Can HardiePlank Be Installed Over a Foam Sheathing?

Lap Siding

• The most common type of HardiePlank is lap siding, long boards nailed horizontally over traditional wood-sheathed walls, typically plywood or oriented strand board. Planks are nailed three-quarters to one inch from the top of each board, working from the bottom of a wall so each plank is overlapped at the top seam by another plank. It closely resembles traditional cedar or other wood planking.

Foam Board Thickness

• HardiePlank is installed over a water-resistant barrier or membrane, with moisture-blocking flashing behind planks where two ends meet. HardiePlank can be installed over thin rigid foam board insulation, fastened to the wall after the moisture barrier is installed. It should be used over foam board no more than 1 inch thick. Since HardiePlank is made to adapt to climate zones, it should be matched with an appropriate foam board for combined insulation effect.

Nailing Over Foam Board

• When using HardiePlank over foam board, care must be taken to avoid overdriving the nails. The nails should be driven in only until the heads are flush with the top of the HardiePlank. Hammering nails too hard can compress the underlying foam board and cause "dimpling," or depressions, in the HardiePlank.

Avoid Power Nailers

• Power nailers generally should be avoided when installing HardiePlank over foam board. Those mechanical devices tend to force nails too hard and far into the foam, creating depressions or even cracks in the HardiePlank. Other manufacturers of fiber cement planking make similar recommendations.