Last Updated on June 26, 2025
A Toronto-focused guide to proper fan venting, cost benchmarks, and quick attic checks
Why Your New Bathroom Can Hurt Your Attic
A 15-minute shower can push ≈0.8 kg (0.8 L) of water vapour into the air web.ornl.gov. If the fan ducts that steam into the attic instead of outside, the warm, damp air condenses on the cold roof deck—prime fuel for mold. Inspectors link “many” (≈½) of attic mold cases to mis-vented bathroom fans.
What Ontario Code Says (and What Contractors Miss)
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Code minimum: Bath exhaust must discharge outdoors, not into soffits or the attic cavity.
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Airflow: At least 50 CFM for a toilet room; bigger rooms need 1 CFM per ft²
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Condensation control: Ducts must slope to drain any water, and joints must stay sealed.
Red flag: “We’ll just vent near an existing roof louvre.” Moisture will loop back through the louvre and feed mold.
Five Common Fan-Venting Mistakes
| Mistake | What Happens | Quick Fix |
|---|---|---|
| Fan left loose in attic | Steam dumps directly on sheathing | Run insulated duct to a roof cap |
| Duct ends at soffit | Damp air re-enters through soffit vents | Vent through roof with dampered cap |
| Duct taped, not clamped | Tape dries, duct falls | Use metal band clamp + foil tape |
| Crushed or extra-long flex duct | Air stalls, condensation pools | Keep runs ≤1.8 m (6 ft) & smooth-wall if possible |
| No insulation on duct | Vapor condenses inside pipe | R-6 insulated duct sleeve |
Cost Check—Venting Done Right vs. Paying Later (Toronto 2025)
| Work Item | Typical Cost (CAD) | Notes |
|---|---|---|
| Roof-vented fan install (new duct) | $350 – $950 | Includes roof cap & dampers angi.com |
| Attic mold remediation (10 m² deck) | $1,800 – $3,200 | Cleaning, HEPA vacuum, stain-block coat (Toronto averages) |
| Savings when venting done at reno | $900 – $2,000 | Avoided remediation & repaint |
Fast Contractor Vetting Checklist
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Shows plan to vent through roof—not soffit.
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Specifies CFM rating & duct length in the quote.
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Allows roofer to handle roof-side work.
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Provides photo proof of vent cap after job.
Download our free 10-Step Attic Moisture Check (PDF) to spot problems before they spread.
DIY Monthly Audit (Takes 5 Minutes)
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Pull down fan grille; vacuum dust.
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Check attic: is duct still attached and insulated?
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Feel roof nails on cold mornings—damp nails hint at condensation.
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Run fan for 20 minutes after each shower to purge steam thespruce.com.
Frequently Asked Questions
| Question | Answer |
|---|---|
| Can I vent into a gable vent? | No. Moist air can cycle back in and still hit wood. Go straight outside through roof or wall. |
| Is flex duct OK? | Yes, if short and fully stretched. Smooth metal is better for runs over 6 ft. |
| How often should I replace a fan? | Good fans last 10–12 years; replace sooner if noise rises or airflow drops. |
Key Takeaways
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A single shower produces enough moisture to trigger mold if the fan vents into the attic.
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Ontario code requires direct outdoor venting and ≥50 CFM airflow.
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Proper roof venting ($350 – $950) costs far less than mold cleanup.
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Check duct attachment and insulation every season to stay mold-free.
Need a certified attic inspection or fresh fan install?
Mold Guy serves the GTA with IICRC-trained techs and same-week slots. Call or book online today.
