Can I seal it as a bivibag?

breath has a large water content

The ShellTA fabric is not a vapour exchange fabric and should not be completely sealed. However you only need a small hole at the face to prevent asphyxiation.

Directing your breath out of the ShellTA (or any bivibag) will reduce the liklihood of condensation forming inside as the exhaled breath has a very large water content, as I’m sure you’ve noticed when you breathe out on a cold day.