XIRR - naming ranges to include monthly cash flows only

Ok I wrote a macro that gave me a range to feed the IRR function (variables are for other IRRs I need to add later) but the last line didn't work Range("E180").Formula = "=XIRR(" & str & ")", the str is ok though, can someone please help me fix it?

PHP Code: 

Sub CalculateIrrs()
Dim c As Range
Dim cRow As Integer, startcol As Integer, lastcol As Integer
Dim rng1 As Range
Dim strSearch As String
   Dim Cell As Range
   Dim i As Integer, j As Integer
   Dim str As String, addi As String
   Dim Columnnumber1, Columnnumber2
   Dim ColumnLetter1 As String, ColumnLetter2 As String
   Worksheets("Model").Activate
   startcol = 6
   lastcol = Worksheets("Model").Cells(6, Columns.Count).End(xlToLeft).Column
     strSearch = "UL PT Net Flows - Unlevered Pre-Tax"
    Set rng1 = Range("B:B").Find(strSearch, , xlValues, xlWhole)
    rng1.Select
   Set c = ActiveCell
   cRow = c.Row
  Columnnumber1 = startcol
  j = Worksheets("Inputs").Range("ModelLastYear").Value - Worksheets("Inputs").Range("ModelFirstYear").Value + 1
 For i = 1 To j
 'MsgBox i
   Columnnumber2 = Columnnumber1 + 11
 ColumnLetter1 = Split(Cells(1, Columnnumber1).Address, "$")(1)
 'MsgBox ColumnLetter1
 ColumnLetter2 = Split(Cells(1, Columnnumber2).Address, "$")(1)
    'MsgBox ColumnLetter2
 addi = ColumnLetter1 & cRow & ":" & ColumnLetter2 & cRow
str = str & "," & addi

Columnnumber1 = Columnnumber2 + 2
Next
str = Right$(str, (Len(str) - Len(",")))
MsgBox str
Range("E180").Formula = "=XIRR(" & str & ")"
 End Sub 


thanks!