How to Fix Attic Mold Caused by Poor Ventilation, Fan Venting and Air Leaks

Step by Step

1. 1Step 1: Look at the pattern and confirm what you are seeing

   Mold usually appears as black, grey or green spotting on the underside of the roof sheathing, often heavier on the north slope where the deck stays cold. Frost on nail tips in winter, rusted nails, dark stains around a bath fan or damp, matted insulation all point to condensation.

   Probe the wood with a screwdriver. Soft or crumbly wood is rot, which is more serious than surface mold and needs the wood replaced. A moisture meter helps: readings well above the normal range for dry lumber, commonly under about 15 percent, point to an active moisture problem.

   Watch out: Do not assume dark staining is mold. Old soot, dust or roof leak stains look similar. A leak stain shows a clear edge and a path from the roof, while condensation mold spreads over a wide area.

2. 2Step 2: Check every fan and vent that discharges into the attic

   This is the most common cause. A bath fan that ends in the open attic, is connected to the soffit, or has a disconnected duct puts litres of water into the attic each week. Mold tends to be heaviest close to the fan.

   Reconnect or replace each duct so it runs to a roof or wall cap, insulated and sealed, following how to vent a bathroom fan through the roof. Also check that no plumbing vent or chimney leaks into the space.

3. 3Step 3: Seal air leaks from the house into the attic

   Warm moist air from the living space rises into the attic through every gap in the ceiling. Seal small gaps with caulk or fire-rated foam, cover recessed lights with approved covers, and add weatherstripping and insulation to the attic hatch.

   Sealing air leaks often does more for attic moisture than adding vents, because it removes the source. Do it before or together with any ventilation changes.

4. 4Step 4: Open and protect the intake

   Blocked intake is the second most common cause. Look along the eaves for painted-over vents, solid soffits and insulation packed into the eave. Open or add vents as described in how to add soffit intake vents.

   Install baffles in every bay, see how to install insulation baffles, and check that insulation is still deep and continuous over the top plate.

5. 5Step 5: Check exhaust, balance and vent mixing

   Calculate what the attic needs using how to calculate attic ventilation. The BC Building Code asks for vent area of not less than 1/300 of the insulated ceiling area, or 1/150 for low slope or joist roofs, split about half intake and half exhaust.

   If the roof has mixed exhaust vents, such as a ridge vent with box vents or a power vent, fix that too. See why you should not mix roof exhaust vents.

6. 6Step 6: Let the wood dry and confirm the cause is fixed

   Dry wood stops growing mold. With the sources fixed, most surface dampness clears in weeks, although in a long Fraser Valley winter it can take longer. Re-check the moisture meter readings and look for frost, damp insulation and new growth.

   If new staining appears or the wood stays wet, there is still a source. A roof leak, a wet wall cavity or a plumbing vent problem may be involved.

7. 7Step 7: Clean light surface mold or replace damaged material

   For light surface mold on solid framing or sheathing, HEPA vacuum the surface, then wipe or scrub with a detergent and water solution and dry quickly. Bag and remove contaminated insulation. Do not use bleach, which does not penetrate wood well and leaves moisture behind.

   Replace sheathing or framing that is soft or rotten. Sheathing replacement is part of roof work, see how to replace roof sheathing. Re-insulate with dry insulation once the space is clean and dry.

8. 8Step 8: Decide whether professional remediation is needed

   Small, light surface growth on solid wood can be handled with care. Widespread growth across large parts of the deck, heavy contamination of insulation, or any concern about occupants' health, particularly asthma, allergies or immune problems, calls for a professional with containment and air filtration.

   If the roof is old and the deck is damaged across a large area, replacing the roof and sheathing with corrected ventilation may be the most practical fix. See roof replacement for when that is worth considering.

Common Causes, in Rough Order of How Often They Are Found

Why the North Slope

Common Mistakes

• Cleaning the mold and leaving the cause. It returns within a season.
• Using bleach. It does not penetrate wood and adds moisture.
• Adding exhaust vents to fix a moisture source. Fix the fan and air leaks first.
• Leaving wet insulation in place. It holds moisture against the deck and loses R-value.
• Working without a respirator. Spores and dust affect your lungs.
• Ignoring soft wood. Rot means the wood has to be replaced, not cleaned.