Ask the Builder: Leaded glass poses weatherization puzzle

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, Tennessee

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.

Let’s talk about the leaded glass first. I grew up around old doors that had true leaded glass panels that leaked vast amounts of air. In the panels made a century ago, the H- and U-shaped pieces of lead stripping surrounding each piece of cut and beveled glass didn’t have any type of sealant. While the air seeping around a given piece of cut glass was not much, when you add it all up for the entire panel, it could be significant.

These days leaded glass panels are almost always sandwiched between two pieces of glass. 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.

Mind you, all modern exterior doors do leak some air. Door manufacturers are trying their best to produce a leak-free product, but it hasn’t been invented yet. High-quality modern doors leak very little air around the door seals when installed properly. So the first question you need to ask is, Was your door installed correctly?

I’m now going to frustrate you a little more by adding some simple physics to the mix. Your current door may be installed well, and the door seals may be doing their job. In that case, what you’re feeling may be simple cold-air convection currents.

If it gets bitterly cold outdoors, the inside temperature of the glass panels in your doors can be significantly lower 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 could see the air moving past your door, it would resemble a slow-moving treadmill. The air layer right next to the door would be constantly falling down toward the floor. This cold air needs to be replaced so warm air at the floor rises to replace the air that’s falling.

I have observed this in my own home by using a lit kitchen match. I have several large French doors that are primarily glass, and I know there’s no air leak through them. I’ve lit a match and blown it out, holding the smoking match at the base of the glass. You can see the match smoke being pulled down toward the floor. It’s pretty dramatic.

There are quite a few steps that have to be followed to eliminate air leaks and ensure the weather stripping 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 weather stripping at one location and the door may not contact it at the opposite diagonal corner.

The most common cause for this is that 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.