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

Created on: 23 Feb 2020 14:53
M
M. Gerd
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.
L
Lumpi_LE
24 Feb 2020 11:09
Vapor retarders (climate membranes) are more forgiving of installation or construction errors than vapor barriers. Therefore, it is generally better to choose them. The additional costs are actually negligible, around 100-200€.

The site manager’s seminar was probably focused on cost savings, which is why vapor barriers were recommended.
P
pffreestyler
24 Feb 2020 14:24
@Nordlys I’ve been wondering this several times looking at your photos: why is the bare membrane visible around an existing window? I always thought that these membranes only withstand UV light for a limited time. Shouldn’t there be at least some protection like drywall or similar installed in the window area?

My cousin left his open for 6 years. The membrane became so fragile that just touching it was enough to tear a hole in it. However, it wasn’t a climate membrane but just a basic vapor barrier.
L
Lumpi_LE
24 Feb 2020 15:09
Most high-quality vapor barriers are UV-resistant.
N
Nordlys
24 Feb 2020 15:44
The window trim will be installed later. It doesn’t crumble that quickly.
M
M. Gerd
27 Feb 2020 18:29
As already mentioned, the issue has nothing to do with the costs.
Bookstar schrieb:

A standard vapor retarder would be wrong! It has a fixed sd value of 2, which could be insufficient in some cases, and your insulation might get wet.
Bookstar also wrote that a vapor retarder would be incorrect and that if anything, a climate membrane would make sense.