What is recommended to hold batts and retain R value?

Dave writes again from Waterbury, Vermont: "Following-up on a previous question that I misstated. I have r21, faced, fastbats going into 2 x 6 ceiling cavities that are 16.5 OC. However, in five of the cavities the width varies from 16.75 OC to 17. The result is the fastbats are a bit too NARROW. Two problems: 1) the bats don't stay in the cavity and; 2) appears that air could leak around the sides all the way along the cavity. What is the recommended technique to hold the bats in and retain R value? I assume if use a metal insulation hanger (u-nails and nylon string or metal spring steel) that would hold the insulation but air would be going around the sides. I also thought I could pad the interior of the joists with a foam board product but worry about condensation." Answer: If the product is too narrow to fill the cavity. It may be necessary to fill the cavity with cut-to-fit unfaced insulation. To maintain the vapor retarder, it may be necessary to cover with a 4- to 6-mil poly.  more