Just got this from James Hardie Products.
"All James Hardie panel cladding products, including HardiePanel® vertical siding, Reveal® panels,
Cempanel vertical siding, and any other James Hardie fiber cement panel siding, require a
rainscreen (3/8 inch air gap behind the cladding) when installed under any of the conditions listed
below:
1 | Exterior walls without eave overhangs; or
2 | Exterior walls that are 3 stories and taller; or
3 | Exterior walls where fiber-cement panels are installed with express seam joints"