how to do variable substitution in 'sed'

I have a 'sed' editor command trying to read contents of one file into another.

v=t1
PX=25

sed '/for/ r /$v/ext/tsfirmfiles.${PX}' /bb/bin/px${PX}.files.2b.deleted > result

I cannot come up with the syntax so 'sed' would do a variable substitution.

Any idea? Thanks a for help. -A

use double-quotes