Career
He was employed at the National Archives of Norway in 1858, and was acting Director-General several times before being formally appointed in 1896. -Kaas published several works on Norwegian history, including several volumes of Diplomatarium Norvegicum. He was also interested in genealogy (particularly noble genealogy), heraldry, sigillography and biographical history.
He started the work Norske Sigiller fra Middelalderen (1899–1950).