LizzyG said:
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I didn't mean the vents were to let the smoke out, but that in the old days with no central heating there would have been more moisture in the air as no central heating to drive the moisture out
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In the old days with no central heating there would have been LESS moisture in the air. Ventilation is needed just as much nowadays, if not more, to ensure that as the moist air moves from warm to cooler areas, the moisture will be removed without causing condensation. If you get mould growing on the walls it is because of condensation, and it is a SERIOUS health hazard. You don't have to block the vents completely.