diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -261,6 +261,13 @@ class abstractvfs(object):
     def islink(self, path=None):
         return os.path.islink(self.join(path))
 
+    def reljoin(self, *paths):
+        """join various elements of a path together (as os.path.join would do)
+
+        The vfs base is not injected so that path stay relative. This exists
+        to allow handling of strange encoding if needed."""
+        return os.path.join(*paths)
+
     def lexists(self, path=None):
         return os.path.lexists(self.join(path))
 
diff --git a/mercurial/transaction.py b/mercurial/transaction.py
--- a/mercurial/transaction.py
+++ b/mercurial/transaction.py
@@ -200,8 +200,8 @@ class transaction(object):
             return
         dirname, filename = os.path.split(file)
         backupfilename = "%s.backup.%s" % (self.journal, filename)
-        backupfile = os.path.join(dirname, backupfilename)
         vfs = self._vfsmap[location]
+        backupfile = vfs.reljoin(dirname, backupfilename)
         if vfs.exists(file):
             filepath = vfs.join(file)
             backuppath = vfs.join(backupfile)