Secret documents from the work by Alan Turing’s team of codebreakers have been found stuffed into the walls and roof of a hut at Bletchley Park.

It appears that after completing a day’s work, the team didn’t securely destroy the papers as was protocol, but instead used them to boost insulation in the far-from-luxurious facility.

The papers were discovered in 2013 during renovations of the hut, which are now part of an exhibition at the Bletchley Park museum, but officials have only just revealed the find. Some of the papers will now be put on display, but others have been sent to GCHQ, one of Britain’s intelligence organizations, for further analysis.

Among the paper are the only known surviving example of a Banbury sheet, named after a printing location. As an extremely simplified explanation of a complex situation, the Banbury sheet took advantage of Turing’s discovery that the encryption settings on Germany’s Enigma machines, although changing daily, were not entirely random.

British codebreakers faced the challenge of discovering each day’s settings as quickly as possible so that they could then decipher intercepted messages for the rest of the day. Turing developed the “Banburismus” technique, which involved punching holes in sheets of paper to represent the cipher in two different intercepted messages. While this took longer than relying solely on writing, overlaying the sheets and looking for holes lining up made it much quicker to then spot repeated patterns.

Intriguingly the haul of papers also includes several notes, possibly related to the decryption, which officials don’t yet fully understand.