Old building codes used to require a vapor barrier on the "warm in winter side" of the wall.
New building science now advises not to install a vapor barrier on either side and definitely not on both sides.
- When your wall gets wet, you want it to be able to dry in both directions.
- A vapor barrier on both sides of your insulation will trap moisture inside your wall.
For more information:
Read Tessa Smith's Q&A "In a passive house wall assembly, is the vapor barrier on the inside or outside?"