Ok DIY's...?

Test it for yourself. Take a candle that is lit and hold it around a window to see whether there is a draft. According to my utility provider's website heat loss through the gl can be significant on an older home. Besides, a rat can enter a hole the size of a quarter and a mouse the size of a dime, so look at sealing up the windows as rodent proofing at the same time that you are saving yourself from heat loss.