Insulating a home involves more than the insulation in your attic. Weather stripping for your door is an important step.
Weather stripping is a simple step to improving energy efficiency of your home. On my home inspections, I find that the weather stripping is damaged or coming off of the door frames after five years of use, so a wise homeowner will inspect their door frames every so often to maintain their home. Weather stripping is such a simple product to install, and such a low cost, that you should be able to do all of your exterior doors in an hour.
Note: you should consider attic doors as exterior doors. Older homes hardly ever have these doors insulated, and rarely do I see them with an insulating weather strip. With the attic doors that drop down from the ceiling, you will want a thin strip (1/8″), because you could be preventing the door from closing.
Step 1: if the door frame has weather stripping which needs to be removed, you can use a sharp knife to cut between the stripping and the wood frame of the door frame. If any area still has some remnants of stripping left, you can use a light sandpaper (fine grit) to clean it off.
Step2: Find the right size weather stripping for your door at the home supply center or hardware store. Weather stripping comes in various sizes, so you need to plan here. When it is too thick, your door will not close. If it is too thin, you will have no effectiveness in the insulation value. Matching to the old product that was on the door is the best solution. If you had no weather stripping, buy a couple of different thicknesses. 3/8″ is fairly typical for doors.
Step 3: These strips have glue strips on the back. Peel backing layer off, while laying the insulation down onto the frame. Start in the top left corner, go down the right side, then along the bottom in one continuous strip. The left side of the frame is where the hinges are located. You can try placing insulation here if the door is not sitting flush against the frame. Frequently though, placing insulation here can prevent the door from closing properly.
Step 4: press the insulating strip in place with your fingers to ensure that it has good contact with the door frame.
Done. Sliding glass doors need a little more planning. Look at the spots where you have gaps, then find an appropriate weather stripping for that gap.