I have double hung windows. One problem is that the top sash tends to slip down, so that cold air blows in the top. The latch does not work on most of the windows because they are not sized correctly.
One thing I have done is to cut a piece of wood the exact size so that it fits between the lower and upper sash tightly and forces the upper sash closed.
It is kind of inconvenient to make these special props. Are there standard products that are designed to be adjustable and can be used to prop up a window sash?