Is the vapor barrier in the roof vapor-permeable? Yes? No!

Hello everyone,

we have a tent roof with the following structure:

- Tiles
- Underlay membrane
- Insulation
- Vapor barrier

Actually, a vapor-retardant membrane with diffusion openness should be used, but the site manager mentioned that in his last training, for our roof, a regular vapor barrier, meaning not diffusion-open, was recommended.

What is your opinion? Are there any reasons why one should not use a diffusion-open vapor barrier?

Thanks in advance

Oh dear, a lot is being confused here. There is no diffusion-open vapor barrier!

Vapor barrier = high SD value and vapor tight!
Vapor retarder = SD value around 1-3 and vapor slowing!
Diffusion-open foil = vapor can pass through unimpeded = diffusion-open

He probably means a diffusion-open underlay membrane. This must necessarily be present in the case of insulation between rafters.

OK, please excuse my ignorance, but I am just a layman in the field, although I am trying to understand. At least now I know the difference.

I do not mean an underlay membrane, but the foil that is installed after the insulation has been placed between the rafters.

At the beginning, a vapor retarder should be used, and the site manager has now mentioned an energy-saving foil with an sd value of > 100. So a vapor barrier.

The question now is whether it makes sense to use a vapor barrier and not a vapor retarder, or when to use which foil.

The climate membrane, which is selectively permeable, is probably too expensive for him. We have it, and I think it's okay.

No, it was nothing financial. He explicitly said that this is not allowed for our roof.