What Causes Mold in an Attic? The 6 Usual Suspects

What causes attic mold is almost always moisture from inside the house, not a hole in the roof. Warm, damp indoor air rises into the attic in winter, meets cold roof sheathing and condenses there. When the wood stays damp long enough, mold grows on it. The six causes below account for nearly every attic we look at in the London area.

What causes attic mold in a cold climate

Mold needs three things: a food source, a suitable temperature and water. An attic is built out of the food source, and it is warm enough for much of the year. Water is the only part you can control. Wood generally has to sit at roughly 20% moisture content or higher for a while before mold takes hold, which is why a dry attic can go fifty years without a spot.

Southwestern Ontario makes this harder than it sounds. Winters are cold enough that the underside of the roof drops well below freezing, and houses are kept warm and fairly humid inside. That temperature gap is what pulls moisture out of the air and onto the wood.

The 6 usual suspects

1. Air leaks through the ceiling. This is the big one. Warm air escapes around the attic hatch, pot lights, plumbing stacks, wiring holes and the tops of interior walls. Each gap is small, but together they can carry a lot of water vapour into the attic every day of the heating season.
2. Bathroom fans that end in the attic. A fan duct that stops short of the roof, or has fallen off its hood, dumps shower steam straight onto the sheathing. Look for a dark patch right above the duct. This one is common enough that it has its own article.
3. Blocked or missing intake vents. Soffit vents get buried when insulation is blown in or topped up. With no air coming in at the eaves, damp air sits in the attic instead of being flushed out.
4. Too much humidity in the house. A furnace humidifier set high in January, a damp basement, lots of indoor laundry drying or an unvented gas appliance all raise the moisture load. The more water in the indoor air, the more ends up overhead.
5. Roof leaks. Failed flashing at a chimney, vent stack or valley, or water backing up behind an ice dam, wets the wood directly. Leak mold is usually confined to one area with a clear water trail.
6. Other exhaust ending in the attic. Dryer ducts, kitchen range hoods and occasionally a disconnected heat recovery duct get routed into the attic or come apart there. A dryer vent is the worst of these because the air is hot and wet and full of lint.

How to tell which cause you have

The pattern of the staining points to the source. Read it before anything gets cleaned, because once the wood is clean the evidence is gone.

• Even staining across the whole north slope, heavier near the eaves: general condensation from air leaks, humidity and weak ventilation.
• A dark circle or fan shape above one spot: something is exhausting right there, usually a bath fan or a pot light.
• A streak running down from a penetration, with water marks on the framing: a roof leak.
• Staining concentrated above the attic hatch: the hatch is not sealed or insulated.
• Rusty nail tips throughout: the attic gets frost or condensation on a regular basis.

The north slope tends to be worst because it gets the least sun and stays cold and damp longest. Lower sections near the eaves are often worse too, since that is where the air is coldest and where blocked soffits leave dead pockets.

Why it often shows up after a renovation

A lot of attic mold starts within a few years of a change to the house. New windows and doors tighten the lower floors, so indoor humidity climbs and more of it exits through the ceiling. Added attic insulation makes the attic colder, which is good for the hydro and gas bills but means the sheathing condenses more easily, and the top-up often covers the soffit vents at the same time. A new bathroom adds a fan and a shower without anyone checking where the duct ends. Pot lights cut into a top-floor ceiling add a dozen new holes in one afternoon.

Older London homes and attic mold

Many of the older houses in Old North, Wortley Village and Old East were built with plank roof decks, little insulation and leaky ceilings. The heat they wasted kept the attic dry. Once they are insulated and tightened, the old gaps around chimneys and plaster ceilings start feeding moisture to a much colder roof deck. Newer subdivisions have tight walls, deep insulation and several bathroom fans. Both grow mold for the same reason: damp indoor air reaching cold wood.

What to do once you know the cause

Fix the moisture source and remove the mold, in the same project. Cleaning the wood without fixing the cause buys a few winters at most. Fixing the cause without cleaning leaves the growth and the stains in place, which is a problem the day you sell. Start by taking photos of the staining pattern and anything that vents into the attic, and have someone who works in attics confirm the cause before you spend money on either half.

Common questions