Striking a balance between warmth and ventilation is important and done correctly can be very effective.
A common thought is that by opening windows or ventilating you are losing heat – but what you’re actually doing is allowing warm moist air to escape and cool dry air to enter your home. Dry cool air is actually cheaper to heat than warm moist air.
Many homes have double glazing and experience problems with condensation and mould growth that didn’t happen with old draughty window frames, because the natural draughts around the poorly fitted windows have been sealed. Using trickle vents or opening window slightly achieves the necessary air circulation.
Remember the point is to ventilate for an appropriate period of time, not to leave the windows open all day long!