Q: How often does insulation need to be replaced?

A: Spray foam will last as long as the building. Dense-packed cellulose and fiberglass tend to settle down over time and so become less effective. However, taking off your siding or drywall is expensive and the reduction in heating bills will likely not pay for the cost of this work even if your insulation is in poor shape. It is better to add insulation opportunistically see Chapter 2 in Zero Carbon Home.