Attic ventilation: your house cool and dry!

ventilating attic

 Attics are naturally hot spaces due to heat rising and typically dark shingles, controlling that heat through proper attic ventilation will extend the life of your shingles, increase the effectiveness of insulation and most important control moisture. 

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Where does the warm moist air come from?

  • Sun hitting shingles causes the attic space to heat up more than the outside air. The rising hot air creates a vacuum pulling air into the attic through soffit ventilation. 
  • Improper vented bathroom and kitchen fans terminated in the soffit of a house can have that moist warm air being inadvertently pulled back into the attic space
  • Attic hatches and plumbing stacks that are not sealed permit warm air to enter the attic space
  • Poor insulation and/or vapour barriers. 

Older Homes;

  • ​Inspect for proper vapour barrier, climb into attic pull up pieces of insulation to find plastic between insulation and drywall. 
  • Inspect soffits, often solid wood soffits were covered by aluminum for looks. Ensure there is actual air flow into attic space and that insulation is not blocking that air flow. 

How do I elliminate moisture in my attic?

  1. Prevent It​
        • Become a detective, find then Insulate around anything entering attic; hatches, electrical, plumbing, vent ducts and fans
        • Ensure vent ducts are terminated though the roof and not through the soffit. Too often bathroom or kitchen fans designed to remove warm moist air are terminated in the soffit which through summer heating can pull that air into the attic space. 
  2. Exhaust It
        • ​Typical code requires 1 square foot of ventilation for every 300 square feet of attic split equally between soffit vents and ridge or roof vents. If there are no roof vents this amount is split in half i.e. 1 square foot of vent per 150 square feet of attic.
        • Understand that the sun heats the shingles which heats the air in the attic space, hot air rising creates a vacuum. Cooler air is pulled into the vacuum through the soffits if permitted. The more air you let move the cooler the attic will remain
        • Roof vents or whole attic fans can be passive there are no moving parts and air is just permitted to escape whereby mechanical roof vents move by wind action or by motors pulling the warm moist air out of the attic space replacing it more often, keeping the attic space cooler and dryer. The best time to improve attic ventilation is right now, however often address when your replacing the shingles earlier than you planned! 

What happens when I don’t have proper attic ventilation?

  • Roof damage through premature wear of shingles
  • Warm air radiating into living spaces from attic
  • Ice damming in the winter, yes poor attic ventilation can affect your house year around. 
  • Higher cost of heating and cooling
  • Water vapour condensing in attic space
    • Reducing effectiveness of insulation
    • Causing mold and mildew growth
    • Decay development

All too often decisions about the shingle style or colour win over proper attic ventilation. Attic vents are not attractive but critical to the longevity of your shingles, overall performance of your house and yes money


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2 Thoughts to “Attic ventilation: your house cool and dry!”

  1. Mark

    What is the max pitch on the roof that can handle an exhaust fan.

    1. The challenge with exhaust fans or roof vents is with low slope roofs. If your roof is under a 3/12 (3″ of rise for every 12″ of run) consider using a continuous ridge vent or power gable vent. I trust this answered your question effectively. Thank-you and remember to create an account at


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