can Flex duct be installed in a concealed floor/ceiling cavity? I thought that ducting in a concealed cavity had to be rigid, not flex, because flex doesn’t last as long.
If you are referring to a dryer vent, you are correct. However, some listed semi rigid vents/flue liners used for venting combustion products are okay to be concealed. Same for HVAC air ducts. So it depends, you have to be more specific as to which duct you are referring to.