outside condensation?

To stop condensation you must try to prevent warm wet air coming into contact with cold surfaces where the air cools and dumps some of its moisture.

Ventilation helps to cool the air in the roof space - and so does the insulation layer on the floor of the loft.

In the winter the outside air will be cool - it may well have some moisture content - but not as much as if it were warm air.

The physics remains the same even in Brigadoon - it's best to ventilate.

Alternatively you might consider reducing the loft ventilation and watch the result.

Good Luck...
January 2016