diff --git a/steward/backup/contents.py b/steward/backup/contents.py
index dcb35f9..8dadd65 100644
--- a/steward/backup/contents.py
+++ b/steward/backup/contents.py
@@ -36,6 +36,8 @@ EXCLUDED: frozenset[str] = frozenset({
     # Learned from the metrics above, so relearned once they fill again.
     "metric_norms",
     "monitor_results",
+    # A host's downtime as its agent reported it: history, like the metrics.
+    "host_downtime",
     "email_outbox", "email_outbox_events", "password_resets",
     # Run history; its logs live on disk, not in the database.
     "ansible_runs",