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Score 20 of 2969%
An open chimney plus gappy floorboards plus an unsealed hatch is not a draught, it is a chimney effect: cold air pulled in low, warm air pushed out high, continuously.
Do the chimney first — a balloon or cap stops the engine driving the whole thing. Then the hatch, then the floors. Never block air bricks or any vent serving a fuel-burning appliance.