What is this material?

[RoyMunson]

The dark board, that is. It's kind of soft if you stick a knife in it.

The picture is taken from the inside where I've removed plasterboard, wind barrier, and insulation. So outside this dark board is the exterior wall's wooden panel.

Is this something one should remove? The insulation was a bit black where the black boards were joined, so moisture must have gotten in there.

The house is from 1970.

 

[Oldboy]

Could be asfaboard.

No, it's not something you should remove.

 

[Lutte]

Is asfaboard, I would keep it
But the fact that you've got some moisture in the insulation at the joints feels a bit iffy.
Maybe you can seal it with tape or caulk, you most likely had a vacuum indoors and sucked in the moisture.

 

[RoyMunson]

Thank you for the response! That must be it. It looked good otherwise, so I'll seal it with sealant before I put everything back again.

 

[Oldboy]

Personally, I would probably use asfapapp and line the compartments where there are joints, instead of sealant. Asfaboard is, after all, wood-based and likely moves with the moisture level throughout the year.