It was about a year ago that I started using sbuild and sbuild. At the time, I had found that I needed a workaround to get around the fact that aufs doesn’t work on fs, which I use for my rootfs (and most other fs’s, bc ext4 crashed hard the very first day I installed it). Last weekend I had a snafu which caused me to reinstall my rootfs, but I’d forgotten all about needing this workaround.
So here it is – if you have /var on xfs and get errors about mounting when starting schroots, then add
tmpfs /var/lib/schroot/union/overlay/ tmpfs defaults 0 0
to your /etc/fstab.