Compound entrance decaying

[A NUMBAH 9 LARGE]

So I just build my first compound base with high external stone walls and got an airlocked room as the entrance, however this entrance room is decaying(and worryingly rapidly) even though it is well within the TC zone and the TC itself has 60 hours worth of upkeep.
If anybody knows what the issue might be I would really appreciate you helping me with it!

[BENQS]

It's not attached to your base. You have to have a bigger compound and put a TC in your airlock. Otherwise you can attach you base to airlocks with foundations, but it will increase your upkeep cost.