The threshold under the door can be raised or lowered with a screwdriver so the door bottom weatherstripping seals tightly.
Energy losses from inefficient front and back doors can account for a significant portion of your monthly utility bills. When leaky doors create drafts, people tend to set the thermostat higher. This wastes even more energy.
There are ways to improve the efficiency of old doors. But you should consider the possibility of installing new ones. The costs of some well-insulated steel and fiberglass doors, especially those for the back door without glass, are very reasonable.
Inspect before deciding
Before making a decision, carefully inspect your old doors. If they are in very bad condition, it will be difficult to improve their efficiency by a meaningful amount. First, make sure a wood door is not rotting. Then place a long straight edge across the door to see if it is badly warped.
The most common problem with metal doors is rust, not warping. Check along the bottom by the weatherstripping on either side. Rainwater tends to collect there, and it is not always painted well.
If you find small holes rusted through, they can be repaired with car body filler and then painted. First, try to determine the reason water is collecting there and correct the problem. Clean out as much rust as possible before filling.
If the doors are reasonably sound, check for air leaks. At night, have someone shine a flashlight from outdoors around the seals and check for light indoors. This will highlight significant leaks. Or carefully move a lighted match around the seals and watch for movement. Check the astragal on double doors. This is usually the raised half-round overlap where pairs of doors meet and acts as a seal between them.
Often with wood doors, especially ones with compression weatherstripping, the main problem is simply that the latch plate is not holding the door tightly closed against the weatherstripping.
One solution: reposition the latch plate. This will require filling in the old screw holes and drilling new ones. Chisel away some of the wood in the recess for the latch plate. Another option: install an adjustable latch plate. You may want to reposition it for summer and winter as the door and frame expand and contract from temperature and humidity.
Steel doors should feature magnetic weatherstripping, so this is not a major issue because the weatherstripping is drawn against the door edge. Just make sure the door’s edge and its paint are clean and smooth, without gaps.
Check hinges and pins. If they are worn, the door will not seal properly. Take old ones to the store to match size. Don’t just buy the cheapest ones, the quality varies.
It is almost certain the seal on the bottom of the doors against the floor threshold is worn. If it is not worn, adjust the floor threshold higher. There are several height adjustment screws across the threshold, but after years of use, they may be filled in with dirt. Poke around to find them. If the seal itself is bad, there are many generic replacement seals you can install.
Another option is an add-on retractable threshold seal, effective if carpeting is on the floor by the door. The threshold seal is mounted on the inside surface of the lower door edge. When the door starts to open, a pin against the door frame is released and the seal automatically lifts to clear the carpeting. It is easy to install and adjust.