Simple rot13 encoder in vb.net

Question

I am looking for a simple way to encode an inputted text into Rot13. I am hitting a brick wall at the stage of being able to separate out words into individual characters and in

Answer 1

Looks like people are giving good answers to this but here's my try at it.

Dim input As String = "This is a Test!! Guvf vf n Grfg!!"
Dim result As StringBuilder = New StringBuilder()

For Each ch As Char In input

    If (Not Char.IsLetter(ch)) Then
        result.Append(ch)
        Continue For
    End If

    Dim checkIndex As Integer = Asc("a") - (Char.IsUpper(ch) * -32)
    Dim index As Integer = ((Asc(ch) - checkIndex) + 13) Mod 26

    result.Append(Chr(index + checkIndex))

Next

Console.WriteLine(result.ToString())

EDIT: improved to remove need for uppercase check. This will properly handle case and special characters with only 1 if statement inside the loop.