What is it?

[mackan67]

I'm going to close up a dog door in our guest cottage... I've taken it down along with the boards around it... but behind it, there's a board, a black board about 1 cm thick, some kind of insulating board, what do you think it could be or can I use another type of board and place it behind the boards where the dog door was?

Very grateful for your answer

 

[Snailman]

sounds like asfaboard I think?

 

[Mats]

I think so too. Black on the outside and brownish on the backside. Quite porous.