diff --git a/cara/apps/calculator/static/form.html b/cara/apps/calculator/static/form.html
index 37329058..0bcc2d4c 100644
--- a/cara/apps/calculator/static/form.html
+++ b/cara/apps/calculator/static/form.html
@@ -28,7 +28,7 @@ Beta v1.0.0 <span style="float:right; font-weight:bold">Please send feedback to
     <hr width="80%">
     <b>Room data:</b><br>
     <input type="radio" id="room_type_volume" name="volume_type" value="room_volume" onclick="require_fields(this)" required>
-    Room volume: &nbsp;&nbsp; <input type="number" step=0.01 id="room_volume" name="room_volume" placeholder="Room volume (m³)" min="0"><br>
+    Room volume: &nbsp;&nbsp; <input type="number" step=0.01 id="room_volume" name="room_volume" placeholder="Room volume (m³)" min="0.01"><br>
     <input type="radio" id="room_type_dimensions" name="volume_type" value="room_dimensions" onclick="require_fields(this)" required>
     Floor area: &nbsp;&nbsp; <input type="number" step=0.01 id="floor_area" name="floor_area" placeholder="Room floor area (m²)" min="0"><br>
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Ceiling height: &nbsp;&nbsp; <input type="number" step=0.01 id="ceiling_height" name="ceiling_height" placeholder="Room ceiling height (m²)" min="0"><br>
diff --git a/cara/apps/calculator/static/js/form.js b/cara/apps/calculator/static/js/form.js
index 2eb71fdc..588376c3 100644
--- a/cara/apps/calculator/static/js/form.js
+++ b/cara/apps/calculator/static/js/form.js
@@ -22,11 +22,12 @@ function show_hide(show, hide, obj) {
       obj.checked = false;
       ventilation_type.value = "";
       mechanical_ventilation_type.value = "";
+      unrequire_fields(obj);
   } else if (show.style.display === "none") {
       show.style.display = "block";
       hide.style.display = "none";
-      require_fields(obj);
       ventilation_type.value = obj.id;
+      require_fields(obj);
 } }
 
 function update_windows_open(obj) {
@@ -77,6 +78,18 @@ function require_fields(obj){
       break;
 } }
 
+function unrequire_fields(obj){
+  switch(obj.id) {
+    case "mechanical":
+      require_mechanical_ventilation(false);
+      break;
+    case "natural":
+      require_natural_ventilation(false);
+      break;
+    default:
+      break;
+} }
+
 function require_room_volume(option) {
   $("#room_volume").prop('required',option);
 }