This is an amusing and a revealing photo taken recently at an inspection at a large commercial building. It was dark inside the building, and bright sunlight prevailed at the outside. The picture tells the story, pretty poor energy efficiency at this door. You might be thinking that I should have shut the door before snapping the photo. Well, look again, fact of the matter is that door is shut and latched.
All that daylight is the result of a poor fit, lack of weatherstrippings and a threshold. When you put those missing ingredients together, you have a door that does not protect the interior space from moisture, wind or cold weather. This photo brings home the point that not all doors are created equal.
The best way to check your door for gaps is to stand back and take a look at it when it is bright on the outside and dim or dark on the inside. Verifying that doors are a decent fit, and doing their intended job, is a good way to get started at preventative home maintenance.