The fact of the matter is, when covering the chicken with foil, you'll never get the crispiest results possible, as that steams the chicken. However, removing the foil part-way through the process and then broiling at the end can help crisp the skin further. You can also press some butter mixture beneath the skin before roasting. Alternatively, you can remove the skin after baking the chicken and shallow-fry it in oil until crispy.