The first step to sealing air leaks inside your home is to find out where they are. There are a few common locations with which to start. Doors and windows are usually thought of as the most obvious culprits of inside air leaks since they are constructed openings into your home. And they are often sources of air leaks. However, they are not always the worst offenders. Attics and basements usually provide the most entrances for air to get inside and for escape of air to the outside. And these two areas work together to make your home inefficient: Warm air rises up into the attic and sucks cold air in through leaks in the basement. This is often referred to as the “chimney effect.”
But really, there are many places where air can get in and out. Often these are areas where separate materials meet, such as between the foundation and walls or between siding and brick. Other culprits are openings for utilities, including electrical, gas and cable, and openings for dryer vents and HVAC systems, as well as recessed “can” lights.
To find leaks, turn off your HVAC system, preferably during a windy day. Close all windows and doors and turn on any bathroom or kitchen fans that blow outside. Hold a flame around the edges of windows, doors and any other potential sites for air infiltration. If the flame flickers in a way that insinuates air is affecting it, you have an air leak. Using a candle is ideal because you can use its flame as well as blow it out and watch how the resulting smoke moves to determine if there is a leak.
Be careful using the candle method around flammable materials such as drapes and other furnishings. Use your discretion as to whether using a flame is feasible in each area you’re checking.
At night, you can try shining a flashlight over potential leak areas inside while someone outside looks for the light to appear on the other side. Note that this will only work for larger leaks.
If you can’t find leaks using the above methods or you want a thorough leak investigation done, you can hire a professional to perform a blower door test in your home. This is a fan that is mounted into the frame of an exterior door that pulls air out of the house, lowering the air pressure inside. This causes the outside air pressure to flow in through cracks and openings revealing leaks.