Radiant barrier does not take the place of traditional insulation (fiberglass, cellulose, etc.). Traditional insulation has R-value; this works to slow conductive heat while radiant barriers reflect radiant heat. Both types of heat are trying to enter your building on a hot, summer day. Traditional insulation and radiant barrier work together and each do their part. Radiant barrier is your first line of defense (against radiant heat) and traditional insulation (fiberglass or cellulose) is the second line of defense against conductive heat gain. If your building is conditioned (with a/c or a heater) then you should use both radiant barrier and regular insulation.