Maybe try some DIY fixes first for your DH. Take the windows out, by half opening the window, grasp the sides of the windows towards the bottom, push the windows up and then pull the bottom half towards you to release them from the runners. Check that the bottom runner area is clean, you may find a build-up of dust/dirt in there. Also have a look at the bottom of the window you have just taken out, to see if everthing is fine there, in our windows we have a strip of foam/fur type stuff. This is also a good chance for your DH clean both sides of the window.

This could be all that your windows need. I know that I need to do this every 6 months or so in our place.