What is a thermal bridge
A thermal bridge is a localized area of the building envelope where thermal resistance is notably lower than nearby zones. Heat follows the path of least resistance, concentrating losses in these areas.
In attics, thermal bridges form mainly at interfaces between building components and at structural penetrations.
Unconditioned attic floor
In a home with an unconditioned attic, the floor between living space and the attic is the main heat exchange surface. Losses spread across the area but can concentrate in certain zones:
- Poorly covered insulation: near top plates (header joists), where access is difficult and insulation is often missing or thin
- Around access hatches: see dedicated section below
- Around electrical penetrations: recessed light boxes, conduits
Top plates and eave walls
The junction between the exterior wall and roof framing is especially exposed. In wood-frame homes, top plates are conductive structural elements crossing the thermal envelope.
The space between rafters and top plates is often the hardest to reach when blowing loose-fill. Flexible hoses are sometimes needed for retrofits.
Common observation
Ice dams at the roof edge often indicate insufficient perimeter attic insulation. Lost heat melts snow high on the roof; water runs and refreezes at the eaves. This is frequent in Eastern provinces and Quebec.
Attic access hatch
The access hatch is among the most neglected parts of the thermal envelope. A standard untreated hatch has very low thermal resistance compared with the rest of the insulated ceiling.
Typical defects are of two kinds:
- Insufficient insulation: the hatch itself is not insulated on the attic side
- Air leakage: the hatch frame is not sealed, allowing cold air into living space
Insulated hatch covers (attic tent boxes) are available and can treat this point effectively.
Recessed ceiling lights
In homes built before airtight standards, recessed light boxes created direct openings between living space and the attic. Natural convection carries heat and water vapour into the attic.
Airtight-rated housings reduce this issue and can be covered with insulation directly. Older non-airtight boxes need specific thermal protection before being covered.
Ventilation and plumbing ducts
Ducts crossing the attic (kitchen, bathroom, crawl space exhaust) penetrate the thermal envelope. Two problems are common:
- Uninsulated ducts in cold attics, leading to condensation inside the duct
- Unsealed space around the duct where it passes through the attic floor
Ventilation ducts must terminate outside the building (roof or wall), not in the attic. Incorrect installation adds moisture directly to the attic and can damage framing.
Chimneys and flue pipes
Canadian rules (National Building Code and ULC standards) require minimum clearance between flues and combustible materials. That constraint creates a discontinuity in insulation at attic floor crossings.
Insulated metal enclosures (fire stops) can fill this space while meeting required clearances.
Gable walls in attics
In traditional unconditioned attics, gable walls (vertical end walls) are exposed to outdoor conditions. If uninsulated, they are significant exchange surfaces, especially on gables facing prevailing winds.
Repeating structural elements
In wood-frame construction, framing members (joists, rafters, purlins) conduct heat more than surrounding insulation. Together they form a network of repeating thermal bridges.
Their effect on overall performance is smaller than point bridges but is included in standardized thermal calculations.
Systematic approach
ASHRAE 90.2 and CMHC building envelope guides offer reference methods for identifying and quantifying thermal bridges in Canadian residential buildings.