diff --git a/cara/apps/calculator/static/form.html b/cara/apps/calculator/static/form.html
index 4fc8a5e4..37329058 100644
--- a/cara/apps/calculator/static/form.html
+++ b/cara/apps/calculator/static/form.html
@@ -49,15 +49,16 @@ Beta v1.0.0 <span style="float:right; font-weight:bold">Please send feedback to
     </div>
 
     <div id="DIVnatural_ventilation" style="display:none">
+        <input type="hidden" id="windows_open" name="windows_open" value=""/>
         Number of windows: <input type="number" id="windows_number" name="windows_number" min="0"><br>
         Height of window: <input type="number" step=0.01 id="window_height" name="window_height" placeholder="meters" min="0"><br>
         Width of window: <input type="number" step=0.01 id="window_width"  name="window_width" placeholder="meters" min="0"><br>
         Opening distance: <input type="number" step=0.01 id="opening_distance"  name="opening_distance" placeholder="meters" min="0"><br>
-        Windows open: <input type="radio" id="always" name="windows_open" value="always">
+        Windows open: <input type="radio" id="always" name="RADIO_windows_open" value="always", onChange="update_windows_open(this)">
                       <label for="always">Always</label>
-                      <input type="radio" id="interval" name="windows_open" value="interval">
+                      <input type="radio" id="interval" name="RADIO_windows_open" value="interval", onChange="update_windows_open(this)">
                       <label for="interval">10 min / 2h</label>
-                      <!--input type="radio" id="breaks" name="windows_open" value="breaks">
+                      <!--input type="radio" id="breaks" name="RADIO_windows_open" value="breaks">
                       <label for="breaks">Breaks</label--><br>
     </div>
     
diff --git a/cara/apps/calculator/static/js/form.js b/cara/apps/calculator/static/js/form.js
index 649bfa2a..2eb71fdc 100644
--- a/cara/apps/calculator/static/js/form.js
+++ b/cara/apps/calculator/static/js/form.js
@@ -29,6 +29,11 @@ function show_hide(show, hide, obj) {
       ventilation_type.value = obj.id;
 } }
 
+function update_windows_open(obj) {
+  var windows_open = document.getElementById("windows_open");
+  windows_open.value = obj.id;
+}
+
 /* -------Required fields------- */
 function require_fields(obj){
   switch(obj.id) {
@@ -84,10 +89,7 @@ function require_room_dimensions(option) {
 function require_mechanical_ventilation(option) {
   $("#air_type_changes").prop('required',option);
   $("#air_type_supply").prop('required',option);
-  if (!option) {
-    var mechanical_ventilation_type = document.getElementById("mechanical_ventilation_type");
-    mechanical_ventilation_type.value = "";
-} }
+  }
 
 function require_natural_ventilation(option) {
   $("#windows_number").prop('required',option);