Closed-cell foams are also highly water-resistant due to the closed-off nature of their cells, as compared to open-cell insulation alternatives. These qualities make this type of foam perfect for outdoor appliances or for any product that might be exposed to the elements. Closed-cell foam insulation resists water vapor from passing through without the need for a jacket, assisting in inhibiting the growth and spread of mold and mildew within an insulation system.
As long as the line temperature does not exceed 180F, you can use this over the pipe system. You might consider using a rubber based insulation product - Armaflex - HST05812 or APT05812 for line temperature usages up to 220F.