ᐅ Correct Installation of a Vapor Retarder Film

You could either include the rafter or staple it on the sides at the top and bottom as you described. In that case, stapling alone is not enough; you need to ensure an airtight seal using suitable adhesive or tape.

I once read that moisture can diffuse through the rafter or wood. Therefore, it is better to place the rafter with or beneath the vapor barrier. Do you know more about this, @Knallkörper?

Yes, that's true. It would be better to include the beam. The beam is about 16 cm (6 inches) high and has a diffusion resistance factor of 40, which results in an Sd-value of around 6 meters (20 feet). This is considered diffusion-retarding and should work with the assembly if the construction above is designed to be diffusion-open, but a vapor retarder typically has an Sd-value of around 100 meters (330 feet) and therefore provides more "security."

But if we later cover with drywall panels, the wrapped rafter will still be visible. That’s why we decided not to wrap it.