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The 'Good Practice' recommendation of the Building Research Establishment (BRE) is that disused chimneys should be 'blocked off at the bottom (including a small ventilator) and fitted at the top with a ventilated weatherproof hood'
Left uncapped, a disused chimney is prone to the destructive effects of the elements and nesting birds.
In the course of a year, the weather can be relied upon to soak an unprotected chimney with the equivalent of several feet depth of water. This damages the fabric of the stack, while obstructions caused by birds' nests restrict ventilation and inhibit drying out.
Open chimneys provide a route for uncontrolled heat loss from the home - with the warm air flowing (invisibly) from the room having to be replaced by cold air from outside. In winter, heating up this inflowing cold air can be an expensive waste of energy.
Capping off the chimney reduces the outflow, while also keeping the chimney dry, allowing the ventilation requirement for the flue to be reduced to 'trickle ventilation' only.
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