Mould in the attic.

Attic mould grows on the underside of the roof sheathing and on the rafters or trusses. CMHC's fact sheet on attic moisture lists the signs, and most show up in a flashlight beam from the hatch.

• Stains or growth on the wood: on the sheathing, rafters or trusses.
• Flattened or stained insulation: packed down, or stained by water or ice.
• Frost on the sheathing: NRCan says to look during or just after a cold snap.
• Ceiling leaks only in spring: CMHC says this can be ice that formed all winter and melted in a warm spell.
• A musty smell in summer: CMHC says a mouldy attic's smell can reach the house.

Rot comes later. CMHC says prolonged wetness rots out the roof sheathing, often first noticed when the roof is re-shingled. NRCan calls mould or rot sure signs of advanced condensation problems.

Before you go up: leave vermiculite insulation alone, since CMHC says it may contain small amounts of asbestos, and leave bat or bird droppings alone. Step only on the joists, and turn off the power near wiring, as CMHC and NRCan advise.

A heated house works like a chimney. CMHC explains that all winter, warm, moist air is pushed through the top-floor ceiling into the attic. Where it meets sheathing below freezing, the moisture condenses and builds up as frost.

Ottawa gives that frost a long season. Environment and Climate Change Canada's 1991 to 2020 normals for the Ottawa airport count 20.3 days in an average November when it drops to 0 °C or below, 29.4 in December and 30.0 in January. January's daily average is -10.0 °C.

Health Canada lists condensation from excessive humidity, lack of ventilation or low temperature among the major causes of mould growth, and says mould begins to grow within 48 hours where moisture is excessive.

The other route is a roof leak. CMHC says to check whatever pierces the sheathing, such as chimneys and plumbing vents, and that the flashing is quite often defective. If a leak has soaked the ceiling, see our water damage restoration page.

CMHC says the leaks are at the holes and joints, not the middle of a drywall sheet or an unbroken plaster ceiling.

• Plumbing stacks and chimneys: where they pass through the attic floor.
• Pot lights: CMHC says safety rules prevent sealing many types, so they dump house air into the attic.
• Wiring holes and the attic hatch: both on NRCan's list of places to seal.
• Ducts in the attic: CMHC says heating or air conditioning ducts there can be the major source of leakage.
• Wall tops: where partition and outside walls meet the attic floor.

To find the big ones, CMHC suggests searching the attic at night for light from the rooms below, and looking for dirty patches in batt insulation that has been filtering house air.

One check works from the driveway: after the first heavy frost or light snow of fall, CMHC says, watch where the roof melts first and find what is under that spot, such as a leaky hatch or a roof-ducted fan.

Health Canada says exhaust fans should be vented outside, not into the attic, and NRCan adds: not directly below the soffit vents. CMHC says a duct crossing the attic should be solid metal, not flex duct, insulated and sloped outward, and that taping the joints helps.

Then use it. Health Canada advises running the fan for at least 30 minutes after a shower or bath, and holding indoor relative humidity between 30 and 50 percent.

The usual response to a wet attic is more ventilation, and CMHC calls that the wrong approach: in some cases it pulls more moist house air into the attic and makes the problem worse. Once the air leaks are stopped, it says, the existing vents are usually more than enough.

Its reason is winter air: cold outside air cannot hold much humidity or carry moisture away, so CMHC calls attic ventilation overrated. Codes still require it, at a nominal 1 to 300, one square foot of vent per 300 square feet of attic floor.

Vents do have to be clear. NRCan says heavy frost, 10 mm or more, is the cue to check that they are not blocked with insulation.

Ice dams come from the same warm attic: roof snow melts and refreezes at the cold overhang, backing water under the shingles. ECCC's normals give Ottawa 231.9 cm of snow a year. CMHC's fix is the same: seal the air leaks and insulate.

Health Canada says that unless the cause of the moisture problem has been identified and solved, mould will reappear. NRCan says to correct the cause before adding insulation. So the order is:

• Stop the water: fix the roof, seal the attic floor and send the fan outdoors.
• Size the mould: on Health Canada's chart, below.
• Clean or remove it: with goggles, an N95 respirator and gloves, and no bleach.
• Then insulate: CMHC suggests at least 300 mm (10 in.) of insulation once the leaks are sealed.

If you can smell mould but cannot find it, our guide to mould testing in Ottawa covers when a test is worth paying for.

Three patches or fewer, totalling one square metre or less, is small: you can clean it with proper procedures and protective equipment. More than three patches, under three square metres in all, is medium: Health Canada recommends an expert assessment, though occupants can usually clean it.

A single patch over three square metres is large and needs a qualified professional. In an attic, that is about one 4 by 8 foot sheet of sheathing.

Health Canada says not to use bleach, and that porous materials often cannot be properly cleaned and must be discarded.

Two companies that sell mould work publish attic prices. Mold Busters lists an average Ottawa cost of $1,500 to $5,000. Mold Inspection Canada's national 2026 table puts attic mould after condensation at $1,000 to $4,000, $2,000 typical, and says removing the insulation adds $500 to $1,500.

Both were read on 2026-11-02, and neither is our price: we quote after seeing the space. Our guide to what mould removal costs in Ottawa has both full lists, and our guide to whether home insurance covers mould in Ontario explains when a policy pays.