Sealing Air Leaks in Doors

DEAR TIM: My exterior doors have leaded-glass inserts that I don’t feel are insulated well. I’m confused as I can’t feel the leaded glass because it’s sandwiched between two pieces of glass. On cold days when I stand in front of the door I feel air movement. How can I make sure this door is sealed well so that air leakage is at a minimum? What should have been done during the installation to minimize air leakage? Nick P., College Grove, TN

DEAR NICK: Air movement and leaks around exterior doors are not always easy to detect, and to make matters worse, what you think is an air leak may not be one. Are you frustrated yet?

Let’s talk about the leaded glass first. I grew up around old doors that had trued leaded glass panels that leaked vast amounts of air. The H and U-shaped pieces of lead stripping surrounding each piece of cut and beveled glass didn’t have any type of sealant when the panels were made nearly 100 years ago. While the air coming around each piece of cut glass was not much, when you add it all up for the entire panel, it was significant.

This is a new door, but if not installed properly it will leak vast amounts of air causing discomfort and increased fuel bills! Photo credit: Tim Carter

Modern leaded glass panels are sandwiched in between two pieces of glass in almost all cases. That’s what I have at my own home. This insulated glass panel performs well and there’s minimal leakage around the edges of the sandwiched glass if the workers at the factory did their job.

Also, all modern exterior doors do leak some air. Door manufacturers are trying their best to produce a leak-free product, but it’s not yet been invented. Mind you high-quality modern doors leak very little air around the door seals when installed properly. Therein is the rub. The question is: Was your door installed correctly?

I’m now going to frustrate you a little more. Let’s add some simple physics to the mix. Your current door may be installed well, the door seals are doing their job but what you’re feeling is simple cold-air convection currents.

If it gets bitter cold outdoors, the inside temperature of the glass panels in your doors can be significantly different than the air temperature indoors. As your indoor air contacts this cool or cold glass panel, it drops in temperature. We all know that hot air rises, but few remember that cold air falls.

If you put your head against the wall and looked down the wall past your door and you could see the air, it would resemble a slow-moving treadmill. The air layer right next to the door would be constantly falling down towards the floor. This cold air needs to be replaced so warm air at the floor rises to replace the air that’s falling.

I experience this myself at my own home as I have several large french doors that are primarily glass. I’ve blown out a lit kitchen match at the base of the glass where I know there’s no air leak through the door and you can see the match smoke being pulled down towards the floor. It’s pretty dramatic.

There are quite a few steps that have to be followed to eliminate air leaks and ensure the weatherstripping seals on an exterior door work as designed. In the limited space I have, I’ll just hit the high points.

First and foremost, the door frame needs to be installed in the same plane as the door itself. It’s very easy to twist the door frame while installing it. If this happens and you screw and/or nail the frame in place, the flat door will not contact the weather-stripping uniformly. It will crush the weatherstripping at one location and the door may not contact it at the opposite diagonal corner.

The most common cause for this is the rough opening in the wall is twisted. Installers will nail the door to the rough opening so the jamb is flush with the interior and exterior wall coverings so the door trim looks good. But if the opening is twisted, this practice leads to air leakage.

A common source of air leaks is under the door threshold. It’s important for the threshold to be level, but the subfloor may not be level. The installer puts shims under the door to level the threshold and this creates a gap. Caulk is normally used to seal this , but installers can and do goof this up.

The sides and top of the door frame jambs need to be sealed. Expanding foam that’s designed for windows and doors is the product to use. To be safe, apply it from the outside and if it doesn’t extend all the way to the inside wall, fill the remaining gap with lightly compacted fiberglass insulation strips. Practice with the foam because it can expand far more than you realize.

I always use modern rubberized asphalt flashing tape as a final barrier. Once the foam has hardened and I’ve trimmed it flush with the outside of the door frame, I install a 2 or 3-inch-wide piece of weatherproof flashing tape that laps over the exterior sheathing and onto the door jamb. I hold the tape back from the door jamb enough so that the door trim hides it. This flashing tape will not allow air past it, so it’s an excellent product to consider.

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