Mold
Attic mold: causes, removal and how to stop it coming back
By ARO Restorations · Updated
Short answer
Attic mold is almost always caused by warm, moist house air leaking into a cold attic and condensing on the roof sheathing. Removing it means cleaning or treating the sheathing under containment, then fixing the air leaks and ventilation, or it comes back.
In this guide
Why attics grow mold
In an Ontario winter the attic is cold and the house below is warm and humid. Wherever house air leaks up into the attic, it meets the cold underside of the roof and gives up its moisture as condensation or frost. Wood that is wet for weeks at a time grows mold.
The usual sources of that air are:
- Bathroom fans vented into the attic instead of outdoors, or ducts that have come loose
- Gaps around pot lights, the attic hatch, plumbing stacks and wiring
- Dryer vents that end in the attic
- Blocked soffit vents, often covered by insulation, so the attic cannot air out
- Too little roof ventilation for the size of the attic
A roof leak can also cause it, but leak-related mold is usually local, around the leak. Condensation mold tends to spread across large areas of sheathing, often worst on the north side.
What it looks like
Dark grey or black staining on the underside of the roof boards or plywood, sometimes fuzzy, sometimes just a discolouration. In winter you may see frost on the nails and sheathing. Many people only learn about it from a home inspection during a sale.
Is attic mold a health risk?
The attic is outside the living space, so exposure is lower than with mold in a bedroom or basement. Health Canada’s advice is still to remove indoor mold and fix the moisture that caused it, whatever the type. Attic air does reach the house through the same gaps the moisture used.
How it is removed
- Find and fix the moisture source first. Otherwise the work is wasted.
- Contain the work. The attic hatch is the only route out, and it opens into the house, so it is sealed and the attic kept under negative pressure while work is under way.
- Clean the sheathing and framing. Depending on the extent, this is done by HEPA vacuuming and damp wiping, abrasive cleaning or media blasting.
- Deal with the insulation. Insulation that is wet, moldy or covered in debris from cleaning is removed and replaced.
- Correct ventilation and air sealing. Vent bath fans outside, seal ceiling penetrations, clear the soffits, add roof vents where needed.
Staining can remain in the wood after the growth has been removed. That is cosmetic.
What affects the cost
- The area of sheathing affected
- How easy the attic is to work in: height, access and the pitch of the roof
- Whether insulation has to be removed and replaced
- The cleaning method needed
- How much air sealing and ventilation work is required
Ask for the remediation and the ventilation corrections to be itemised separately. Both matter.
One caution before anyone goes up
If the attic insulation is loose, pebbly and grey-brown, it may be vermiculite, which can contain asbestos. Do not disturb it. Have it tested first. See asbestos removal.
Keeping it from coming back
- Confirm every bathroom and kitchen fan discharges outdoors
- Weatherstrip and insulate the attic hatch
- Keep soffit vents clear with baffles
- Keep indoor humidity moderate in winter
- Look in the attic each winter for frost on the nails
Why it is so often found during a home sale
Few people go into their attic. Mold on the underside of the roof can develop for years without anyone noticing, because it does not smell in the living space and does not show on the ceilings. A buyer’s home inspector looks, finds dark staining on the sheathing, and it becomes a condition of the sale.
If you are selling, it is worth checking the attic before listing. If you are buying, ask for the cause to be identified as well as the mold removed.
Ventilation: what a healthy attic needs
A roof stays dry when outside air can enter low at the soffits and leave high at the ridge or roof vents, sweeping moisture out. Problems start when:
- Insulation is pushed into the eaves and blocks the soffit vents
- There are too few roof vents for the size of the attic
- Different vent types are mixed and short-circuit the airflow
- A bathroom, kitchen or dryer exhaust dumps moist air into the attic
Ventilation only removes the moisture that gets in. Stopping house air leaking up through the ceiling matters as much. The usual leak points are the attic hatch, pot lights, plumbing stacks, wiring holes and the tops of interior walls.
Winter and spring are when to look
In deep cold, look for frost on the nail tips and the underside of the sheathing. That frost is house moisture that has condensed and frozen. When it melts, it wets the wood. Dark staining concentrated on the north side, or around a bathroom fan duct, points to the source.
What remediation does and does not do
Cleaning removes the growth from the wood. It does not undo staining that has soaked in, and it does not change the conditions that caused the problem. A job that cleans the sheathing without fixing the air leaks and ventilation will need doing again in a few winters.
Ask any contractor to list the corrections separately: which fans will be vented outside, what will be sealed, and what ventilation will be added.
Questions people ask
What causes mold in the attic?
Almost always warm, moist air from the house leaking into a cold attic and condensing on the roof sheathing. Bathroom fans vented into the attic, gaps around lights and hatches, and blocked soffit vents are the usual causes.
Is attic mold dangerous?
Exposure is lower than with mold in living space because the attic is outside it, but Health Canada advises removing indoor mold and fixing the moisture source whatever the type.
Can I remove attic mold myself?
A very small patch can be cleaned by a homeowner. A large area of sheathing needs containment at the attic hatch so that spores are not drawn into the house, and the insulation may need replacing.
Will attic mold come back after removal?
It will if the air leaks and ventilation are not corrected. Removing the mold without fixing the cause only resets the clock.
Does attic mold affect a home sale?
Often. It is a common finding in home inspections. Buyers usually ask for remediation and for the cause to be corrected, so it is worth checking before you list.