There are three main causes of condensation
1) Moisture content in the window timbers
If you have had cracked or broken putty (even with fine hair line cracks) for some time, rain will have been running down the glass and soaking into the putty, particularly along the lower edges of the glass, and this will have soaked into the window timbers and glazing bars.
This moisture then creeps underneath the glass and comes out on the inside between the primary and secondary glazing, condensing on the inside of the glass and causing condensation to be trapped in the void. If the window timbers have already become saturated with water, then even if the windows are subsequently restored, moisture already in the wood will evaporate into the void and cause condensation. The only way to ascertain if the window timbers are saturated is to take readings with a moisture content detector. Perfect joinery-quality timber has a moisture content of around 12% and we have found that windows suffering from condensation can have moisture content as high as 30-40%.
One solution to saturated timbers is to paint the window frames inside the void between the glass and the magnetic strips with a waterproof yacht primer. This prevents moisture evaporating into the void. However, the rest of the window frame is still painted with normal paint which is micro-porous and allows the timber to breathe.
2) Background humidity levels of the house
Every house will vary, even two houses next door to each other, because the ground underneath the house will have different moisture content in different places, with spring-like activity occurring in the soil beneath the house. Saturated ground and moisture ingresses particularly, are strong where a property is below a hill or the outside ground level is higher than the floor level on any of the walls.
3) Low ventilation levels
Other factors that contribute to moisture in the house are low ventilation levels. Larger rooms have more capacity to absorb the moisture created by cooking, washing and laundry (drying laundry in the house emits 2-3 litres of water per load, so drying laundry in the house significantly raises the humidity levels of the house).
To reduce humidity levels and ventilate the house regularly, consider damp proofing measures such as injected damp proof coarse or tanking floors. Also consider drying laundry outside if possible and opening the windows while cooking and bathing. In particular, throw open all the windows for a couple of hours on dry days.