layers perpendicular to each other to eliminate joist-level thermal bridging. This cross-layering approach brings the joist fraction into compliance with the required assembly performance.
Spray foam in attic applications
Spray polyurethane foam (SPF) comes in two formulations relevant to attics: open-cell and closed-cell. Their application zones within the attic assembly differ significantly.
Closed-cell spray foam on rafters
When converting a vented attic to a conditioned unvented attic — a common approach in cold climates when mechanical equipment is located in the attic space — closed-cell SPF is applied directly to the underside of the roof deck. This creates a warm roof assembly. The foam acts as both insulation and vapour retarder.
Under the National Building Code for Zone 6 and 7, the closed-cell SPF layer must be thick enough to keep the interior face of the foam above the dew point temperature. For Zone 7, this typically requires a minimum of 4 to 5 inches of closed-cell SPF before adding any permeable insulation below.
Ventilation requirements
For vented attics, the National Building Code requires a minimum 1:300 net free ventilation area ratio (inlet to outlet), with at least 25% of ventilation area at the ridge and 25% at the soffit. Blown-in insulation must not block soffit ventilation channels; baffles (also called wind baffles or rafter vents) installed at the eaves maintain the airflow path between the top of the insulation and the roof deck.
In practice, a common error is installing insulation flush to the top of the exterior wall plate without baffles, blocking soffit vents entirely. This leads to moisture accumulation at the eaves, ice dam formation, and eventual roof deck deterioration.
Vapour barriers in attic assemblies
For vented attic floor assemblies, the vapour barrier (polyethylene sheet, minimum 6-mil) belongs at the warm-in-winter side of the insulation — the ceiling surface below the attic floor. In existing homes, this is often difficult to add without disturbing the ceiling finish, so a coat of vapour-retarder primer paint on the ceiling is sometimes used as an alternative in moderate climate zones.
R-value targets by zone
| Climate Zone | Representative cities | Minimum attic R-value (NBC 2020) | Recommended retrofit target |
|---|---|---|---|
| Zone 4 | Victoria, Vancouver (mild) | R-38 | R-40 to R-50 |
| Zone 5 | Toronto, Ottawa (suburbs) | R-40 | R-50 to R-60 |
| Zone 6 | Ottawa, Winnipeg (south) | R-50 | R-60 |
| Zone 7 | Edmonton, Saskatoon | R-50 | R-60 to R-70 |
| Zone 8 | Yellowknife, Whitehorse | R-60 | R-70+ |
Inspection before and after
Before adding insulation to an existing attic, a thermal inspection or blower door test identifies air leakage locations. Addressing air leakage first yields more energy savings per dollar than adding insulation on top of a leaky attic floor. After insulation is installed, a post-retrofit EnerGuide audit documents the as-built performance and is required for Canada Greener Homes grant claims.