Cause · the moisture that comes from below

Attic mould: where the moisture comes from, what the frost means, and why insurance treats it as maintenance

Attic mould is water damage that rarely starts with water. It starts with warm, damp air from the house — a shower, a pot on the stove, people breathing — leaking up through gaps in the ceiling into an attic that is below freezing. Natural Resources Canada describes what follows: the vapour condenses on cold surfaces and shows as frost, and where it keeps happening the wood ends up with mould or rot.

This page sets out what Natural Resources Canada and Health Canada say about where attic moisture comes from, how to read the frost, the bathroom fan that vents into the attic, what ventilation can and cannot do, what two companies publish as removal prices, and why insurance treats attic mould differently from a roof that leaked in a storm.

What the inspector will call it

Where the moisture comes from

NRCan names two sources and asks you to tell them apart. Moisture from outside comes through failed roofing or flashing — at a hip, a valley, a chimney — or an ice dam, and shows as water marks on the sheathing or along the rafters near the leak. Moisture from inside arrives as vapour through every hole in the ceiling, and shows as frost spread across the cold side of the roof. Its test is simple: look during or just after a cold snap for frost, and during or just after a rainstorm for leaks. If the problem shows in the cold and not in the rain, the source is the house.

That distinction decides the repair. A roof leak is fixed on the roof. Condensation is fixed in the ceiling — by sealing the air leaks NRCan lists — and adding vents or insulation on top of an unsealed ceiling can make it worse: NRCan warns that more insulation means a colder attic, in which escaping vapour condenses before it can be vented.

Reading the frost

Some frost in a Canadian attic in January is normal, on NRCan’s own account. Heavy frost is not, and its first check is the simplest one: that the vents exist and are not buried under insulation. NRCan’s retrofit instructions say not to block the ventilation at the eaves when insulation is added. The page’s unit conversion does not hold together (10 mm is not 2 inches), so read it as “thick frost”, not as a line to measure against.

Inside the house, Health Canada’s humidity target is the other half of the same equation:

A house kept well above that range in winter has more vapour to push into the attic. Condensation streaming down the inside of windows is the visible sign; Health Canada asks for frames and sills to be dried daily where it forms.

The bathroom fan into the attic

Both federal sources say it, and it is easy to check. Go into the attic and follow the duct from the bathroom fan. It should end at a hood on the roof or the wall, not lie loose in the insulation, and not end directly under the soffit vents, where the moist air is drawn straight back in. A fan that empties into the attic puts the bathroom’s moisture exactly where NRCan says it must not go. NRCan also asks for ducts to be kept below the insulation or wrapped, so they do not condense themselves.

What ventilation can and cannot do

Ventilation is the second line of defence in NRCan’s order, after air sealing, and it gives a ratio — roughly 1 square unit of unobstructed vent to 300 of ceiling — with a caveat that screens and baffles reduce the clear area. It also warns against two instincts. Do not automatically add vents. And do not install a powered attic fan, which can pull more air than the soffits supply and draw warm, moist house air up into the attic — the opposite of the intent.

One warning stands apart. NRCan says repeatedly not to disturb vermiculite insulation, a loose-fill type found in some older attics. If the attic you need to clean has it, the job changes, and it is a conversation for a qualified contractor before anyone goes up with a vacuum.

What removal costs, where anyone publishes it

No public body publishes an attic remediation price, and this site does not invent an average. Two companies publish theirs, reproduced here as theirs.

Both ranges are wide because the bill is decided by the same few things: how much sheathing is affected, whether the insulation has to come out, and how hard the attic is to work in. Health Canada’s threshold for calling a professional applies here as anywhere — a single patch larger than three square metres. Ask any quote what it does about the cause. A cleaned attic above an unsealed ceiling and a fan venting into the insulation will grow mould again; NRCan’s order is cause first.

Why insurance treats it differently

GIO’s list of typical water exclusions includes wear and tear, slow leaks or seepage, and mould. Condensation in an attic is the slow kind by definition: it builds up over winters, not in an afternoon. A roof that leaks suddenly in a storm is a different claim, and IBC describes interior water damage from roof leaks as covered, excluding the roof itself where the damage is wear and tear.

So the first question an adjuster asks about attic mould is which of NRCan’s two sources caused it. Frost-pattern mould across the cold side of the roof points to the house; a stain spreading from one flashing points to the roof. Photograph both before anything is cleaned. The roof leak page covers the storm side, and mould removal covers what Health Canada says about cleanup in general.

Questions people actually ask

Where to read next

• Roof leaks and ice dams
• Mould removal
• Eavestroughs and downspouts
• The other end of the house: crawl spaces
• Twelve water situations compared