Warm air collects near the ceiling, so an under-insulated attic lets a steady stream of heat pass into an unconditioned roof space and out through the roof. Because attic floors are often open and accessible, they are also among the simpler areas to improve, which is why energy assessments frequently start there.

How attic insulation is measured

Thermal resistance in Canada is given as an RSI value, the metric figure in square-metre-kelvins per watt, and is also widely quoted as the imperial R-value. A higher number means the material slows conductive heat flow more. Building codes describe minimum effective values for the assembly as a whole, which accounts for framing and gaps rather than the insulation alone.

Common attic materials

  • Loose-fill — blown fibreglass or cellulose that settles into an even layer and fills irregular spaces around joists and wiring.
  • Batts — pre-cut fibreglass or mineral wool sections fitted between joists; straightforward to handle but need careful placement to avoid gaps.
  • Mineral wool — dense stone or slag fibre that resists fire and holds its shape well.
  • Spray foam — expands to seal and insulate at once, sometimes used at the roofline rather than the attic floor.
Stacked panels of mineral wool insulation
Mineral wool insulation panels. Photo via Wikimedia Commons (CC BY-SA 3.0 DE).

Ventilation and moisture

In a cold climate, a vented attic is usually kept cold so that the underside of the roof deck stays near outdoor temperature. This reduces the chance of snowmelt refreezing at the eaves, the pattern behind ice dams. Soffit and ridge or roof vents allow air to move through, and insulation should not block the airflow path at the eaves.

Why air sealing comes first. Adding insulation over leaky ceiling penetrations — light fixtures, plumbing stacks, attic hatches — lets warm, moist indoor air keep rising into the cold attic, where it can condense. Sealing those gaps before topping up insulation is a recurring recommendation. This connects directly to air sealing and weatherproofing.

Practical points for retrofits

  • Measure the existing depth before deciding how much to add; older homes are often far below current expectations.
  • Keep insulation clear of recessed fixtures unless they are rated for contact.
  • Maintain baffles at the eaves so soffit vents stay open.
  • Insulate and weatherstrip the attic hatch, which is easy to overlook.

None of these steps require specialised judgement to understand, but the right combination depends on the specific house. A building professional or an energy advisor can assess the assembly as a whole, including how it ties into the walls discussed in the windows and heat loss article.

Publicly available references