diff --git a/cara/apps/calculator/model_generator.py b/cara/apps/calculator/model_generator.py
index 68b060ab..1d4f51ec 100644
--- a/cara/apps/calculator/model_generator.py
+++ b/cara/apps/calculator/model_generator.py
@@ -265,11 +265,15 @@ class FormData:
                 ventilation = models.HVACMechanical(
                     active=always_on, q_air_mech=self.air_supply)
 
+        # this is a minimal, always present source of ventilation, due
+        # to the air infiltration from the outside.
+        # See CERN-OPEN-2021-004, p. 12.
+        infiltration_ventilation = models.AirChange(active=always_on, air_exch=0.25)
         if self.hepa_option:
             hepa = models.HEPAFilter(active=always_on, q_air_mech=self.hepa_amount)
-            return models.MultipleVentilation((ventilation, hepa))
+            return models.MultipleVentilation((ventilation, hepa, infiltration_ventilation))
         else:
-            return ventilation
+            return models.MultipleVentilation((ventilation, infiltration_ventilation))
 
     def mask(self) -> models.Mask:
         # Initializes the mask type if mask wearing is "continuous", otherwise instantiates the mask attribute as
diff --git a/cara/tests/apps/calculator/test_model_generator.py b/cara/tests/apps/calculator/test_model_generator.py
index eb216727..53045564 100644
--- a/cara/tests/apps/calculator/test_model_generator.py
+++ b/cara/tests/apps/calculator/test_model_generator.py
@@ -50,7 +50,7 @@ def test_ventilation_slidingwindow(baseline_form: model_generator.FormData):
     baseline_form.opening_distance = 0.6
 
     ts = np.linspace(8, 16, 100)
-    np.testing.assert_allclose([window.air_exchange(room, t) for t in ts],
+    np.testing.assert_allclose([window.air_exchange(room, t)+0.25 for t in ts],
                                [baseline_form.ventilation().air_exchange(room, t) for t in ts])
 
 
@@ -73,7 +73,7 @@ def test_ventilation_hingedwindow(baseline_form: model_generator.FormData):
     baseline_form.opening_distance = 0.6
 
     ts = np.linspace(8, 16, 100)
-    np.testing.assert_allclose([window.air_exchange(room, t) for t in ts],
+    np.testing.assert_allclose([window.air_exchange(room, t)+0.25 for t in ts],
                                [baseline_form.ventilation().air_exchange(room, t) for t in ts])
 
 
@@ -88,7 +88,7 @@ def test_ventilation_mechanical(baseline_form: model_generator.FormData):
     baseline_form.air_supply = 500.
 
     ts = np.linspace(8, 16, 100)
-    np.testing.assert_allclose([mech.air_exchange(room, t) for t in ts],
+    np.testing.assert_allclose([mech.air_exchange(room, t)+0.25 for t in ts],
                                [baseline_form.ventilation().air_exchange(room, t) for t in ts])
 
 
@@ -103,7 +103,7 @@ def test_ventilation_airchanges(baseline_form: model_generator.FormData):
     baseline_form.air_changes = 3.
 
     ts = np.linspace(8, 16, 100)
-    np.testing.assert_allclose([airchange.air_exchange(room, t) for t in ts],
+    np.testing.assert_allclose([airchange.air_exchange(room, t)+0.25 for t in ts],
                                [baseline_form.ventilation().air_exchange(room, t) for t in ts])
 
 
@@ -131,7 +131,7 @@ def test_ventilation_window_hepa(baseline_form: model_generator.FormData):
     baseline_form.hepa_option = True
 
     ts = np.linspace(9, 17, 100)
-    np.testing.assert_allclose([ventilation.air_exchange(room, t) for t in ts],
+    np.testing.assert_allclose([ventilation.air_exchange(room, t)+0.25 for t in ts],
                                [baseline_form.ventilation().air_exchange(room, t) for t in ts])