Icynene LDC-50 is airtight but breathable.
The best analogy is a Gore-tex Jacket. With Gore-tex the fabric allows moisture produced by the body to diffuse out through the material but stops wind from the outside blowing through. That’s exactly the same as icynene which allows vapour and moisture to diffuse but also acts as an air barrier.
By breathable we mean icynene allows this moisture vapour diffusion.
Because of the foams open cell nature vapour diffuses through the foam from moist areas to dry.
This is called Bi-Directional drying, allowing any moisture within the foam to evaporate out.