ᐅ Vapor barrier on the roof: vapor-permeable? Yes or no?

Hello everyone,

We have a tent roof with the following structure:

- Tiles
- Underlay membrane
- Insulation
- Vapor barrier

Originally, a vapor-permeable vapor barrier was supposed to be used, but the site manager mentioned that in his last training, for our roof type, a standard vapor barrier, meaning not vapor-permeable, was recommended.

What is your opinion? Are there any reasons why a vapor-permeable vapor barrier should not be used?

Thanks in advance.

Oh dear, there is a lot of confusion here. There is no vapor retarder that is vapor permeable!

Vapor retarder = high SD value and vapor tight!
Vapor barrier = SD value around 1-3 and vapor retarding!
Vapor-permeable membrane = vapor can pass through freely = vapor permeable

He probably means a vapor-permeable underlay membrane. This is absolutely necessary when using insulation between rafters.

OK, please excuse my lack of knowledge, as I am a layperson in this field, but I am trying to understand. At least now I know the difference.

I am not referring to a roofing underlayment but to the membrane installed after placing insulation between the rafters.

Initially, a vapor retarder was supposed to be used, but the construction manager has now specified an energy-saving foil with an sd-value of over 100, so a vapor barrier.

The question is whether it makes sense to use a vapor barrier instead of a vapor retarder, and when to use each type of membrane.

He probably finds the climate membrane, which is selectively permeable, too expensive. We have it, and I think it’s fine.

No, it was not about finances. He explicitly said that this is not allowed for our roof.