Show alert and Redirect in ASP.Net

after clicking  the button it is redirectiong without displaying alert box

please tel the changes in the code.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;

public partial class feedback : System.Web.UI.Page
{
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
    protected void Page_Load(object sender, EventArgs e)
    {
        con.Open();
    }
    protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
    {
        SqlCommand cmd = new SqlCommand("insert into feed values('" + TextBox1.Text + "','" + TextBox4.Text + "','" + TextBox5.Text + "')", con);
        cmd.ExecuteNonQuery();
        con.Close();
        string message = "Thank You For Your Feedback";
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        sb.Append("<script type = 'text/javascript'>");
        sb.Append("window.onload=function(){");
        sb.Append("alert('");
        sb.Append(message);
        sb.Append("')};");
        sb.Append("</script>");
        ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", sb.ToString());
        Response.Redirect("home.aspx");
    }
  
}