Should I nail the boards?

[Curt]

I took the time to inspect my crawlspace. Besides finding loose insulation, I noticed two small boards in between each floor joist that are just hanging by a nail.

The two boards are crossed but not nailed. Should I nail the boards and what are they for?

[home improvement guy]

Not all floor joists require the X-bracing you described, but you should nail the loose bracing to the joists to take some of the bounce out of your floors.

The depth of the joists determines if X-bracing is required and the code basically states that the bracing is required if the joists are greater than 12 inches in depth.

When weight is added to the floors above, the joists tend to deflect or bend slightly. Secure the boards to the bottom sides of the joists using one to two 8-penny nails on each board.

When the X boards are secured, the weight applied to the floor is deflected from the side of one joist to the side of the next joist and so on and so on.

What I think has happened to your house, and several others that I have seen, is that the carpenter set the X-bracing before the subflooring was installed and had every intention of completing the bracing once the home was under roof.

Sometimes minor things that are out of sight are out of mind and are left incomplete.

The bracing is not an immediate repair, but you could make it a rainy day project and at the same time check your crawlspace for potential leaks.