Can they withstand weather for a few months? I want to use it to wrap a swim/spa in the winter to help lower the power bill.

MidWest Canvas concrete curing foam insulating blankets are weather-resistant and durable enough for temporary outdoor use, but they are not designed for long-term exposure over several months. They can handle rain, cold, and short winter periods, but extended UV, wind, and snow will cause the outer material to degrade. You could use one short-term to reduce heat loss, but it’s not ideal or guaranteed for a full winter around a swim spa.