Airtightness of the building envelope is critical to high performance.
Stopping draughts is only part of the reason for striving for this objective. Getting control over heat flows in and out of the building, managing ventilation for the best indoor air quality and controlling moisture flows whilst still allowing breathability are part of this approach to quality in the construction process.
This video from Dow Corning uses infra red photography to show where heat losses can occur around poorly installed bulk insulation. What it also shows are the effects of thermal bridging in the thermal envelope.
Look out for the cold spots at the base of the external corners – just the spot for condensation & mould to occur – and the lower temperatures on the inside of the timber studs.