diff --git a/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_FourElement b/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_FourElement
index 2e39a2c27..47e948ae1 100644
--- a/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_FourElement
+++ b/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_FourElement
@@ -15,7 +15,7 @@ record ${zone.parent.name}_${zone.name} "${zone.parent.name}_${zone.name}"
     alphaWin = ${zone.model_attr.alpha_conv_inner_win},
     RWin = ${zone.model_attr.r1_win},
     gWin = ${zone.model_attr.weighted_g_value},
-    UWin= ${zone.model_attr.ua_value_win/zone.model_attr.area_win},
+    UWin= ${zone.model_attr.u_value_win},
     ratioWinConRad = ${zone.model_attr.ratio_conv_rad_inner_win},
     AExt = ${get_list(zone.model_attr.facade_areas)},
     alphaExt = ${zone.model_attr.alpha_conv_inner_ow},
diff --git a/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_OneElement b/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_OneElement
index 0b08a9f25..b0d3eff7a 100644
--- a/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_OneElement
+++ b/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_OneElement
@@ -14,7 +14,7 @@ record ${zone.parent.name}_${zone.name} "${zone.parent.name}_${zone.name}"
     alphaWin = ${zone.model_attr.alpha_conv_inner_win},
     RWin = ${zone.model_attr.r1_win},
     gWin = ${zone.model_attr.weighted_g_value},
-    UWin= ${zone.model_attr.ua_value_win/zone.model_attr.area_win},
+    UWin= ${zone.model_attr.u_value_win},
     ratioWinConRad = ${zone.model_attr.ratio_conv_rad_inner_win},
     AExt = ${get_list(zone.model_attr.facade_areas)},
     alphaExt = ${zone.model_attr.alpha_conv_inner_ow},
diff --git a/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_ThreeElement b/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_ThreeElement
index 3b49042b2..407c4a5e9 100644
--- a/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_ThreeElement
+++ b/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_ThreeElement
@@ -14,7 +14,7 @@ record ${zone.parent.name}_${zone.name} "${zone.parent.name}_${zone.name}"
     alphaWin = ${zone.model_attr.alpha_conv_inner_win},
     RWin = ${zone.model_attr.r1_win},
     gWin = ${zone.model_attr.weighted_g_value},
-    UWin= ${zone.model_attr.ua_value_win/zone.model_attr.area_win},
+    UWin= ${zone.model_attr.u_value_win},
     ratioWinConRad = ${zone.model_attr.ratio_conv_rad_inner_win},
     AExt = ${get_list(zone.model_attr.facade_areas)},
     alphaExt = ${zone.model_attr.alpha_conv_inner_ow},
diff --git a/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_TwoElement b/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_TwoElement
index dd7c01f2a..c1cce2861 100644
--- a/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_TwoElement
+++ b/teaser/data/output/modelicatemplate/AixLib/AixLib_ThermalZoneRecord_TwoElement
@@ -14,7 +14,7 @@ record ${zone.parent.name}_${zone.name} "${zone.parent.name}_${zone.name}"
     alphaWin = ${zone.model_attr.alpha_conv_inner_win},
     RWin = ${zone.model_attr.r1_win},
     gWin = ${zone.model_attr.weighted_g_value},
-    UWin= ${zone.model_attr.ua_value_win/zone.model_attr.area_win},
+    UWin= ${zone.model_attr.u_value_win},
     ratioWinConRad = ${zone.model_attr.ratio_conv_rad_inner_win},
     AExt = ${get_list(zone.model_attr.facade_areas)},
     alphaExt = ${zone.model_attr.alpha_conv_inner_ow},
diff --git a/teaser/data/output/modelicatemplate/Annex60/Annex60_FourElements b/teaser/data/output/modelicatemplate/Annex60/Annex60_FourElements
index ca19bbaf8..f01612998 100644
--- a/teaser/data/output/modelicatemplate/Annex60/Annex60_FourElements
+++ b/teaser/data/output/modelicatemplate/Annex60/Annex60_FourElements
@@ -24,7 +24,7 @@ model ${zone.parent.name}_${zone.name}
     "Calculates direct solar radiation on titled surface for all directions"
     annotation (Placement(transformation(extent={{-68,52},{-48,72}})));
   Annex60.ThermalZones.ReducedOrder.SolarGain.CorrectionGDoublePane corGDoublePane(n=${zone.model_attr.n_outer},
-  UWin=${zone.model_attr.ua_value_win/zone.model_attr.area_win})
+  UWin=${zone.model_attr.u_value_win})
     "Correction factor for solar transmission"
     annotation (Placement(transformation(extent={{6,54},{26,74}})));
   Annex60.ThermalZones.ReducedOrder.RC.FourElements
diff --git a/teaser/data/output/modelicatemplate/Annex60/Annex60_OneElement b/teaser/data/output/modelicatemplate/Annex60/Annex60_OneElement
index 7e918345d..b93b5caba 100644
--- a/teaser/data/output/modelicatemplate/Annex60/Annex60_OneElement
+++ b/teaser/data/output/modelicatemplate/Annex60/Annex60_OneElement
@@ -24,7 +24,7 @@ model ${zone.parent.name}_${zone.name}
     "Calculates direct solar radiation on titled surface for all directions"
     annotation (Placement(transformation(extent={{-68,52},{-48,72}})));
   Annex60.ThermalZones.ReducedOrder.SolarGain.CorrectionGDoublePane corGDoublePane(n=${zone.model_attr.n_outer},
-  UWin=${zone.model_attr.ua_value_win/zone.model_attr.area_win})
+  UWin=${zone.model_attr.u_value_win})
     "Correction factor for solar transmission"
     annotation (Placement(transformation(extent={{6,54},{26,74}})));
   Annex60.ThermalZones.ReducedOrder.RC.OneElement
diff --git a/teaser/data/output/modelicatemplate/Annex60/Annex60_ThreeElements b/teaser/data/output/modelicatemplate/Annex60/Annex60_ThreeElements
index 48f2c7855..e33722702 100644
--- a/teaser/data/output/modelicatemplate/Annex60/Annex60_ThreeElements
+++ b/teaser/data/output/modelicatemplate/Annex60/Annex60_ThreeElements
@@ -24,7 +24,7 @@ model ${zone.parent.name}_${zone.name}
     "Calculates direct solar radiation on titled surface for all directions"
     annotation (Placement(transformation(extent={{-68,52},{-48,72}})));
   Annex60.ThermalZones.ReducedOrder.SolarGain.CorrectionGDoublePane corGDoublePane(n=${zone.model_attr.n_outer},
-  UWin=${zone.model_attr.ua_value_win/zone.model_attr.area_win})
+  UWin=${zone.model_attr.u_value_win})
     "Correction factor for solar transmission"
     annotation (Placement(transformation(extent={{6,54},{26,74}})));
   Annex60.ThermalZones.ReducedOrder.RC.ThreeElements
diff --git a/teaser/data/output/modelicatemplate/Annex60/Annex60_TwoElements b/teaser/data/output/modelicatemplate/Annex60/Annex60_TwoElements
index 747e62c10..6489dc9de 100644
--- a/teaser/data/output/modelicatemplate/Annex60/Annex60_TwoElements
+++ b/teaser/data/output/modelicatemplate/Annex60/Annex60_TwoElements
@@ -24,7 +24,7 @@ model ${zone.parent.name}_${zone.name}
     "Calculates direct solar radiation on titled surface for all directions"
     annotation (Placement(transformation(extent={{-68,52},{-48,72}})));
   Annex60.ThermalZones.ReducedOrder.SolarGain.CorrectionGDoublePane corGDoublePane(n=${zone.model_attr.n_outer},
-  UWin=${zone.model_attr.ua_value_win/zone.model_attr.area_win})
+  UWin=${zone.model_attr.u_value_win})
     "Correction factor for solar transmission"
     annotation (Placement(transformation(extent={{6,54},{26,74}})));
   Annex60.ThermalZones.ReducedOrder.RC.TwoElements
diff --git a/teaser/logic/buildingobjects/calculation/four_element.py b/teaser/logic/buildingobjects/calculation/four_element.py
index 7ecdc43c6..2dbb7f0e4 100644
--- a/teaser/logic/buildingobjects/calculation/four_element.py
+++ b/teaser/logic/buildingobjects/calculation/four_element.py
@@ -288,6 +288,8 @@ class FourElement(object):
         the ambient.
     ua_value_win : float [W/K]
         U-Value times outer wall area.
+    u_value_win : float [W/(m2K)]
+        Area weighted U-Value of windows.
     r_conv_inner_win : float [K/W]
         Sum of convective resistances for all windows facing the
         inside of this thermal zone.
@@ -530,6 +532,7 @@ def __init__(self, thermal_zone, merge_windows, t_bt):
 
         # UA-Value
         self.ua_value_win = 0.0
+        self.u_value_win = 0.0
 
         # resistances for heat transfer facing the inside of this thermal zone
         self.r_conv_inner_win = 0.0
@@ -583,15 +586,41 @@ def calc_attributes(self):
             win.calc_ua_value()
 
         self.set_calc_default()
+        if len(self.thermal_zone.outer_walls) < 1:
+            warnings.warn("No walls are defined as outer walls for thermal " +
+                          "zone " + self.thermal_zone.name + " in building " +
+                          self.thermal_zone.parent.name +
+                          ", please be careful with results. In addition " +
+                          "this might lead to RunTimeErrors")
         self._sum_outer_wall_elements()
-        self._sum_ground_floor_elements()
-        self._sum_rooftop_elements()
-        self._sum_inner_wall_elements()
-        self._sum_window_elements()
-        self._calc_outer_elements()
-        self._calc_ground_floor_elements()
+        if len(self.thermal_zone.inner_walls) < 1:
+            warnings.warn('For thermal zone ' + self.thermal_zone.name +
+                          ' in building ' + self.thermal_zone.parent.name +
+                          ', no inner walls have been defined.')
+        else:
+            self._sum_inner_wall_elements()
+            self._calc_inner_elements()
+        if len(self.thermal_zone.windows) < 1:
+            warnings.warn('For thermal zone ' + self.thermal_zone.name +
+                          ' in building ' + self.thermal_zone.parent.name +
+                          ', no windows have been defined.')
+        else:
+            self._sum_window_elements()
+        if len(self.thermal_zone.ground_floors) < 1:
+            warnings.warn('For thermal zone ' + self.thermal_zone.name +
+                          ' in building ' + self.thermal_zone.parent.name +
+                          ', no ground floors have been defined.')
+        else:
+            self._sum_ground_floor_elements()
+            self._calc_ground_floor_elements()
+        if len(self.thermal_zone.ground_floors) < 1:
+            warnings.warn('For thermal zone ' + self.thermal_zone.name +
+                          ' in building ' + self.thermal_zone.parent.name +
+                          ', no rooftops have been defined.')
+        else:
+            self._sum_rooftop_elements()
         self._calc_rooftop_elements()
-        self._calc_inner_elements()
+        self._calc_outer_elements()
         self._calc_wf()
         self._calc_mean_values()
         self._calc_number_of_elements()
@@ -958,6 +987,7 @@ def _sum_window_elements(self):
         self.area_win = sum(win.area for win in self.thermal_zone.windows)
         self.ua_value_win = sum(
             win.ua_value for win in self.thermal_zone.windows)
+        self.u_value_win = self.ua_value_win/self.area_win
 
         # values facing the inside of the thermal zone
 
@@ -1048,8 +1078,9 @@ def _calc_outer_elements(self):
         if self.merge_windows is False:
             try:
 
-                self.r1_win = (1 / sum((1 / win.r1) for
-                                       win in self.thermal_zone.windows))
+                if len(self.thermal_zone.windows) > 0:
+                    self.r1_win = (1 / sum((1 / win.r1) for win in
+                                           self.thermal_zone.windows))
 
                 conduction = (1 / sum((1 / element.r_conduc) for element in
                                       outer_walls))
@@ -1063,17 +1094,20 @@ def _calc_outer_elements(self):
         if self.merge_windows is True:
 
             try:
-                self.r1_win = 1 / sum(1 / (win.r1 / 6) for win in
-                                      self.thermal_zone.windows)
-
-                self.r1_ow = 1 / (1 / self.r1_ow + 1 / self.r1_win)
-                self.r_total_ow = 1 / (self.ua_value_ow + self.ua_value_win)
-                self.r_rest_ow = (self.r_total_ow - self.r1_ow - 1 / (
-                    ((1 / self.r_conv_inner_ow)
-                     + (1 / self.r_conv_inner_win)
-                     + (1 / self.r_rad_inner_ow)
-                     + (1 / self.r_rad_inner_win)))) - 1 / (
-                    self.alpha_comb_outer_ow * self.area_ow)
+                if len(self.thermal_zone.windows) > 0:
+                    self.r1_win = 1 / sum(1 / (win.r1 / 6) for win in
+                                          self.thermal_zone.windows)
+
+                    self.r1_ow = 1 / (1 / self.r1_ow + 1 / self.r1_win)
+
+                    self.r_total_ow = 1 / (self.ua_value_ow +
+                                           self.ua_value_win)
+                    self.r_rest_ow = (self.r_total_ow - self.r1_ow - 1 / (
+                        ((1 / self.r_conv_inner_ow)
+                         + (1 / self.r_conv_inner_win)
+                         + (1 / self.r_rad_inner_ow)
+                         + (1 / self.r_rad_inner_win)))) - 1 / (
+                        self.alpha_comb_outer_ow * self.area_ow)
 
                 self.ir_emissivity_inner_ow = (
                     (self.ir_emissivity_inner_ow * self.area_ow
@@ -1558,6 +1592,7 @@ def set_calc_default(self):
 
         # UA-Value
         self.ua_value_win = 0.0
+        self.u_value_win = 0.0
 
         # resistances for heat transfer facing the inside of this thermal zone
         self.r_conv_inner_win = 0.0
diff --git a/teaser/logic/buildingobjects/calculation/one_element.py b/teaser/logic/buildingobjects/calculation/one_element.py
index 1eb5a3e0c..188a26e1e 100644
--- a/teaser/logic/buildingobjects/calculation/one_element.py
+++ b/teaser/logic/buildingobjects/calculation/one_element.py
@@ -141,7 +141,9 @@ class OneElement(object):
         Area-weighted combined coefficient of heat transfer of windows facing
         the ambient.
     ua_value_win : float [W/K]
-        U-Value times outer wall area.
+        U-Value times window wall area.
+    u_value_win : float [W/(m2K)]
+        Area weighted U-Value of windows.
     r_conv_inner_win : float [K/W]
         Sum of convective resistances for all windows facing the
         inside of this thermal zone.
@@ -278,6 +280,7 @@ def __init__(self, thermal_zone, merge_windows, t_bt):
 
         # UA-Value
         self.ua_value_win = 0.0
+        self.u_value_win = 0.0
 
         # resistances for heat transfer facing the inside of this thermal zone
         self.r_conv_inner_win = 0.0
@@ -328,8 +331,19 @@ def calc_attributes(self):
             win.calc_ua_value()
 
         self.set_calc_default()
+        if len(outer_walls) < 1:
+            warnings.warn("No walls are defined as outer walls for thermal " +
+                          "zone " + self.thermal_zone.name + " in building " +
+                          self.thermal_zone.parent.name +
+                          ", please be careful with results. In addition " +
+                          "this might lead to RunTimeErrors")
         self._sum_outer_wall_elements()
-        self._sum_window_elements()
+        if len(self.thermal_zone.windows) < 1:
+            warnings.warn('For thermal zone ' + self.thermal_zone.name +
+                          ' in building ' + self.thermal_zone.parent.name +
+                          ', no windows have been defined.')
+        else:
+            self._sum_window_elements()
         self._calc_outer_elements()
         self._calc_wf()
         self._calc_mean_values()
@@ -545,6 +559,7 @@ def _sum_window_elements(self):
         self.area_win = sum(win.area for win in self.thermal_zone.windows)
         self.ua_value_win = sum(
             win.ua_value for win in self.thermal_zone.windows)
+        self.u_value_win = self.ua_value_win/self.area_win
 
         self.r_total_win = 1 / self.ua_value_win
         # values facing the inside of the thermal zone
@@ -630,16 +645,13 @@ def _calc_outer_elements(self):
             # more than one outer wall, calculate chain matrix
             self.r1_ow, self.c1_ow = self._calc_parallel_connection(outer_walls,
                                                                     omega)
-        else:
-            warnings.warn("No walls are defined as outer walls, please be "
-                          "careful with results. In addition this might lead "
-                          "to RunTimeErrors")
 
         if self.merge_windows is False:
             try:
 
-                self.r1_win = (1 / sum((1 / win.r1) for
-                                       win in self.thermal_zone.windows))
+                if len(self.thermal_zone.windows) > 0:
+                    self.r1_win = (1 / sum((1 / win.r1) for win in
+                                           self.thermal_zone.windows))
 
                 conduction = (1 / sum((1 / element.r_conduc) for element in
                                       outer_walls))
@@ -654,17 +666,20 @@ def _calc_outer_elements(self):
 
             try:
 
-                self.r1_win = 1 / sum(1 / (win.r1 / 6) for win in
-                                      self.thermal_zone.windows)
+                if len(self.thermal_zone.windows) > 0:
+                    self.r1_win = 1 / sum(1 / (win.r1 / 6) for win in
+                                          self.thermal_zone.windows)
+
+                    self.r1_ow = 1 / (1 / self.r1_ow + 1 / self.r1_win)
 
-                self.r1_ow = 1 / (1 / self.r1_ow + 1 / self.r1_win)
-                self.r_total_ow = 1 / (self.ua_value_ow + self.ua_value_win)
-                self.r_rest_ow = (self.r_total_ow - self.r1_ow - 1 / (
-                    ((1 / self.r_conv_inner_ow)
-                     + (1 / self.r_conv_inner_win)
-                     + (1 / self.r_rad_inner_ow)
-                     + (1 / self.r_rad_inner_win)))) - 1 / (
-                    self.alpha_comb_outer_ow * self.area_ow)
+                    self.r_total_ow = 1 / (self.ua_value_ow +
+                                           self.ua_value_win)
+                    self.r_rest_ow = (self.r_total_ow - self.r1_ow - 1 / (
+                        ((1 / self.r_conv_inner_ow)
+                         + (1 / self.r_conv_inner_win)
+                         + (1 / self.r_rad_inner_ow)
+                         + (1 / self.r_rad_inner_win)))) - 1 / (
+                        self.alpha_comb_outer_ow * self.area_ow)
 
                 self.ir_emissivity_inner_ow = (
                     (self.ir_emissivity_inner_ow * self.area_ow
@@ -931,6 +946,7 @@ def set_calc_default(self):
 
         # UA-Value
         self.ua_value_win = 0.0
+        self.u_value_win = 0.0
 
         # resistances for heat transfer facing the inside of this thermal zone
         self.r_conv_inner_win = 0.0
diff --git a/teaser/logic/buildingobjects/calculation/three_element.py b/teaser/logic/buildingobjects/calculation/three_element.py
index 882264aa9..d3441b6a0 100644
--- a/teaser/logic/buildingobjects/calculation/three_element.py
+++ b/teaser/logic/buildingobjects/calculation/three_element.py
@@ -217,6 +217,8 @@ class ThreeElement(object):
         the ambient.
     ua_value_win : float [W/K]
         U-Value times outer wall area.
+    u_value_win : float [W/(m2K)]
+        Area weighted U-Value of windows.
     r_conv_inner_win : float [K/W]
         Sum of convective resistances for all windows facing the
         inside of this thermal zone.
@@ -410,6 +412,7 @@ def __init__(self, thermal_zone, merge_windows, t_bt):
 
         # UA-Value
         self.ua_value_win = 0.0
+        self.u_value_win = 0.0
 
         # resistances for heat transfer facing the inside of this thermal zone
         self.r_conv_inner_win = 0.0
@@ -462,13 +465,34 @@ def calc_attributes(self):
             win.calc_ua_value()
 
         self.set_calc_default()
+        if len(outer_walls) < 1:
+            warnings.warn("No walls are defined as outer walls for thermal " +
+                          "zone " + self.thermal_zone.name + " in building " +
+                          self.thermal_zone.parent.name +
+                          ", please be careful with results. In addition " +
+                          "this might lead to RunTimeErrors")
         self._sum_outer_wall_elements()
-        self._sum_ground_floor_elements()
-        self._sum_inner_wall_elements()
-        self._sum_window_elements()
+        if len(self.thermal_zone.inner_walls) < 1:
+            warnings.warn('For thermal zone ' + self.thermal_zone.name +
+                          ' in building ' + self.thermal_zone.parent.name +
+                          ', no inner walls have been defined.')
+        else:
+            self._sum_inner_wall_elements()
+            self._calc_inner_elements()
+        if len(self.thermal_zone.windows) < 1:
+            warnings.warn('For thermal zone ' + self.thermal_zone.name +
+                          ' in building ' + self.thermal_zone.parent.name +
+                          ', no windows have been defined.')
+        else:
+            self._sum_window_elements()
+        if len(self.thermal_zone.ground_floors) < 1:
+            warnings.warn('For thermal zone ' + self.thermal_zone.name +
+                          ' in building ' + self.thermal_zone.parent.name +
+                          ', no ground floors have been defined.')
+        else:
+            self._sum_ground_floor_elements()
+            self._calc_ground_floor_elements()
         self._calc_outer_elements()
-        self._calc_ground_floor_elements()
-        self._calc_inner_elements()
         self._calc_wf()
         self._calc_mean_values()
         self._calc_number_of_elements()
@@ -785,6 +809,7 @@ def _sum_window_elements(self):
         self.area_win = sum(win.area for win in self.thermal_zone.windows)
         self.ua_value_win = sum(
             win.ua_value for win in self.thermal_zone.windows)
+        self.u_value_win = self.ua_value_win/self.area_win
 
         # values facing the inside of the thermal zone
 
@@ -868,16 +893,13 @@ def _calc_outer_elements(self):
             # more than one outer wall, calculate chain matrix
             self.r1_ow, self.c1_ow = self._calc_parallel_connection(outer_walls,
                                                                     omega)
-        else:
-            warnings.warn("No walls are defined as outer walls, please be "
-                          "careful with results. In addition this might lead "
-                          "to RunTimeErrors")
 
         if self.merge_windows is False:
             try:
 
-                self.r1_win = (1 / sum((1 / win.r1) for
-                                       win in self.thermal_zone.windows))
+                if len(self.thermal_zone.windows) > 0:
+                    self.r1_win = (1 / sum((1 / win.r1) for win in
+                                           self.thermal_zone.windows))
 
                 conduction = (1 / sum((1 / element.r_conduc) for element in
                                       outer_walls))
@@ -891,17 +913,20 @@ def _calc_outer_elements(self):
         if self.merge_windows is True:
 
             try:
-                self.r1_win = 1 / sum(1 / (win.r1 / 6) for win in
-                                      self.thermal_zone.windows)
-
-                self.r1_ow = 1 / (1 / self.r1_ow + 1 / self.r1_win)
-                self.r_total_ow = 1 / (self.ua_value_ow + self.ua_value_win)
-                self.r_rest_ow = (self.r_total_ow - self.r1_ow - 1 / (
-                    ((1 / self.r_conv_inner_ow)
-                     + (1 / self.r_conv_inner_win)
-                     + (1 / self.r_rad_inner_ow)
-                     + (1 / self.r_rad_inner_win)))) - 1 / (
-                    self.alpha_comb_outer_ow * self.area_ow)
+                if len(self.thermal_zone.windows) > 0:
+                    self.r1_win = 1 / sum(1 / (win.r1 / 6) for win in
+                                          self.thermal_zone.windows)
+
+                    self.r1_ow = 1 / (1 / self.r1_ow + 1 / self.r1_win)
+
+                    self.r_total_ow = 1 / (self.ua_value_ow +
+                                           self.ua_value_win)
+                    self.r_rest_ow = (self.r_total_ow - self.r1_ow - 1 / (
+                        ((1 / self.r_conv_inner_ow)
+                         + (1 / self.r_conv_inner_win)
+                         + (1 / self.r_rad_inner_ow)
+                         + (1 / self.r_rad_inner_win)))) - 1 / (
+                        self.alpha_comb_outer_ow * self.area_ow)
 
                 self.ir_emissivity_inner_ow = (
                     (self.ir_emissivity_inner_ow * self.area_ow
@@ -943,10 +968,6 @@ def _calc_ground_floor_elements(self):
             # more than one outer wall, calculate chain matrix
             self.r1_gf, self.c1_gf = self._calc_parallel_connection(
                 self.thermal_zone.ground_floors, omega)
-        else:
-            warnings.warn("No walls are defined as ground floors, please be "
-                          "careful with results. In addition this might lead "
-                          "to RunTimeErrors")
         try:
             conduction = (1 / sum((1 / element.r_conduc) for element in
                                 self.thermal_zone.ground_floors))
@@ -991,10 +1012,6 @@ def _calc_inner_elements(self):
             self.r1_iw, self.c1_iw = self._calc_parallel_connection(
                 inner_walls,
                 omega)
-        else:
-            warnings.warn("No walls are defined as outer walls, please be "
-                          "careful with results. In addition this might lead "
-                          "to RunTimeErrors")
 
     def _calc_wf(self):
         """Weightfactors for outer elements(walls, roof, ground floor, windows)
@@ -1284,6 +1301,7 @@ def set_calc_default(self):
 
         # UA-Value
         self.ua_value_win = 0.0
+        self.u_value_win = 0.0
 
         # resistances for heat transfer facing the inside of this thermal zone
         self.r_conv_inner_win = 0.0
diff --git a/teaser/logic/buildingobjects/calculation/two_element.py b/teaser/logic/buildingobjects/calculation/two_element.py
index 744afc717..4cf479a9b 100644
--- a/teaser/logic/buildingobjects/calculation/two_element.py
+++ b/teaser/logic/buildingobjects/calculation/two_element.py
@@ -177,7 +177,9 @@ class TwoElement(object):
         Area-weighted combined coefficient of heat transfer of windows facing
         the ambient.
     ua_value_win : float [W/K]
-        U-Value times outer wall area.
+        U-Value times window wall area.
+    u_value_win : float [W/(m2K)]
+        Area weighted U-Value of windows.
     r_conv_inner_win : float [K/W]
         Sum of convective resistances for all windows facing the
         inside of this thermal zone.
@@ -341,6 +343,7 @@ def __init__(self, thermal_zone, merge_windows, t_bt):
 
         # UA-Value
         self.ua_value_win = 0.0
+        self.u_value_win = 0.0
 
         # resistances for heat transfer facing the inside of this thermal zone
         self.r_conv_inner_win = 0.0
@@ -391,11 +394,28 @@ def calc_attributes(self):
             win.calc_ua_value()
 
         self.set_calc_default()
+        if len(outer_walls) < 1:
+            warnings.warn("No walls are defined as outer walls for thermal "
+                          "zone " + str(self.thermal_zone.name) +
+                          " in building " +
+                          str(self.thermal_zone.parent.name) +
+                          ", please be careful with results. In addition " +
+                          "this might lead to RunTimeErrors")
         self._sum_outer_wall_elements()
-        self._sum_inner_wall_elements()
-        self._sum_window_elements()
+        if len(self.thermal_zone.inner_walls) < 1:
+            warnings.warn('For thermal zone ' + self.thermal_zone.name,
+                          ' in building ' + self.thermal_zone.parent.name +
+                          ', no inner walls have been defined.')
+        else:
+            self._sum_inner_wall_elements()
+            self._calc_inner_elements()
+        if len(self.thermal_zone.windows) < 1:
+            warnings.warn('For thermal zone ' + self.thermal_zone.name +
+                          ' in building ' + self.thermal_zone.parent.name +
+                          ', no windows have been defined.')
+        else:
+            self._sum_window_elements()
         self._calc_outer_elements()
-        self._calc_inner_elements()
         self._calc_wf()
         self._calc_mean_values()
         self._calc_number_of_elements()
@@ -686,6 +706,7 @@ def _sum_window_elements(self):
         self.area_win = sum(win.area for win in self.thermal_zone.windows)
         self.ua_value_win = sum(
             win.ua_value for win in self.thermal_zone.windows)
+        self.u_value_win = self.ua_value_win/self.area_win
 
         self.r_total_win = 1 / self.ua_value_win
         # values facing the inside of the thermal zone
@@ -771,16 +792,13 @@ def _calc_outer_elements(self):
             # more than one outer wall, calculate chain matrix
             self.r1_ow, self.c1_ow = self._calc_parallel_connection(outer_walls,
                                                                     omega)
-        else:
-            warnings.warn("No walls are defined as outer walls, please be "
-                          "careful with results. In addition this might lead "
-                          "to RunTimeErrors")
 
         if self.merge_windows is False:
             try:
 
-                self.r1_win = (1 / sum((1 / win.r1) for
-                                       win in self.thermal_zone.windows))
+                if len(self.thermal_zone.windows) > 0:
+                    self.r1_win = (1 / sum((1 / win.r1) for win in
+                                           self.thermal_zone.windows))
 
                 conduction = (1 / sum((1 / element.r_conduc) for element in
                                       outer_walls))
@@ -795,18 +813,20 @@ def _calc_outer_elements(self):
 
             try:
 
-                self.r1_win = 1 / sum(1 / (win.r1 / 6) for win in
-                                      self.thermal_zone.windows)
+                if len(self.thermal_zone.windows) > 0:
+                    self.r1_win = 1 / sum(1 / (win.r1 / 6) for win in
+                                          self.thermal_zone.windows)
 
-                self.r1_ow = 1 / (1 / self.r1_ow + 1 / self.r1_win)
-                self.r_total_ow = 1 / (self.ua_value_ow + self.ua_value_win)
+                    self.r1_ow = 1 / (1 / self.r1_ow + 1 / self.r1_win)
 
-                self.r_rest_ow = (self.r_total_ow - self.r1_ow - 1 / (
-                    ((1 / self.r_conv_inner_ow)
-                     + (1 / self.r_conv_inner_win)
-                     + (1 / self.r_rad_inner_ow)
-                     + (1 / self.r_rad_inner_win)))) - 1 / (
-                    self.alpha_comb_outer_ow * self.area_ow)
+                    self.r_total_ow = 1 / (self.ua_value_ow +
+                                           self.ua_value_win)
+                    self.r_rest_ow = (self.r_total_ow - self.r1_ow - 1 / (
+                        ((1 / self.r_conv_inner_ow)
+                         + (1 / self.r_conv_inner_win)
+                         + (1 / self.r_rad_inner_ow)
+                         + (1 / self.r_rad_inner_win)))) - 1 / (
+                        self.alpha_comb_outer_ow * self.area_ow)
 
                 self.ir_emissivity_inner_ow = (
                     (self.ir_emissivity_inner_ow * self.area_ow
@@ -862,10 +882,6 @@ def _calc_inner_elements(self):
             self.r1_iw, self.c1_iw = self._calc_parallel_connection(
                 inner_walls,
                 omega)
-        else:
-            warnings.warn("No walls are defined as outer walls, please be "
-                          "careful with results. In addition this might lead "
-                          "to RunTimeErrors")
 
     def _calc_wf(self):
         """Weightfactors for outer elements(walls, roof, ground floor, windows)
@@ -1139,6 +1155,7 @@ def set_calc_default(self):
 
         # UA-Value
         self.ua_value_win = 0.0
+        self.u_value_win = 0.0
 
         # resistances for heat transfer facing the inside of this thermal zone
         self.r_conv_inner_win = 0.0