ᐅ Insulation for a Two-Family House – Installing a Climate Membrane Beneath the Insulation

Hello everyone,

I am new to this forum and hope you can help me regarding the insulation of a duplex house.

I bought a house built in 1920 last summer. Originally, it was a single-family home, which I now want to divide into two separate apartments. So the house will then have a basement, ground floor (first apartment), and first floor (second apartment).

At the moment, I want to finish renovating the ground floor (first apartment) to make it ready for renting. For this, I have just installed insulation with Isover stone wool in the ceiling of the ground floor (i.e., insulation above the ground floor ceiling).

Since I plan to renovate the first floor only after two years, I have the following question:

Because the upper apartment will remain unoccupied and cold for two years, I will need to install a vapor control layer beneath the insulation on the ground floor (I have chosen the Isover Vario KM vapor control membrane for this). After two years, I will apply the same on the first floor.

Are there any known issues with installing vapor control membranes on every level (i.e., both ground floor and first floor)?

Thank you very much for your help, best regards
Oliver