How air should move
Air comes in through the soffits, under the roof overhang. It rises along the underside of the decking, then leaves near the ridge through roof vents or a continuous ridge vent.
Three conditions have to be met.
- Clear intakes. Perforated soffits that are not blocked by insulation, paint or an old wood soffit left behind.
- Exhausts up high. Placed near the peak, where warm air collects.
- A clear path between the two. Baffles between the rafters keep a channel open above the insulation.
The balance between intakes and exhausts matters. An attic with plenty of vents at the top but no intake at the bottom draws its air from somewhere else. It pulls it from the house, through leaks in the ceiling.
What a poorly ventilated attic does to the roof
When ventilation fails, the damage comes from both sides of the decking.
In summer, heat stays trapped under the roof. It speeds up the aging of the shingles, which curl and lose their granules sooner. The top floor becomes hard to keep cool.
In winter, moist air from the house rises into the attic, meets the frozen decking and settles there as frost. During a thaw, the frost melts and wets the wood and the insulation. Over time, the decking turns black, grows mould, then goes soft.
At the roof edge, an attic that is too warm melts the snow from below. The water refreezes above the colder overhang and forms an ice dam.
Signs to watch
Some clues can be seen without going on the roof.
- thick icicles and a band of ice at the roof edge in winter
- snow melting in patches on your roof while the neighbours' roofs stay white
- a top floor much warmer than the ground floor in summer
- ceiling stains that appear during a thaw, without any rain
From the access hatch, without going into the attic, shine a flashlight on the underside of the decking. Frost, black stains, rusty nails or packed, damp insulation confirm the problem. Do not walk in the attic, and do not touch granular insulation that looks like vermiculite.
Possible fixes
A roof replacement is the best time to fix the ventilation. We then have access to the decking. The most common work includes the following.
- Clearing the soffits and installing baffles between the rafters.
- Adding exhausts near the ridge or replacing the existing ones with a better suited model.
- Connecting bathroom fans that blow into the attic to the outdoors.
The number and size of vents are calculated from the attic area, according to the building code and the manufacturer's instructions. Ask us to write in the quote what we plan and why.
Frequently asked questions
Are more roof vents always better?
No, more vents do not mean better ventilation. Too many exhausts, or exhausts of different types, can short-circuit each other. Air then comes in through one and leaves through another without sweeping the attic. What counts is a continuous path from the soffit to the ridge.
Does ventilation alone solve attic condensation?
Ventilation does not always solve condensation on its own. Moving air carries off the moisture that reaches the attic, but it does not replace an airtight ceiling. If a lot of warm, moist air escapes from the house, those leaks have to be sealed first.
Do shingle manufacturers require attic ventilation?
Manufacturer warranties generally set conditions on attic ventilation. Read the warranty for your shingle line and ask us to confirm that the planned work meets them.