Add empty row on setValues() Google Script

Question

Column A :

Number
1
2
...
31

Hello,

Actually I'm looking for a mean to add empty row under conditions with a short running time on Google Apps Script. So I first tried to test a very simple example :

function insertRow(){
    var ss = SpreadsheetApp.getActive();
    var sh = ss.getSheets()[5];

    var temp = sh.getRange("A1:A"+sh.getRange("A1").getDataRegion().getLastRow()).getValues();
    Logger.log("Temp : " + temp);

    i = 0;
    while (i < temp.length && temp[i][0] != ""){
      if(temp[i][0] == 20){
        var old1 = temp.slice(0,i);
        var old2 = temp.slice(i);
        Logger.log("Old1 : " + old1+"\nOld2 : " +old2);
        old1.push([""]);
        temp = old1.concat(old2);
        Logger.log(temp);
      }
      i++;
    }
  }

But it adds me infinite row in my array and I'm blowing my minds to understand.

At the end, to make my script working,

I have to put sh.getRange(1,1,temp.length,temp[0].length).setValues(temp);

I want to use setValues() Format because making work this if statement cell by cell could take a very long time if there is too much data.

Can you please, help me on this ?