Permanent venting through tiny holes in the frame is the worst that one can do against enrgy wasting . Reason : The permanent cooling down of the window frames makes them colder as they are any how and that causes additional condensation .Standard PVC frames as commonly installed here in Ireland are the weakest part of the insulation at any modern home .The legal demand for U-values for the pane as well as for the wall is higher than the (still unregulated !) U-value of a standard plastic frame .
Condensation is caused when the surface temperature is 6 degrees Celsius lower than the air temperature . That is a physical law .
Proper - energy saving - aeration is done for a short time , if necessary repeatedly. But thouroughly: all windows fully open as well as the doors to all rooms in need of ventilation. Reason : warm air transports more moisture than cold air .Once exchanged it has to warm up again to take the moisture from the interior (getting it out from furniture and carpets etc. ) .This warming up of the fresh air can't happen if it is blown out straight away . Cold air entering the room , blowing through and leaving it does not dry the moisture containing things but cools down the room . A physical law .
The best/most efficient time to have a thouroughly ventilation of the house is at night before bedtime . The fresh air would be drier/colder and the house wouldn't need extra heat from the heating system to warm up the temperature very quick to comfort level , it's bedtime any how . And turn off the radiators when having the windows open .
There are heat recovering systems available that are ventilating the rooms and recovering some of the energy contained in the stale air . But don't forget to build in a condensation trap if installing those .
Bofarr01 did not say if it is the glass pane or the frame showing condensation. Or both . A frame can be insulated by filling those trickle-vent holes and by adding an other layer of PVC to the outside. As I have done after having the problem with condensation on badly insulating PVC window frames . I solved this problem for good . If advice is needed how to do the extra insulation of the frame ( plenty of work, but cheap ) feel free to ask .
If the double gazing pane has aluminia spacers nothing can be done about the condensation along the spacers exept to exchange the panes for better quality ones .
If the double gazing pane has a leak than condensation can be seen between the panes . As well as oxidation of the aluminia spacer at the bottom of the pane where the condensed water goes . This oxidation is a white , frost like apearing dust on the spacer.