It depends on the climate you live in. Radiant barriers should be installed only after you have air sealed and insulated, and generally help keep a building cool, not warm.
There are three primary ways heat is transferred:
- Convection (moving of warm air)
- Conduction (heat flowing through solid materials)
- Radiation
Radiant barriers do indeed block radiation, but radiation is by far the smallest source of heat loss and gain
Contrary to popular belief, most building heat loss is through convection. An building with no air leaks and no insulation costs less to heat or cool than a building with normal amounts of air leaks and lots of insulation.
The most cost-effective way to seal and insulate a building is usually to finish a ceiling, seal all cracks between it and the roof thoroughly with canned foam, and blow cellulose on top of it without a vapor barrier.
Spray foam is loved by building scientists because it both insulates and creates a very airtight seal if applied properly. It's very expensive, however, and will generally not pay for itself compared to cellulose.