diff --git a/cara/apps/calculator/model_generator.py b/cara/apps/calculator/model_generator.py
index 89366b04..2e10b1f2 100644
--- a/cara/apps/calculator/model_generator.py
+++ b/cara/apps/calculator/model_generator.py
@@ -254,6 +254,11 @@ class FormData:
                     "window_opening_regime cannot be 'not-applicable' if "
                     "ventilation_type is 'natural_ventilation'"
                 )
+            if (self.window_opening_regime == 'windows_open_periodically' and
+                self.windows_duration > self.windows_frequency):
+                raise ValueError(
+                    'Duration cannot be bigger than frequency.'
+                )
 
         if (self.ventilation_type == 'mechanical_ventilation'
                 and self.mechanical_ventilation_type == 'not-applicable'):
diff --git a/cara/tests/apps/calculator/test_model_generator.py b/cara/tests/apps/calculator/test_model_generator.py
index f925497e..c1d1f2c2 100644
--- a/cara/tests/apps/calculator/test_model_generator.py
+++ b/cara/tests/apps/calculator/test_model_generator.py
@@ -508,6 +508,14 @@ def test_key_validation_natural_ventilation_window_opening_regime_na(baseline_fo
         model_generator.FormData.from_dict(baseline_form_data)
 
 
+def test_natural_ventilation_window_opening_periodically(baseline_form: model_generator.FormData):
+    baseline_form.window_opening_regime = 'windows_open_periodically'
+    baseline_form.windows_duration = 20
+    baseline_form.windows_frequency = 10
+    with pytest.raises(ValueError, match='Duration cannot be bigger than frequency.'):
+        baseline_form.validate()
+
+
 def test_key_validation_mech_ventilation_type_na(baseline_form_data):
     baseline_form_data['ventilation_type'] = 'mechanical_ventilation'
     baseline_form_data['mechanical_ventilation_type'] = 'not-applicable'