A.Mike Keogh, a specialist in energy conservation and ventilation from Campbellford, Ont., responds: Upgrading existing ventilation when reroofing is a smart move -- an opportunity that is missed all too frequently. But should you add a ridge vent to a roof with gable vents? No way -- that's a prescription for disaster. Gable vents will alter the air flow around the ridge vent and, especially when wind is parallel to the ridge (at right angles to the gable), can actually reverse air flow through the ridge vent, pulling rain or snow into the attic.

Unfortunately, however, even when they're matched with continuous eaves venting, gable vents are not very effective because most of the air flow is along the floor of the attic. This leaves much of the attic volume unvented (see illustration), with pockets of dead air that can store summer heat and radiate it into the living space below.

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