Attic condensation and frost: causes and fixes

What is going on

In winter the attic is meant to be cold, close to the outdoor temperature. The house below is warm, and its air carries moisture from breathing, cooking, showers and laundry. Warm air rises, and it finds every gap in the top-floor ceiling.

When that air reaches the cold underside of the roof, the moisture in it condenses, or freezes straight into frost. Over a cold spell the frost builds up on the roof boards, nails and rafters. On the first mild or sunny day it melts all at once and drips onto the insulation and through the ceiling.

Homeowners often take this for a roof leak. The clue is the timing: water appearing in a thaw after a cold snap, with no rain.

Signs

In the attic

• White frost on the underside of the roof deck and on nail points
• Rusty nails with dark rings round them on the wood
• Dark staining or mold on the roof boards, usually worst on the north slope and near the eaves
• Damp, matted or stained insulation
• Water droplets on the underside of the roof on a mild day
• Frost or ice around a bathroom fan duct or plumbing stack

In the house

• Ceiling stains that appear in late winter
• Drips from pot lights, bathroom fans or the attic hatch during a thaw
• Stains along the top of outside walls
• Heavy condensation on windows, which points to high indoor humidity
• Ice dams at the eaves

Where the air gets in

Insulation does not stop air. Fibreglass and cellulose slow heat, but air passes through them. The leaks are in the ceiling below:

• The attic hatch, unsealed and uninsulated
• Pot lights of the older, unsealed kind
• Bathroom fans and their ducts
• Plumbing stacks and electrical wires where they pass through the top plates of walls
• The chimney chase
• Dropped ceilings and bulkheads over kitchens and bathrooms, which are often open to the attic
• Tops of interior and exterior walls
• Ductwork running through the attic

Bathroom fans

A bathroom fan that discharges into the attic, or whose duct has come loose, pours shower steam straight onto the cold roof deck. It is one of the commonest causes of a patch of frost or mold. The duct should be insulated, as short and straight as practical, and connected to a proper hood through the roof or wall. Aiming it at a soffit vent is not enough, because the moist air is drawn back in.

Kitchen range hoods and clothes dryers must vent outdoors too, never into the attic.

Humidity in the house

The more moisture in the indoor air, the more reaches the attic. Health Canada recommends keeping indoor relative humidity between 30 and 50 percent, and in an Ontario winter the lower end of that range, about 30 percent, is the sensible target. Signs that yours is too high: water or ice on the windows in the morning.

• Run bathroom fans during showers and for 20 minutes or so after
• Use the range hood when cooking
• Turn a furnace humidifier down, or off, in very cold weather
• Do not hang laundry to dry indoors in winter
• Use the ventilation system if the house has one

Newly air-sealed houses and houses with new windows often show higher humidity, because the leaks that used to carry moisture away have been closed.

Ventilation

Attic ventilation removes the moisture that does get in. It needs intake low down, at the soffits, and exhaust high up, at ridge or roof vents.

• Blocked soffits are common. Insulation gets pushed out over the eaves and closes them off. Baffles hold a clear air channel.
• Too few vents, or exhaust without intake.
• Mixed systems that short-circuit, such as a powered fan beside a ridge vent.

Ventilation matters, but it does not make up for a leaky ceiling. More venting on a ceiling full of holes can even pull more house air up. Seal first, then ventilate.

The fix, in order

1. Find the leaks. Look for dirty, darkened insulation, which marks air passing through it, and frost patterns that point to the source. An energy adviser can test the house with a blower door.
2. Air seal the ceiling from the attic side: foam or caulk at wiring and plumbing holes, sealed covers over pot lights that are rated for it, weatherstripping and insulation on the hatch, and blocking over open bulkheads. Around a chimney or flue, only fire-rated materials with the required clearances.
3. Vent the fans outdoors.
4. Clear the soffits and fit baffles.
5. Top up the insulation once the sealing is done.
6. Control indoor humidity.

Dealing with the damage

• Wet insulation. Lightly damp insulation dries when the cause is fixed. Soaked, compressed insulation is replaced.
• Stained ceilings. Let them dry fully, then use a stain-blocking primer before painting.
• Mold on the roof deck. Light staining with no further moisture may only need monitoring. Visible growth over a wider area is removed. See attic mold removal.
• Rot. Roof boards that are soft or delaminating are replaced.

Before working in an attic, check the insulation. Loose, pebbly, grey-brown or gold granules may be vermiculite, which can contain asbestos and should not be disturbed.

Condensation or a roof leak?

• Condensation: shows in cold weather and thaws, with no rain. Widespread frost or staining, often worst on the north side. Many rusty nail tips.
• Roof leak: shows during or after rain or melting snow on the roof. A localised stain, usually below a penetration, valley or damaged shingles.
• Ice dam: water at the eaves and the top of outside walls after heavy snow and freeze-thaw weather. See ice dam removal.

Insurance

Damage from condensation is generally treated as a maintenance or gradual problem and is not usually covered. Sudden damage from an ice dam or a storm-damaged roof is a different matter. Check your policy.

More: mold removal and remediation.