The primary downside of radiant barriers is their limited effectiveness in cold climates. They are designed to reflect radiant heat, making them less beneficial in cooler conditions where heat retention is more important. 

Radiant barriers require proper installation to maximize effectiveness. Incorrect placement can lead to minimal benefits. They also do not provide insulation, meaning they should be used in conjunction with traditional insulation for optimal energy efficiency. 

Several types of insulation are effective for crawl spaces, with spray foam and rigid foam boards being two of the best options. Closed-cell spray foam insulation is a strong choice because it effectively repels moisture, resists biological growth, seals the air, and provides excellent thermal protection, especially in hard-to-reach areas. Rigid foam insulation boards are another excellent material as they insulate well, resist water damage, and will not settle over time like fiberglass or cellulose.

The primary reason that insulation saves you money over time is that it reduces your energy bills by making your home more energy efficient. A crucial part of an insulation system is a properly installed vapor barrier. Local building codes dictate whether your insulation requires a vapor barrier based on the type installed. While some high-quality wool insulation may not technically need a vapor barrier, most municipalities still require one to meet code.

Reflective and high R-value batt or roll insulation are often considered energy-efficient, offering significant thermal resistance benefits.