Perfect Airtight Seal
A gap in the vapour check is comparable in energy loss terms to a continious gap between the window frame & masonry. The effects of this are perceptible & produce discomfort.
Example:- On an insulation area of 1m2 & using 140mm thick insulation, a 1mm wide gap in the airtight seal on the inside (assuming standard temperature, humidity & a low air pressure differential), will produce an air loss via this gap of 4.8 times greater than across the entire insulation area.
The result is that the u-value calculated for the design is no longer correct.
When a building is complete a Blower Door test is performed to ensure total airtightness has been achieved.