What is this???

[frityreldarn]

Have 8 rolls of this. Adhesive strip on both sides it seems. But what is it??

 

[GoC]

Strips for external air-permeable sealing around windows during installation

 

[frityreldarn]

Strips for external air-permeable sealing around windows during installation
[media]

Not sure if I understand. Watched the video, it's not the expanding strip. But they place one on the outside. But if it is that one, why should it be permeable? Does it let through air, moisture?

 

[GoC]

I'm not quite sure if I understand. I watched the film, it's not the expanding strip. But they put one on the outside. But if it is that, why should it be permeable? Does it let through air, moisture?

The exterior of the building envelope should be vapor permeable but wind and waterproof to prevent condensation inside.
Search for the name of the strip, and you will find others from the same manufacturer. It wasn't entirely clear in the picture.
But it looks like a "sticky strip" that is mounted on the outside during window installation.
Think of it as functioning like a Goretex garment that is waterproof against rain but lets through humid sweaty air so you don't get wet on the inside.