Why does an attic get so hot inland?
Heat rises and collects under the roof, and the sun on the covering warms the air beneath it. Without a path for that air to leave, the attic becomes a hot box above the ceiling.
Inland sun on a dark roof is the classic case. The covering absorbs the sun, the deck warms, and the attic air warms with it. A vent that lets the hot air out and the cooler air in breaks the cycle, and the difference is felt in the rooms below the ceiling.
Heat in an attic also builds up in the framing itself, and that stored heat keeps the attic warm after dark. A vent that clears the air during the day lets the framing release less heat into the evening, which is why the difference is often noticed in the hours after sunset as much as at the hottest part of the afternoon.
How do soffit intake and ridge exhaust work as a pair?
Cool air enters through the soffit at the eave and rises through the attic, carrying heat with it, and it leaves through the ridge. The pair forms a path that moves air across the whole attic.
A ridge vent with no soffit intake draws air from the house below or from the nearest gap, which can pull conditioned air up from the ceiling. The intake is therefore added or cleared first, so the exhaust works as intended.
The net free area of the intake and exhaust should be roughly balanced, with the intake a little larger. The technician calculates the openings on the roof and writes the figures on the scope.
A gable vent is a useful addition on a house with a short roof run, but it works only where the wind and the sun allow the air to move across the attic. A technician places each gable vent so it does not short-circuit the flow from the soffit to the ridge, and each one is checked for a path through the framing that the air can actually use.
Why is insulation a factor in the vent design?
Insulation can block the eave openings, and it can also trap heat in the attic when laid too thickly against the deck. Both affect how well the vents work.
Baffles are small channels fixed at the eave that keep the insulation back from the soffit, so air can pass into the attic above it. They are a cheap and effective part of the work.
Where the attic floor is insulated, the technician checks that the insulation is not compressed around the vents, since a crushed layer can reduce its value and still block the air.
Vapour from the house below is a second reason to keep the attic moving. Cooking, bathing and laundry all add moisture to the air, and an attic without vents collects some of it. Over time that moisture can condense on the cold underside of the sheathing, and the technician looks for the dark staining and musty smell that show it has happened.
What changes after the work is done?
The attic runs cooler in the afternoon, the covering is less exposed to heat from below and the rooms under the attic feel more even. The owner can see the difference in the readings taken before and after.
The change is gradual rather than dramatic. The most noticeable effect is in the hottest part of the day, when the attic would otherwise be at its peak.
A ventilation change does not stop the covering from ageing from the sun above it. It removes one source of heat and moisture from below, which helps the roof last as its manufacturer and its age allow.
Owners often ask whether a fan should be added to the attic. A powered fan can move a large volume of air, but it draws replacement air from wherever it can reach, so an attic with no intake can pull air from the house below and raise the energy used by the cooling system. The technician measures the intake before any fan is considered and explains the trade-off on the scope.
