Air needs to flow around a building to keep it free of condensation. The following are beneficial sources of ventilation and therefore should not be blocked:
To mitigate unwanted draughts, fill cracks with concrete or hard-setting fillers, including around windows, electrical fittings on ceilings and walls, and at joists. If the crack is large, check that there is not an underlying structural problem. Fill small gaps around pipework with silicone filler, and larger gaps with polyurethane foam.
Draughts from windows can be prevented with self-adhesive foam strips. These strips are inexpensive but not especially durable. A more expensive and durable option is metal or plastic strips with brushes or wipers attached. Similar strips can also be fitted to doors.
(1) Energy Saving Trust, Reducing Home Heat Loss, Draught-Proofing. Available from: https://energysavingtrust.org.uk/advice/draught-proofing/ [Accessed 26th January 2021].