Insulating an outbuilding

[ClareG1964]

OK, found a scrap of the sarking / membrane / whatever it is! Photos of both sides below. One side is smooth-ish and the other is gritty, like sandpaper. This has been fitted inside the timber cladding. Also managed to establish that the shed was built between 20 and 27 years ago. So my question is, is this stuff breathable, or not, do you think?

 

[JohnD]

the gritty surface is typical of the roofing felt sold for sheds, rabbit-hutches etc.

it is not breathable.

Can you see the joints in the felt from inside the shed? horizontal or vertical?

 

[ClareG1964]

Can't see the joints I'm afraid.