Attic Rain in Calgary: The Ceiling Leak That Isn't a Roof Leak

Every winter we get the same call, usually the day a chinook rolls in: a brown ring on the bedroom ceiling, water dripping out of a pot light, and no rain in the forecast. Nine times out of ten the shingles are fine. What is failing is the lid of the house.

Prairie tradespeople call it attic rain. Warm, moist air from inside your home leaks up into the attic all winter. It hits the underside of a roof deck that is sitting at outdoor temperature, and it freezes there as frost. Nothing drips while it is cold. Then the temperature jumps 15 or 20 degrees in an afternoon, the frost melts faster than the attic can dry, and the meltwater runs down the trusses, soaks the insulation and shows up on your ceiling.

It is a moisture problem, not a roofing problem. Which is why re-shingling a house with attic rain does absolutely nothing, and why we will tell you that before we quote you anything.

Where the water actually comes from

Attic frost is house air that got out. Every litre of it came from somewhere inside. In Calgary homes, in rough order of how often we find them:

• Bathroom and kitchen fans terminating in the attic. A duct that ends over the insulation, or has fallen off its roof cap, is a humidifier pointed at your roof deck. CMHC is specific: fans must be ducted outside, in solid metal duct, insulated, sloped to the outside, joints taped or mastic-sealed — and never wrapped in plastic, because that traps moisture.
• Fans dumping under a perforated soffit. Technically outside. Practically, the plume gets pulled back in through the soffit vents 300 mm away.
• A humidifier turned up too high. Health Canada recommends indoor relative humidity of 30–35% in winter, and below 50% in summer. We routinely see 45–50% in winter, often from a furnace humidistat nobody has touched in years plus a hot tub or a basement suite.
• Missing, torn or unsealed vapour barrier. Older Calgary homes may have nothing but kraft paper or a few coats of paint. Newer homes have poly that was cut for wiring and never taped.
• Bypasses. Pot lights, plumbing stacks, chimney chases, bath fan housings, top plates of interior partition and exterior walls, dropped ceilings and soffits, the attic hatch, and the split-level or storey-and-a-half discontinuity where a knee wall meets the ceiling.

CMHC describes the driving force well: a heated house behaves like a chimney. It pulls air in low and pushes it out high, all winter, through any hole in the ceiling plane. That is why the leaks that matter are the ones at the top of the house.

The fix order that works: air seal, then insulate, then ventilate

This sequence is not our opinion. It is the order CMHC and Natural Resources Canada have published for decades, and getting it backwards is the single most common mistake we see on Calgary attics.

1. Air seal the ceiling plane

CMHC is blunt: the usual response to a wet attic is to add ventilation, and that is the wrong approach — in some cases more venting pulls more moist house air up and makes it worse. Stop the air movement and existing ventilation is usually more than enough. In practice that means pulling insulation back at the perimeter and at every penetration, then sealing with caulking, foam, poly and rigid board: top plates, partition walls, plumbing stacks with a flexible collar, wiring, bath fan housings, chimney chases with non-combustible fire stops, and a gasketed, insulated attic hatch. Old non-rated pot lights get replaced with sealed, airtight fixtures. The City of Calgary makes the same point from the heat side — swap incandescent ceiling fixtures below the attic for LEDs, because they can put out enough heat to melt snow on the roof.

2. Insulate, properly, right out to the eave

Once the ceiling is tight, add depth. CMHC’s minimum benchmark is at least 300 mm (10 in.) of loose fill or batts; for Calgary we build to R-50 to R-60 because the marginal cost of another 100 mm of blown fibre is small next to what a wet attic costs. The critical zone is the outer 600 mm above the exterior wall, where a baffle can leave only about 100 mm of room. NRCan recommends the best available insulation there — spray foam or cut extruded polystyrene held 25 mm off the sheathing — because a cold spot there is exactly what starts snowmelt above the overhang. That perimeter detail is most of what we do on an attic insulation job.

3. Ventilate to code, and no more

Only now does venting help. Section 9.19 of the National Building Code, adopted in Alberta as the National Building Code – 2023 Alberta Edition, requires unobstructed vent area of at least 1/300 of the insulated ceiling area — the “1/300 rule” — tightening to 1/150 where the slope is less than 1 in 6 or the roof is framed with joists. Vents must be spread out, with at least 25% of the opening at the top of the space and at least 25% at the bottom, and there must be at least 63 mm of clear space between the top of the insulation and the underside of the sheathing. Practically: continuous intake at the soffits, roughly half the area up high, baffles at every truss bay, and nothing blocked with blown-in fibre.

NRCan is also clear on what not to install: powered attic exhaust fans can draw more air than the soffits can supply and end up pulling house air into the attic, and revolving turbine vents are no more effective than a capped static vent.