Did you know that your siding ISN'T water tight? Did you also know that siding is NOT the water barrier for your home? It doesn't matter what kind of siding you have, water WILL penetrate behind it at some point in time. This is true for vinyl, fibered cement, and all other forms of siding, EVEN brick! Even though the siding guy caulked all the joints, materials expand and contract with changes in the weather and eventually it WILL crack. There are many types of water barriers currently used in building; one of the most commonly know products is house wrap (All house wraps are not created equal BTW). Your siding is only intended to protect the water barrier from damage from weather and UV rays. Once water gets behind the siding you need to get it dry ASAP. Water can come in several forms, from rain to condensation. Moisture inside wall assemblies is responsible for all kinds of nefarious situations ranging from wood rot to harmful mold, mildew and fungus. I don't think anyone wants that inside their walls. Building scientists agree that the best way to get rid of moisture behind the siding is a rain screen. A rain screen is an air space between the siding and the water barrier that allows any moisture to drain out by gravity and dry without getting trapped behind tightly nailed siding before it has a chance to soak into the water barrier and wet your wood. Remember, house wrap is not water tight either! Siding, wood, and paint will last much longer when it stays dry than it will when the back stays damp. Many sidings are expensive and so is painting; don't you want them to last as long as possible?
Notice the metal flashing over the window trim. The flashing will direct any surface water out and over the windows where it will safely drain away. The window is sealed to the sheathing to keep any water out. Any water that does get behind the siding will drain out the bottom of the wall. Keeping water out of your home is the top priority. If water gets in nothing else matters!
The picture shows a home with pre-painted fibered cement siding. The water barrier is the green board seen at the top behind the siding. The windows, doors, and all joints are all taped and sealed to keep water from getting inside the home and causing damage. The vertical strips behind the siding are the rain screen. The rain screen spaces the siding 1/2" off the water barrier sheathing which allows it to be vented at the top and bottom. Any water that gets behind the siding can drain out and dry making the siding, the paint, and the structure behind last much longer than they would if they were damp though-out the year. Water intrusion is much more likely when the walls are taller and the overhangs are smaller.