diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -10,6 +10,7 @@
 use Symfony\Component\DependencyInjection\Exception\RuntimeException as DependencyInjectionRuntimeException;
 use Symfony\Component\HttpFoundation\Request;
 use Drupal\Core\Language\Language;
+use Exception;
 
 /**
  * @file
@@ -483,11 +484,11 @@
 }
 
 /**
- * Returns the path of the configuration directory.
+ * Returns the path of a configuration directory.
  *
  * @param string $type
- *   Type of config directory to return. Drupal core provides 'active' and
- *   'staging'.
+ *   (optional) The type of config directory to return. Drupal core provides
+ *   'active' and 'staging'. Defaults to CONFIG_ACTIVE_DIRECTORY.
  *
  * @return string
  *   The configuration directory path.
@@ -503,7 +504,7 @@
     $path = conf_path() . '/files/' . $config_directories[$type];
   }
   else {
-    throw new Exception(t('The configuration directory type %type does not exist.', array('%type' => $type)));
+    throw new Exception(format_string('The configuration directory type %type does not exist.', array('%type' => $type)));
   }
   return $path;
 }
diff -u b/core/includes/config.inc b/core/includes/config.inc
--- b/core/includes/config.inc
+++ b/core/includes/config.inc
@@ -129,14 +129,14 @@
 }
 
 /**
- * Imports configuration from FileStorage to the active store.
+ * Imports configuration into the active store.
  *
  * @return bool|null
  *   TRUE if configuration was imported successfully, FALSE in case of a
  *   synchronization error, or NULL if there are no changes to synchronize.
  */
 function config_import() {
-  // Retrieve a list of differences between FileStorage and the active store.
+  // Retrieve a list of differences between staging and the active store.
   $source_storage = new FileStorage(array('directory' => config_get_config_directory(CONFIG_STAGING_DIRECTORY)));
   $target_storage = drupal_container()->get('config.storage');
 
@@ -214,10 +214,10 @@
 }
 
 /**
- * Exports configuration from the active store to FileStorage.
+ * Exports configuration from the active store to staging.
  */
 function config_export() {
-  // Retrieve a list of differences between the active store and FileStorage.
+  // Retrieve a list of differences between the active store and staging.
   $source_storage = drupal_container()->get('config.storage');
   $target_storage = new FileStorage(array('directory' => config_get_config_directory(CONFIG_STAGING_DIRECTORY)));
 
diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -320,7 +320,7 @@
 
   // Check existing settings.php.
   $install_state['database_verified'] = install_verify_database_settings();
-  $install_state['config_verified'] = install_ensure_config_directory(CONFIG_ACTIVE_DIRECTORY) && install_ensure_config_directory(CONFIG_STAGING_DIRECTORY);
+  $install_state['config_verified'] = install_verify_config_directory(CONFIG_ACTIVE_DIRECTORY) && install_verify_config_directory(CONFIG_STAGING_DIRECTORY);
   $install_state['settings_verified'] = $install_state['config_verified'] && $install_state['database_verified'];
 
   if ($install_state['database_verified']) {
diff -u b/core/includes/install.inc b/core/includes/install.inc
--- b/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -261,13 +261,12 @@
   // manually defined in the existing already.
   if (empty($config_directories)) {
     $settings['config_directories'] = array(
-      'required' => TRUE,
       'value' => array(
         CONFIG_ACTIVE_DIRECTORY => 'config/active_' . drupal_hash_base64(drupal_random_bytes(55)),
         CONFIG_STAGING_DIRECTORY => 'config/staging_' . drupal_hash_base64(drupal_random_bytes(55)),
-      )
+      ),
+      'required' => TRUE,
     );
-
     // Rewrite settings.php, which also sets the value as global variable.
     drupal_rewrite_settings($settings);
   }
@@ -290,6 +289,34 @@
 }
 
 /**
+ * Checks whether a config directory name is defined, and if so, whether it
+ * exists and is writable.
+ *
+ * This partically duplicates install_ensure_config_directory(), but is required
+ * since the installer would create the config directory too early in the
+ * installation process otherwise (e.g., when only visiting install.php when
+ * there is a settings.php already, but not actually executing the installation).
+ *
+ * @param string $type
+ *   Type of config directory to return. Drupal core provides 'active' and
+ *   'staging'.
+ *
+ * @return bool
+ *   TRUE if the config directory exists and is writable.
+ */
+function install_verify_config_directory($type) {
+  global $config_directories;
+  if (!isset($config_directories[$type])) {
+    return FALSE;
+  }
+  $config_directory = config_get_config_directory($type);
+  if (is_dir($config_directory) && is_writable($config_directory)) {
+    return TRUE;
+  }
+  return FALSE;
+}
+
+/**
  * Ensures that the config directory exists and is writable, or can be made so.
  *
  * @param string $type
diff -u b/core/modules/config/lib/Drupal/config/Tests/ConfigImportTest.php b/core/modules/config/lib/Drupal/config/Tests/ConfigImportTest.php
--- b/core/modules/config/lib/Drupal/config/Tests/ConfigImportTest.php
+++ b/core/modules/config/lib/Drupal/config/Tests/ConfigImportTest.php
@@ -64,6 +64,8 @@
   function testDeleted() {
     $name = 'config_test.system';
     $dynamic_name = 'config_test.dynamic.default';
+    $active_storage = new DatabaseStorage();
+    $staging_storage = new FileStorage(array('directory' => config_get_config_directory(CONFIG_STAGING_DIRECTORY)));
 
     // Verify the default configuration values exist.
     $config = config($name);
@@ -75,17 +77,15 @@
     config_export();
 
     // Delete the configuration objects from the staging directory.
-    $file_storage = new FileStorage(array('directory' => config_get_config_directory(CONFIG_STAGING_DIRECTORY)));
-    $file_storage->delete($name);
-    $file_storage->delete($dynamic_name);
+    $staging_storage->delete($name);
+    $staging_storage->delete($dynamic_name);
 
     // Import.
     config_import();
 
     // Verify the values have disappeared.
-    $database_storage = new DatabaseStorage();
-    $this->assertIdentical($database_storage->read($name), FALSE);
-    $this->assertIdentical($database_storage->read($dynamic_name), FALSE);
+    $this->assertIdentical($active_storage->read($name), FALSE);
+    $this->assertIdentical($active_storage->read($dynamic_name), FALSE);
 
     $config = config($name);
     $this->assertIdentical($config->get('foo'), NULL);
@@ -107,34 +107,36 @@
   function testNew() {
     $name = 'config_test.new';
     $dynamic_name = 'config_test.dynamic.new';
+    $staging_storage = new FileStorage(array('directory' => config_get_config_directory(CONFIG_STAGING_DIRECTORY)));
 
     // Export.
     config_export();
 
-    // Verify the new configuration does not exist in the staging directory.
-    $file_storage = new FileStorage(array('directory' => config_get_config_directory(CONFIG_STAGING_DIRECTORY)));
-    $this->assertIdentical($file_storage->exists($name), FALSE, $name . ' not found.');
-    $this->assertIdentical($file_storage->exists($dynamic_name), FALSE, $dynamic_name . ' not found.');
+    // Verify the configuration to create does not exist yet.
+    $this->assertIdentical($staging_storage->exists($name), FALSE, $name . ' not found.');
+    $this->assertIdentical($staging_storage->exists($dynamic_name), FALSE, $dynamic_name . ' not found.');
 
     // Create new configuration objects in the staging directory.
-    $file_storage->write($name, array(
+    $original_name_data = array(
       'add_me' => 'new value',
-    ));
-    $file_storage->write($dynamic_name, array(
+    );
+    $staging_storage->write($name, $original_name_data);
+    $original_dynamic_data = array(
       'id' => 'new',
       'label' => 'New',
-    ));
-    $this->assertIdentical($file_storage->exists($name), TRUE, $name . ' found.');
-    $this->assertIdentical($file_storage->exists($dynamic_name), TRUE, $dynamic_name . ' found.');
+    );
+    $staging_storage->write($dynamic_name, $original_dynamic_data);
+    $this->assertIdentical($staging_storage->exists($name), TRUE, $name . ' found.');
+    $this->assertIdentical($staging_storage->exists($dynamic_name), TRUE, $dynamic_name . ' found.');
 
     // Import.
     config_import();
 
     // Verify the values appeared.
     $config = config($name);
-    $this->assertIdentical($config->get('add_me'), 'new value');
+    $this->assertIdentical($config->get('add_me'), $original_name_data['add_me']);
     $config = config($dynamic_name);
-    $this->assertIdentical($config->get('label'), 'New');
+    $this->assertIdentical($config->get('label'), $original_dynamic_data['label']);
 
     // Verify that appropriate module API hooks have been invoked.
     $this->assertFalse(isset($GLOBALS['hook_config_test']['load']));
@@ -151,22 +153,26 @@
   function testUpdated() {
     $name = 'config_test.system';
     $dynamic_name = 'config_test.dynamic.default';
+    $staging_storage = new FileStorage(array('directory' => config_get_config_directory(CONFIG_STAGING_DIRECTORY)));
 
     // Export.
     config_export();
 
+    // Verify that the configuration objects to import exist.
+    $this->assertIdentical($staging_storage->exists($name), TRUE, $name . ' found.');
+    $this->assertIdentical($staging_storage->exists($dynamic_name), TRUE, $dynamic_name . ' found.');
+
     // Replace the file content of the existing configuration objects in the
     // staging directory.
-    $file_storage = new FileStorage(array('directory' => config_get_config_directory(CONFIG_STAGING_DIRECTORY)));
-    $this->assertIdentical($file_storage->exists($name), TRUE, $name . ' found.');
-    $this->assertIdentical($file_storage->exists($dynamic_name), TRUE, $dynamic_name . ' found.');
-    $file_storage->write($name, array(
+    $original_name_data = array(
       'foo' => 'beer',
-    ));
-    $file_storage->write($dynamic_name, array(
+    );
+    $staging_storage->write($name, $original_name_data);
+    $original_dynamic_data = array(
       'id' => 'default',
       'label' => 'Updated',
-    ));
+    );
+    $staging_storage->write($dynamic_name, $original_dynamic_data);
 
     // Verify the active store still returns the default values.
     $config = config($name);
@@ -183,6 +189,10 @@
     $config = config($dynamic_name);
     $this->assertIdentical($config->get('label'), 'Updated');
 
+    // Verify that the original file content is still the same.
+    $this->assertIdentical($staging_storage->read($name), $original_name_data);
+    $this->assertIdentical($staging_storage->read($dynamic_name), $original_dynamic_data);
+
     // Verify that appropriate module API hooks have been invoked.
     $this->assertTrue(isset($GLOBALS['hook_config_test']['load']));
     $this->assertTrue(isset($GLOBALS['hook_config_test']['presave']));
diff -u b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -721,12 +721,19 @@
     // uses drupal_valid_test_ua() to adjust the config directory paths to
     // a test-prefix-specific directory within the public files directory.
     // @see config_get_config_directory()
-    require_once DRUPAL_ROOT . '/core/includes/install.inc';
-    foreach (array(CONFIG_ACTIVE_DIRECTORY, CONFIG_STAGING_DIRECTORY) as $config_env) {
-      $GLOBALS['config_directories'][$config_env] = 'simpletest/' . substr($this->databasePrefix, 10) . '/config_' . $config_env;
-      install_ensure_config_directory($config_env);
+    $GLOBALS['config_directories'] = array();
+    foreach (array(CONFIG_ACTIVE_DIRECTORY, CONFIG_STAGING_DIRECTORY) as $type) {
+      $GLOBALS['config_directories'][$type] = 'simpletest/' . substr($this->databasePrefix, 10) . '/config_' . $type;
+    }
+
+    $this->configDirectories = array();
+    include_once DRUPAL_ROOT . '/core/includes/install.inc';
+    foreach ($GLOBALS['config_directories'] as $type => $path) {
+      if (!install_ensure_config_directory($type)) {
+        return FALSE;
+      }
+      $this->configDirectories[$type] = $this->originalFileDirectory . '/' . $path;
     }
-    $this->configDirectories = $GLOBALS['config_directories'];
 
     // Log fatal errors.
     ini_set('log_errors', 1);
diff -u b/sites/default/default.settings.php b/sites/default/default.settings.php
--- b/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -257,10 +257,12 @@
  * @todo Flesh this out, provide more details, etc.
  *
  * Example:
+ * @code
  *   $config_directories = array(
  *     CONFIG_ACTIVE_DIRECTORY => '/some/directory/outside/webroot',
  *     CONFIG_STAGING_DIRECTORY => '/another/directory/outside/webroot',
  *   );
+ * @endcode
  */
 $config_directories = array();