Yes, essential to seal the cardboard from the risk of condensation, unless you have a ventilation gap etc.
On a related note, A few years ago, I was interested in making insulated panels to build metal frame & mgo board Hexayurts from 'Ecobricks' so experimented with 'Making an Insulated Eco Brick by Filling Empty Plastic Bottles with Rolled Cardboard or Plastic & Evacuating Air (VIB))' which was tough as I could not make a strong enough matrix out of the cardboard filler to resist the outside air pressure.
I then moved onto helping the bricks with filling bottles with smaller bottles / cardboard & with expanding foam - Pics at my blog link)
https://cabinznet.blogspot.com/2018/09/making-better-insulated-eco-brick-by.html
THis lead later to a more adventurous bottles and cardboard insulated panels in a 3d Weave fabric
https://cabinznet.blogspot.com/2022/07/more-thoughts-on-reusing-plastic.html
