"Are radiant barriers a good thing to invest in?"
The answer depends on where you live and what properties are already existing in your home.
Radiant barriers are designed to block the sun rays from being absorbed by your home. This can be confused with thinking it has R-value, or it as a type of insulation. Radiant barriers do not insulate your home, it reflects heat waves.
This is why they are placed in your attic. Radiant barriers are ideal for warm weather climates. Climates where homes are designed to cool inhabitants will benefit most from radiant barriers. Places like Arizona or Miami can see savings stack up over the years using this technology when implemented properly. This is because they receive 300 plus days of sun a year reaching well over 100 degree weather daily! That's a lot of radiant heat hitting their attic space. And that makes for a lot of heat for the radiant barriers to reflect.
In climates that homes are designed to maximize the sun's radiation and heat the home's inhabitants, radiant barriers can be counter intuitive. Homes built with lots of insulation are working to keep heat in, not reflect it away. Our marine climate, here in the Pacific Northwest meets this scenario. With only 120 days of sun each year, we want to keep our homes as dry as possible.
Following EnergyStar's advice, we recommend these good home practices:
Check you insulation, make sure you have R-38 or better in your attic spaces. Walls and crawl spaces should also be addressed.
Whole house sealing leaks reduces drafts, will make you feel and overall comfort, and will drastically reduce energy consumption.
Properly seal any ducting in your home. This can make up to 40% of heat loss if ducting runs in an uninsulated area.
Update old windows to EnergyStar windows on older homes.