diff --git a/custom_components/heating_mpc/compatibility.py b/custom_components/heating_mpc/compatibility.py
index 00cd6ae..71a62c9 100644
--- a/custom_components/heating_mpc/compatibility.py
+++ b/custom_components/heating_mpc/compatibility.py
@@ -15,7 +15,7 @@ everything else is kept. What each fingerprint covers:
 * ventilation terms: the measurement plus the zone's windows (sensor, polarity,
   opening factor); "vent_wind_dir" also the windows' facing directions;
   "recovery" also the recovery time constant; "vent_cross" also the other zones'
-  windows.
+  windows (sensor, polarity, opening factor).
 
 Comfort weight, target offset and names are not physical and are never included.
 """
@@ -71,7 +71,7 @@ def zone_fingerprints(zones: Mapping[str, Any], sources: Iterable[Any], windows:
         m = measured[key]
         plain = zone_windows(key)
         # Cross-ventilation scales with the other zones' openings.
-        others = sorted([w.sensor, w.opening] for w in windows if w.zone != key and w.enabled)
+        others = sorted([w.sensor, w.inverted, w.opening] for w in windows if w.zone != key and w.enabled)
         prints = {"base": m, "loss": m, "heat": m, "bias": m,
                   "internal": _hash([m, sorted(source_signature(s, sources) for s in sources
                                                 if s.zone == key and s.enabled)]),
diff --git a/tests/test_compatibility.py b/tests/test_compatibility.py
index f269b71..746c58e 100644
--- a/tests/test_compatibility.py
+++ b/tests/test_compatibility.py
@@ -72,6 +72,12 @@ class FingerprintTests(unittest.TestCase):
         self.assertEqual(diff['bed'], ['recovery', 'vent_cross', 'vent_gust', 'vent_natural', 'vent_wind',
                                        'vent_wind_dir'])
         self.assertEqual(diff['living'], ['vent_cross'])  # sees the bedroom's windows
+        # A flipped polarity reverses what every term saw as open, the neighbour's too.
+        flipped = [{'key': 'w1', 'sensor': 'binary_sensor.w1', 'zone': 'bed', 'azimuth': 345, 'inverted': True},
+                   {'key': 'w2', 'sensor': 'binary_sensor.w2', 'zone': 'living', 'azimuth': 345}]
+        diff = differing(self.base, setup(windows=flipped)[0])
+        self.assertIn('vent_natural', diff['bed'])
+        self.assertEqual(diff['living'], ['vent_cross'])
         self.assertEqual(differing(self.base, setup(recovery=40)[0]), {'living': ['recovery'], 'bed': ['recovery']})
 
     def test_baseline_prints_and_change_detection(self):