ᐅ Which type of vapor retarder is recommended for this construction assembly?

Hello,

we have renovated the roof structure in our older house and have now installed the insulation between the rafters. The rooms under the roof are a living room, a bathroom, and an office.

This is the roof composition we have or plan to have:

· Clay tiles
· 80 mm (3 inches) insulation above the rafters (wood fiber boards Pavatex)
· 240 mm (9.5 inches) insulation between the rafters (wood fiber Pava-Flex)
· Vapor retarder
· Battens
· Gypsum plasterboard

Yesterday, we visited the local building materials supplier regarding a vapor retarder. They offered two options:

1. A classic one with an sd-value of 100 (this is "tight" and allows little to no moisture through)
2. A breathable one with an sd-value of 2 (Delta Luxx by Doerken)

According to our carpenter, it doesn’t matter which type we choose—that is, either a vapor retarder that allows moisture to pass on both sides like the Delta Luxx or one that is mostly airtight (with an sd-value of 100).

We are now unsure and would like to ask for your opinion on which type of membrane you would recommend for this roof setup. Especially considering that one of the rooms will be a bathroom.

I would also appreciate it if you could provide the exact designation of the vapor retarder (manufacturer and type) with your recommendation. There are many producers such as Isover, Würth, etc. So, which products are commonly used, have a good reputation, and offer a good price-performance ratio?

Thank you in advance for any suggestions.

Best regards,
Markus

No one has an answer? 🙁

As far as I know, it does not matter. However, whether this is really the case is another matter.
A bathroom in a typical house is usually only exposed to higher humidity for short periods compared to other rooms, due to regular ventilation.

The best option is probably to seek advice from a professional.