Since you are talking about air leaks and reducing heating/cooling costs I thought I'd pipe in with some basics for people, many overlooked.
Places to look for air infiltration: Around door and window frames, not just the closures on the doors and windows themselves, these leaks are often easily solved with a simple paintable latex caulk.
All outside wall switches and electrical outlets. There is a thin foam insulation that is pre-cut to fit behind the covers and is found at most hardware stores. This is an easy fix that is often overlooked!
Around the perimeter of the foundation where the wall sits on it. In modern construction, a foam strip is placed between the plate and the concrete foundation, However on most older homes this is not the case!.
Infared imaging or photography can be used to find "hotspots" by taking pictures of the outside of the structure when the temperature differential is great enough.
Of course, by now we all know the importance of attic and crawlspace insulation, also insulating any heating ducts (and water pipes)that pass through unheated areas.
One way to find areas of air infiltration is to take a candle on a windy day and move slowly around ANYTHING you suspect of air leakage. The candle will flicker, of course.
That is a cheap D.I.Y. way of doing it.
I try to add to my insulation and air infiltration problems every year, and now it is much harder to find problems to fix, some of them are places that older solutions have failed over time!
When I bought my current house 15 years ago, it had very little insulation but quite typical amounts for an old house in this area. Over the years, I have lowered my dependence on energy to stay comfortable. My current usage is 25% of what the original use was.
Of course, there is such a thing as too tight a house! The solution to that is a heat exchanger designed to swap inside air for outside..
