diff --git a/etc/bashrc b/etc/bashrc
index 700599f..a4a01ef 100644
--- a/etc/bashrc
+++ b/etc/bashrc
@@ -107,7 +107,8 @@ export FOAM_SIGFPE=
 ################################################################################
 
 # The old dirs to be cleaned from the environment variables
-foamOldDirs="$WM_PROJECT_DIR $WM_THIRD_PARTY_DIR $HOME/$WM_PROJECT/$USER \
+foamOldDirs="$WM_PROJECT_DIR $WM_THIRD_PARTY_DIR \
+    $HOME/$WM_PROJECT/$USER $FOAM_USER_APPBIN $FOAM_USER_LIBBIN \
     $WM_PROJECT_SITE $FOAM_SITE_APPBIN $FOAM_SITE_LIBBIN"
 
 # Location of installation
diff --git a/etc/config.sh/mpi b/etc/config.sh/mpi
index 36b651e..8c679f6 100644
--- a/etc/config.sh/mpi
+++ b/etc/config.sh/mpi
@@ -37,6 +37,12 @@ SYSTEMOPENMPI)
     # Use the system installed openmpi, get library directory via mpicc
     export FOAM_MPI=openmpi-system
 
+    # Undefine OPAL_PREFIX if set to one of the paths on foamOldDirs
+    if [ -z "$($foamClean "$OPAL_PREFIX" "$foamOldDirs")" ]
+    then
+        unset OPAL_PREFIX
+    fi
+
     libDir=`mpicc --showme:link | sed -e 's/.*-L\([^ ]*\).*/\1/'`
 
     # Bit of a hack: strip off 'lib' and hope this is the path to openmpi
diff --git a/etc/config.sh/unset b/etc/config.sh/unset
index fa0a65b..75aefc6 100644
--- a/etc/config.sh/unset
+++ b/etc/config.sh/unset
@@ -34,7 +34,8 @@ foamClean=$WM_PROJECT_DIR/bin/foamCleanPath
 [ -f "$foamClean" -a -x "$foamClean" ] || unset foamClean
 
 # The old dirs to be cleaned from the environment variables
-foamOldDirs="$WM_PROJECT_DIR $WM_THIRD_PARTY_DIR $HOME/$WM_PROJECT/$USER \
+foamOldDirs="$WM_PROJECT_DIR $WM_THIRD_PARTY_DIR \
+    $HOME/$WM_PROJECT/$USER $FOAM_USER_APPBIN $FOAM_USER_LIBBIN \
     $WM_PROJECT_SITE $FOAM_SITE_APPBIN $FOAM_SITE_LIBBIN $ParaView_DIR"
 
 #------------------------------------------------------------------------------
@@ -101,7 +102,12 @@ unset FOAM_UTILITIES
 
 unset MPI_ARCH_PATH
 unset MPI_BUFFER_SIZE
-unset OPAL_PREFIX
+
+# Undefine OPAL_PREFIX if set to one of the paths on foamOldDirs
+if [ -z "$($foamClean "$OPAL_PREFIX" "$foamOldDirs")" ]
+then
+    unset OPAL_PREFIX
+fi
 
 
 #------------------------------------------------------------------------------