While not every wall requires a vapor barrier, if you do need one and don’t install it, moisture can get between the walls of your home. This can lead to water damage from mold growth or rot, and the excess moisture could also attract pests, including termites, cockroaches, or ants.

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. 

Yes, insulating ductwork in a crawl space is a worthwhile investment, especially if you live in a colder climate or run your air conditioning for most of the year. Uninsulated ducts can lose up to 30% of their heating and cooling efficiency, which can significantly impact your electricity bill. The project is a relatively small investment and typically only takes a few hours. It's a particularly simple add-on if you are already hiring someone to insulate the entire crawl space. An added benefit is that insulated ducts put off less hot or cold air, making them less attractive to pests.

Insulation can be easy to install, depending on the condition of your walls, but there are many safety considerations and potential challenges involved. For example, most types of insulation are made from materials that can cause irritation if inhaled or if they come into contact with your skin. Additionally, installing insulation incorrectly can actually make moisture issues in your home worse.