Air Sealing — The Step That Makes Insulation Actually Work

Your ceiling is full of holes you can’t see

Every pot light, plumbing stack, bathroom fan, duct boot, and wiring run is a hole in the barrier between your heated house and your freezing (or baking) attic. Warm air rises, so in winter your ceiling works like a slow chimney: conditioned air escapes through every gap, and your furnace pays to replace it around the clock. The same leaks carry household moisture into the attic, where it condenses on cold roof sheathing — which is how attics grow mold.

We documented a textbook example in Willowdale: thin, patchy insulation with open penetrations everywhere — every one of them leaking warm, humid house air into the attic. Sealing them first is what made the new R-60 perform.

What we seal on a standard attic job

• Pot lights & fixture boxes — sealed with proper clearances and covers (never just buried)
• Plumbing stacks & wiring penetrations — spray-foam sealed at the ceiling plane
• Bathroom fan housings — sealed to the drywall, with the duct verified to exhaust outside (venting into the attic is the #1 defect we find)
• Duct boots & HVAC penetrations — sealed where they pass through the ceiling
• Top plates of interior walls — the long, invisible gaps that leak the most in older homes
• Chimney & flue chases — closed with fire-rated materials where required
• The attic hatch — weatherstripped and insulated; it’s the biggest single hole in most ceilings

Then baffles go in at the eaves to protect the soffit airflow, and only after that does the insulation get blown to R-60.

When the answer is a spray-foam vapour barrier

Much of the GTA’s pre-1980 housing was built with no vapour barrier at all — nothing separating house air from attic air but a layer of plaster. On those homes, patching individual leaks still leaves a ceiling that breathes everywhere, which is how a Lawrence Park attic ended up with 350 sq ft of mold and a musty smell in the bedrooms below.

The fix there — and on many older Toronto homes — is 2 inches of closed-cell spray foam applied across the entire attic floor after removal: a seamless air seal and vapour barrier in one, with fresh blown insulation over top to R-60. It’s the modern envelope a century home never had.

Air sealing, answered