In many older homes, the windows still have the old style sash weights and pulleys and are single pane glass units. Not only are these type windows inefficient against heat loss, most are glazed with glazing putty that has cracked and dried from age and is most likely missing. Take a good look at the your windows and check the sash weight cords to see if they are broken as well. Missing or badly frayed sash ropes can be dangerous as it makes the window difficult to open and close and by using a prop stick to hold the window up in warm weather, you are inviting an accident when someone accidentally knocks out the stick and the window comes crashing down on tiny fingers perhaps or even worse a neck!
If your survey shows the windows to indeed be old style single pane windows it is time to replace them with new thermally efficient and much easier to operate units. Replacement windows come in all shapes and styles today. Most commonly seen are white or brown vinyl types as these are also the cheapest to buy. Many manufacturers will make custom colored units for your project. All brands are not the same however.