I've always used a diagonal* cross-hatch. Regular foam rigid insulation is typically shown as a square-on cross-hatch, so... For close-up large-scale details, l might add a stippled-dot overlay like "cement" or "gyp. drywall". Where there's an open exposed surface like when applied between or over rafters, I might use "treeline" as a boundry, redrawn if necessary to keep the bumpies on the outer exposed surface. Clarity of intent being more important than literal accuracy.