"Any quick and easy ideas on how I can find the source of drafts in this house" Hold a lighted candle near an area where you suspect air is leaking and the flame will quiver if there is air movement. Sometimes people tape plastic over the fireplace front to prevent heat from leaking out. You have the damper closed I hope? " and whether caulking would hold up around a window that's icy to the touch?" As long as the surfaces are dry, silicone can be applied when it's cold but it gives the minimum temp it can be applied on the tube(I can't remember what it was) so you will need to check it out.