I know that in German the passive voice is formed with both sein and werden:
Das Haus wird gebaut
Das Haus ist gebaut
Are these two forms of the passive voice interchangeable?
Don't mix this, I guess this is a "false friend" for native English speakers. The passive of "I build the house/Ich baue das Haus" is "The house is built (by me)/Das Haus wird (von mir) gebaut" and not "Das Haus ist gebaut". The latter would rather mean "The house has been built" or to exaggerate that the "ist" is for describung a state "The house is something that has the property of having come to existence by a process of building". Or you can view "Das Haus ist gebaut" as meaning the same as "Das Haus ist bezugsfertig/The house is ready to move in"