Figure 8-2 Sectional view of a double hung window showing rooms and areas of air leakage
Condensation and frost problems on the interior surface of windows are common. It could be a light mist on some windows or a persistent frost covering the entire glass. Many homeowners buy new windows and find that the problem only got worse because the old leaking window was actually helping to reduce humidity. The new windows make the house more waterproof, which increases the humidity level. One of the solutions to this problem is to reduce the humidity level in the house.
You can also increase the surface temperature of the window and frame by installing another layer of glazing. New energy efficient windows are the best solution.
When the condensation lodges between the panes of the unsealed window panes, the humidity level in the house has escaped around the interior pane to escape condense on the outer glass.
Even with a low humidity level, a house can experience this kind of problem which often occurs on the second floor where there is more air coming out the window due to the chimney effect. It is therefore necessary to install a weatherstripping on the inner frame to prevent air leaks; but make sure the storm window drain holes, which allow water to drain, are open to the outside.
If condensation occurs inside a sealed double-glazed window, the problem can only be corrected by replacing the glazing. Although some specialist companies offer the refurbishment service for poorly sealed glass, this repair is temporary and will not match the initial level of energy efficiency. Check if the window is still under warranty.
(Source : http://www.rncan.gc.ca/node/15644)