diff --git a/src/pip/_internal/build_env.py b/src/pip/_internal/build_env.py
index 1989c8aad05..04e1861ebb0 100644
--- a/src/pip/_internal/build_env.py
+++ b/src/pip/_internal/build_env.py
@@ -24,6 +24,7 @@ def __enter__(self):
 
         self.save_path = os.environ.get('PATH', None)
         self.save_pythonpath = os.environ.get('PYTHONPATH', None)
+        self.save_nousersite = os.environ.get('PYTHONNOUSERSITE', None)
 
         install_scheme = 'nt' if (os.name == 'nt') else 'posix_prefix'
         install_dirs = get_paths(install_scheme, vars={
@@ -48,20 +49,23 @@ def __enter__(self):
         else:
             os.environ['PYTHONPATH'] = lib_dirs
 
+        os.environ['PYTHONNOUSERSITE'] = '1'
+
         return self.path
 
     def __exit__(self, exc_type, exc_val, exc_tb):
         if not self._no_clean:
             self._temp_dir.cleanup()
-        if self.save_path is None:
-            os.environ.pop('PATH', None)
-        else:
-            os.environ['PATH'] = self.save_path
 
-        if self.save_pythonpath is None:
-            os.environ.pop('PYTHONPATH', None)
-        else:
-            os.environ['PYTHONPATH'] = self.save_pythonpath
+        def restore_var(varname, old_value):
+            if old_value is None:
+                os.environ.pop(varname, None)
+            else:
+                os.environ[varname] = old_value
+
+        restore_var('PATH', self.save_path)
+        restore_var('PYTHONPATH', self.save_pythonpath)
+        restore_var('PYTHONNOUSERSITE', self.save_nousersite)
 
     def cleanup(self):
         self._temp_dir.cleanup()
diff --git a/src/pip/_internal/wheel.py b/src/pip/_internal/wheel.py
index 17d52911698..7118558a56b 100644
--- a/src/pip/_internal/wheel.py
+++ b/src/pip/_internal/wheel.py
@@ -625,14 +625,11 @@ def _build_one(self, req, output_dir, python_tag=None):
         # Install build deps into temporary directory (PEP 518)
         with req.build_env:
             return self._build_one_inside_env(req, output_dir,
-                                              python_tag=python_tag,
-                                              isolate=True)
+                                              python_tag=python_tag)
 
-    def _build_one_inside_env(self, req, output_dir, python_tag=None,
-                              isolate=False):
+    def _build_one_inside_env(self, req, output_dir, python_tag=None):
         with TempDirectory(kind="wheel") as temp_dir:
-            if self.__build_one(req, temp_dir.path, python_tag=python_tag,
-                                isolate=isolate):
+            if self.__build_one(req, temp_dir.path, python_tag=python_tag):
                 try:
                     wheel_name = os.listdir(temp_dir.path)[0]
                     wheel_path = os.path.join(output_dir, wheel_name)
@@ -647,20 +644,18 @@ def _build_one_inside_env(self, req, output_dir, python_tag=None,
             self._clean_one(req)
             return None
 
-    def _base_setup_args(self, req, isolate=False):
-        flags = '-u'
-        # The -S flag currently breaks Python in virtualenvs, because it relies
-        # on site.py to find parts of the standard library outside the env. So
-        # isolation is disabled for now.
-        # if isolate:
-        #     flags += 'S'
+    def _base_setup_args(self, req):
+        # NOTE: Eventually, we'd want to also -S to the flags here, when we're
+        # isolating. Currently, it breaks Python in virtualenvs, because it
+        # relies on site.py to find parts of the standard library outside the
+        # virtualenv.
         return [
-            sys.executable, flags, '-c',
+            sys.executable, '-u', '-c',
             SETUPTOOLS_SHIM % req.setup_py
         ] + list(self.global_options)
 
-    def __build_one(self, req, tempd, python_tag=None, isolate=False):
-        base_args = self._base_setup_args(req, isolate=isolate)
+    def __build_one(self, req, tempd, python_tag=None):
+        base_args = self._base_setup_args(req)
 
         spin_message = 'Running setup.py bdist_wheel for %s' % (req.name,)
         with open_spinner(spin_message) as spinner:
@@ -671,13 +666,8 @@ def __build_one(self, req, tempd, python_tag=None, isolate=False):
             if python_tag is not None:
                 wheel_args += ["--python-tag", python_tag]
 
-            env = {}
-            if isolate:
-                env['PYTHONNOUSERSITE'] = '1'
-
             try:
                 call_subprocess(wheel_args, cwd=req.setup_py_dir,
-                                extra_environ=env,
                                 show_stdout=False, spinner=spinner)
                 return True
             except: