How should attic ventilation work in an old house?
Short answer. Many older houses were built with gable louvers only and boxed or open eaves with no real intake, so their attics run hot and damp by modern standards. The usual fix, best done during a reroof, is to add soffit intake with baffles, add ridge exhaust, close off competing gable vents, and seal the ceiling first.
What you usually find in an older Georgia attic
- Gable louvers only: common in houses from the 1940s through the 1970s. They vent the ends of the attic but leave the middle stagnant.
- Board or plank decking: the gaps between boards once let some air through. Newer layers of shingles, felt or plywood overlays sealed those gaps.
- Boxed eaves without vents, or bungalow style open eaves where the blocking between rafter tails was never drilled for air.
- Leaky ceilings: plaster cracks, open chases and old light fixtures push humid house air into the attic.
Cautions before anyone starts
- Vermiculite insulation: loose, gray brown, pebble like fill can contain asbestos. The EPA advises treating it as if it contains asbestos and leaving it undisturbed; hire a trained professional if work would disturb it.
- Old wiring: knob and tube wiring buried in insulation is a fire risk; have an electrician look before insulation is added.
- Thin rafters: older framing may sag, so a roofer should check before cutting a ridge slot.
The right order of work
- Seal the attic floor: chases, fixture holes and the hatch.
- Open intake: cut vent openings in the soffit or drill blocking, and install baffles in each bay.
- Add exhaust high, usually a ridge vent if the ridge is long enough.
- Close the gable louvers from inside so they do not short circuit the ridge vent, unless the attic has no other exhaust.
Historic district rules may limit changes you can see from the street, so check before altering eaves or gables. For more, see roof ventilation basics and OSB vs plywood roof decking.