Jack's neighbors called the cops - that's how loud the noise was when the top plate of his cripple wall broke - and they worried that he had shot himself. . Before we get into the details of Jack's case, who knows what a cripple wall is? . . . . Yeah, about what I figured. I'll keep it short.
The cripple wall is a short structural wall built within the crawlspace and designed to provide support to the home. It replaces the old post and beam system that we used in home up through the 1970's. The cripple wall is less expensive to install, less likely to move (it's usually built on a concrete footing), and easier to build. I've put in a picture to help you visualize it. As you can see, it looks just like any other framed wall - a sole plate, a top plate, studs. The size will vary depending on how much load needs to be managed but most are either 2x4 or 2x6 pieces of lumber.
There is another similarity to the walls of your home - since this is a structural wall, it needs headers when the studs are removed just as the exterior walls have at the doors and windows.
Most of the time, these are installed automatically to make moving through the crawlspace easier - even for a skinny inspector, trying to squeeze my butt through a 14 inch opening takes a little wiggling.
Sometimes though, I find that either a plumber or the HVAC installers decided that the studs are in the way so they knock them out. No biggie, they think, quite erroneously.
Whoever installed Jack's ductwork knocked out two of the supports so instead of bracing every 16 inches as needed, the top plate spanned 4 feet. That's a long way for a 2x4 to carry load.
The 2x4 also had a knot in it at about the midpoint of the span. Knots are weaker and more prone to split. When they do, it happens fast and sounds explosive.
The effects of the HVAC contractors abuse of the foundation system aren't over. Each part of the system relies on the other parts to perform. Now the studs at either end of the opening are doing more work than they were intended so they will become increasingly likely to fail as well, and, like a set of very slow moving dominos, may affect the whole cripple wall and the interior walls above it in the living spaces.
The good news is that repairs are both possible and not very challenging or expensive. We need more support for the cripple wall on both sides of the ductwork. We also need a fix for the broken top plate - which is where I punt and suggest you need an engineer or really good structural contractor who's done these types of repairs before.
For those that are curious, this house is less than seven years old. If an inspection had been done on it at the time of purchase, the damaged cripple wall likely would have been spotted and fixed.