Barbara writes from Bellingham, Washington: "Our home is 29 years old and the insulation is getting thin, a local home inspector suggests adding R19 blown insulation on the top of the old insulation. Do we need to add plastic on top of this blown insulation?"

Answer: No we do not recommend installing plastic over existing insulation before you add blow-in insulation. Nor do we recommend installing it over the new insulation. The vapor retarder should be on the warm-in-winter side and putting it on top of either insulation would not place it properly. When adding insulation to existing insulation, you generally don't need to add a vapor retarder.