diff --git a/cara/models.py b/cara/models.py
index 918ca439..12f85c7c 100644
--- a/cara/models.py
+++ b/cara/models.py
@@ -203,6 +203,9 @@ class WindowOpening(Ventilation):
     #: used to exchange air (0 <= cd_b <= 1)
     cd_b: float = 0.6
 
+    #: Minimum difference between inside and outside temperature
+    min_deltaT: float = 0.1
+
     def transition_times(self) -> typing.Set[float]:
         transitions = super().transition_times()
         transitions.update(self.inside_temp.transition_times)
@@ -214,12 +217,17 @@ class WindowOpening(Ventilation):
         if not self.active.triggered(time):
             return 0.
 
+        # Reminder, no dependence on time in the resulting calculation.
         inside_temp = self.inside_temp.value(time)
         outside_temp = self.outside_temp.value(time)
 
-        # Reminder, no dependence on time in the resulting calculation.
-        temp_delta = abs(inside_temp - outside_temp) / outside_temp
-        root = np.sqrt(9.81 * self.window_height * temp_delta)
+        # The inside_temperature is forced to be always at least min_deltaT degree
+        # warmer than the outside_temperature. Further research needed to
+        # handle the buoyancy driven ventilation when the temperature gradient
+        # is inverted.
+        inside_temp = max(inside_temp, outside_temp + self.min_deltaT)
+        temp_gradient = (inside_temp - outside_temp) / outside_temp
+        root = np.sqrt(9.81 * self.window_height * temp_gradient)
         window_area = self.window_height * self.opening_length * self.number_of_windows
         return (3600 / (3 * room.volume)) * self.cd_b * window_area * root
 
diff --git a/cara/tests/test_known_quantities.py b/cara/tests/test_known_quantities.py
index 336751f9..d621f31d 100644
--- a/cara/tests/test_known_quantities.py
+++ b/cara/tests/test_known_quantities.py
@@ -471,7 +471,7 @@ def build_exposure_model(concentration_model):
     "month, expected_r0",
     [
         ['Jan', 91.06953],
-        ['Jun', 99.46692],
+        ['Jun', 99.995335],
     ],
 )
 def test_r0_hourly_dep(month,expected_r0):
@@ -489,7 +489,7 @@ def test_r0_hourly_dep(month,expected_r0):
     "month, expected_r0",
     [
         ['Jan', 91.19912],
-        ['Jun', 99.59226],
+        ['Jun', 99.997324],
     ],
 )
 def test_r0_hourly_dep_refined(month,expected_r0):