Context for choosing materials
Insulation choice for attics depends on building-specific factors: framing layout, existing vapour barrier, access to the space, local humidity, and target RSI by climate zone.
In Canada, recommended RSI values vary by province and region. Natural Resources Canada’s guide on home energy efficiency is a useful public reference for applicable targets.
Fibreglass (glass fibre)
Made from spun recycled glass, fibreglass is among the most common attic insulations in Canada. It comes in two main forms:
- Batts or panels: installed manually between framing members
- Blown loose-fill: machine-applied for even coverage on irregular surfaces
Blown fibreglass is often used to top up existing layers or reach greater depths in unconditioned attics.
Installation note
Fibreglass loses insulating value if compressed. It should remain in its uncompressed state to maintain nominal RSI.
Rock wool (mineral wool)
Produced from basalt or blast-furnace slag, rock wool has higher thermal resistance than fibreglass. It is naturally water-repellent at the surface, which suits areas with humidity variation.
As panels, it is sometimes used in finished attics for sloped-roof insulation, with a ventilated air space.
Blown cellulose
Made from treated recycled newsprint with fire retardants and antifungal agents, blown cellulose is common in unconditioned attic retrofits.
Characteristics that suit Canadian attics include:
- Good fill in irregular spaces around joists
- High recycled content
- Stable thermal performance in cold conditions
It is more sensitive to moisture than mineral wool. Proper vapour control (vapour barrier on the warm side) is recommended. Water intrusion can cause settling and loss of thickness.
Spray polyurethane foam (SPF)
Spray polyurethane foam is used in specific cases: insulating the underside of the roof deck in conditioned attics, sealing penetrations, or treating complex thermal bridges.
Two variants exist:
- Open-cell foam: more flexible, vapour-permeable, lower RSI per centimetre
- Closed-cell foam: rigid, vapour-impermeable, higher RSI per centimetre
Application requires professional equipment and precise temperature conditions. It is generally costlier than other materials at equivalent RSI.
Rigid insulation panels
Expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate panels are sometimes used in finished attics for sloped-roof insulation with an air space, or continuous deck insulation.
In unconditioned attics, use is less common because of difficulty achieving uniform coverage on irregular surfaces.
General characteristic comparison
- DIY ease: fibreglass batts ≥ rigid panels; blown cellulose and SPF need specialized equipment
- Moisture sensitivity: mineral wool < cellulose < fibreglass (at comparable density)
- RSI per centimetre: closed-cell SPF > polyisocyanurate > XPS > EPS ≈ mineral wool ≈ fibreglass ≈ cellulose (approximate; always check data sheets)
- Recycled content: blown cellulose leads
Reference source
CMHC publishes technical sheets on residential building materials on its site: cmhc-schl.gc.ca
Practical considerations for Canada
Prairie provinces (Alberta, Saskatchewan, Manitoba) see winters that can drop below −30 °C. Required RSI values there are generally higher than on the coastal British Columbia mainland.
Water vapour management is central in the Canadian context. Insulation chosen and installed with an adequate vapour barrier reduces interstitial condensation that can damage framing over time.