How Attic Ventilation Works: Intake, Exhaust and the 1 in 300 Rule
Air in low, air out high, and a clear path between. What each part does and why balance beats quantity.
- Beginner
- 1 hour to assess
- Updated
The short answer
Attic ventilation works by letting outside air enter low at the soffits and leave high at the ridge or roof vents, so rising warm air pulls fresh air through. It removes moisture in winter and heat in summer. The BC Building Code asks for unobstructed vent area of at least 1 in 300 of the insulated ceiling area, or 1 in 150 for low slope or joist roofs, with intake and exhaust balanced.
An attic is a buffer between the house and the weather. Its job is to stay close to outside temperature and stay dry. Attic ventilation is how that happens: outside air enters low, moves up under the roof deck and leaves high, carrying moisture and heat with it. When the flow works, the roof deck stays dry in winter and the shingles do not bake in summer. When it does not, you get frost on nails, damp insulation, mold on the deck and ice dams.
In the Fraser Valley the main job of attic ventilation is moisture, not heat. Winters are long, wet and mild, so the attic spends months cool and damp, with warm humid air leaking up from the house. Without air moving through, that moisture condenses on the coldest surface, usually the north-facing roof deck, and stays there.
This guide explains how the system works. To work out how much you need, see how to calculate attic ventilation. For what goes wrong, see how to fix attic mold caused by poor ventilation.
Tools
- Flashlight
- Tape measure
- Ladder
- Notepad
- Camera or phone for photos
Materials
- No materials needed to assess. Vents, baffles and ducts are chosen after you know what is wrong.
Safety first
- Step only on ceiling joists or a board laid across them in an attic, never on the drywall between them.
- Wear a dust mask and eye protection in an attic, and gloves around old insulation.
- If you need to go on the roof, follow WorkSafeBC fall protection rules: fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
Step by Step
Step 1: Understand what ventilation is meant to do
In winter, the warm air in a house carries water vapour, and some of it escapes into the attic through gaps around light fixtures, hatches, fans and wall tops. Cold outside air moving through the attic dilutes and carries that moisture out before it condenses on the roof deck.
In summer, moving air takes away some of the heat under the roof. It lowers the attic temperature and the load on the shingles, although in the Fraser Valley the moisture job matters more.
Step 2: Learn how air moves: the stack effect
The stack effect is the natural movement that drives passive ventilation. Air in the attic warms slightly and rises to the ridge. As it leaves through exhaust vents, it pulls replacement air in through the intake vents low in the eaves.
Wind helps. Wind blowing across a ridge vent creates a low-pressure zone that draws air out, and pushes air into windward soffit vents. Good ventilation uses both effects together, with no fans needed.
Step 3: Identify the intake vents
Intake goes at the lowest part of the roof: perforated or vented soffits, continuous soffit vents, or vents at the fascia. Look from the ground for vent openings, then in the attic for daylight or airflow at the eaves.
Check if the air can get past the top of the wall. Blown-in or batt insulation pushed into the eave blocks the path. An open soffit vent with a blocked eave does nothing. See how to install insulation baffles.
Step 4: Identify the exhaust vents
Exhaust goes high so that rising air has a way out. A continuous ridge vent gives the most even exhaust. Box vents work in a row near the ridge. Gable vents, turbines and power vents are also used.
Count the exhaust types. One type works best. Mixing them can short circuit the airflow, see why you should not mix roof exhaust vents.
Step 5: Check the code minimum: the 1 in 300 rule
For most pitched roofs, the vent area required is not less than 1/300 of the insulated ceiling area. Where the roof slope is less than 1 in 6, or the roof is built with roof joists, the requirement is 1/150. The code also asks for vents to be distributed uniformly on opposite sides of the building, with at least 25 percent of the required vent area at the top of the space and at least 25 percent at the bottom, and for at least 63 mm (about 2-1/2 inches) of clear space between the top of the insulation and the underside of the sheathing.
Some guidance elsewhere treats 1/150 as the default and allows 1/300 only when intake and exhaust are balanced and part of the exhaust is high. The safest practice is the same either way: balanced intake and exhaust, sized from the net free area of the vents. The arithmetic is in how to calculate attic ventilation.
Step 6: Check the balance between intake and exhaust
If exhaust is bigger than intake, the exhaust vents pull replacement air from wherever they can, including through ceiling leaks from the house below, which brings moist indoor air into the attic. If intake is bigger than exhaust, air moves slowly and moisture lingers.
A common target is roughly equal intake and exhaust, by net free area. Many guidance documents allow somewhat more intake than exhaust, but never the reverse.
Step 7: Look for the path between intake and exhaust
From the soffit, air needs a channel at least about an inch deep along the deck, which baffles provide. At the top, the ridge slot or vent openings must be clear.
Look for compressed insulation, bird nests, paint-clogged perforated soffits and framing blocks that seal off bays. Each bay between rafters is its own channel and needs its own open path.
Step 8: Stop moisture at the source
The most effective step for a damp attic is often to stop moisture getting in. Seal around light fixtures, the attic hatch, plumbing and wiring penetrations and the tops of walls. Insulate the hatch.
Never let a bath fan, range hood or dryer vent discharge into the attic. A single fan can put litres of water per day into the space. See how to vent a bathroom fan through the roof.
Why Fraser Valley Attics Are Damp
Long, mild, wet winters keep attics at temperatures where humid air condenses on the roof deck. Mold often shows first on the north-facing deck, which never gets winter sun. Moss on the shingles outside can also be a hint that the roof is staying cold and wet.
Heavier snow at Hope and in the canyon adds another pattern: snow on a warm roof melts, runs to the cold eave and refreezes into an ice dam. A cold, well-ventilated roof deck helps stop that. See ice dams.
Common Vent Types at a Glance
- Soffit vents: intake low at the eaves. Continuous or individual.
- Ridge vent: exhaust along the peak. Even, low profile.
- Box vents: exhaust in a row near the ridge. Count depends on net free area.
- Gable vents: intake or exhaust at the ends. Airflow depends on wind direction.
- Turbines and power vents: active exhaust. Usually not mixed with other exhaust types.
Common Mistakes
- Adding exhaust without intake. The roof pulls air from the house instead of from the eaves.
- Blocked soffits. Insulation or paint plugs the intake, and the ridge vent does nothing.
- Mixing exhaust types. Air short circuits between them and the lower roof stays stagnant.
- Venting fans into the attic. The moisture load overwhelms any ventilation.
- Treating ventilation as a fix for air leaks. Sealing the ceiling plane matters as much as adding vents.
- Assuming more is better. Oversized or unbalanced ventilation can draw in rain, snow and indoor air.
When to Call a Professional
If you find mold, wet insulation, stained or soft sheathing or frost on the nails, or if the roof is steep or hard to reach, have a roofer assess it. The fix may involve adding intake, baffles, new vents and air sealing, and the order matters.
A roofer can also check the exhaust layout on a reroof, which is the easiest time to correct the system.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How Attic Ventilation Works: Questions
How much attic ventilation does a house need?
The BC Building Code sets unobstructed vent area at not less than 1/300 of the insulated ceiling area, or 1/150 where the roof slope is less than 1 in 6 or the roof is built with roof joists. A 1,500 square foot insulated ceiling needs about 5 square feet of net free vent area, split between intake and exhaust.
Do soffit vents and ridge vents work together?
Yes, and it is the best passive combination. Air enters at the soffit, rises under the roof deck and leaves at the ridge. The two must be balanced by net free area, and the soffit vents must not be blocked by insulation. Baffles in each rafter bay keep the channel clear.
What is the stack effect in an attic?
It is the natural tendency of warm air to rise. As air in the attic warms, it rises to the top and leaves through exhaust vents, pulling cooler outside air in through the intake vents at the eaves. It needs no fan, and wind across the ridge adds to the pull.
Why is my attic damp in winter?
Warm humid air from the house leaks into the attic through gaps and condenses on the cold roof deck. Common causes are bath fans venting into the attic, leaky ceilings and blocked soffit vents. Ventilation carries some moisture away, but sealing leaks and venting fans outside are just as important.
Related Guides
All Ventilation and Attics guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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