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Welcome to the Bunnings Workshop community @MalcolmS. It's sensational to have you join us, and many thanks for your question about insulating a hot water tank.
This is a great question, and you’ve made some very valid observations. Before going any further, the most important first step is to contact the manufacturer of your heat pump hot water system. Many manufacturers have strict guidelines on insulating their units, and some may explicitly advise against it. There could be warranty or performance issues if any modifications are made without their guidance, including the addition of insulation.
You’re absolutely right to be concerned about water getting trapped between a blanket and the tank. Since your unit is fully exposed to the elements, any insulation that doesn’t completely and tightly enclose the top could allow rainwater to run down and get caught between the blanket and the tank body. That trapped moisture, especially in a metal-on-metal or insulation-on-metal scenario, could accelerate corrosion significantly. Even condensation, particularly in Canberra's cold winters and frosty mornings, could pose the same risk.
Manufacturers may be able to advise whether condensation is a known issue in these setups and whether they have approved insulation kits or rigid covers designed for their systems. In some cases, they may even recommend or supply weatherproof enclosures or jackets that allow insulation while protecting the system from water ingress.
If the manufacturer gives the green light, an insulated housing or shelter around the unit, such as a simple timber-framed enclosure with a waterproof roof and adequate ventilation, could be a smart way to reduce thermal losses without introducing moisture risk. Just make sure any structure allows for airflow, service access, and doesn't affect the unit's performance.
Please let me know if you have any questions.
Mitchell
