How long do siding nails have to be?
 
The CRA recommends and the Uniform Building Code now requires that the siding nails be of sufficient length to penetrate 1-1/2 inches into combined solid wood sheathing and framing. So, for example 1/2x6 plain bevel siding over -inch plywood sheathing and 2x4 framing would require a 2-inch long or 6d nail.

Remember that rigid foam sheathing has no nail-holding power, so these installations require a longer nail. For example, 1x8 T&G siding over 1-inch rigid foam sheathing, -inch plywood and 2x6 framing would require a 3-1/4-inch or 12d nail.