diff --git a/subiquity/server/apt.py b/subiquity/server/apt.py
index d06a498d3..d18e4911e 100644
--- a/subiquity/server/apt.py
+++ b/subiquity/server/apt.py
@@ -268,9 +268,9 @@ async def configure_for_install(self, context):
     async def overlay(self):
         overlay = await self.mounter.setup_overlay(
             [
-                self.install_tree.upperdir,
-                self.configured_tree.upperdir,
                 self.source_path,
+                self.configured_tree.upperdir,
+                self.install_tree.upperdir,
             ]
         )
         try:
diff --git a/subiquity/server/mounter.py b/subiquity/server/mounter.py
index 2139b7a55..53d45d2ca 100644
--- a/subiquity/server/mounter.py
+++ b/subiquity/server/mounter.py
@@ -155,6 +155,15 @@ async def unmount(self, mountpoint: Mountpoint, remove=True):
                 path.unlink(missing_ok=True)
 
     async def setup_overlay(self, lowers: List[Lower]) -> OverlayMountpoint:
+        """Setup a RW overlay FS over one or more lower layers.
+        Be careful, when multiple lower layers are specified, they are stacked
+        from the leftmost one and going right. This is the opposite of what the
+        lowerdir mount option expects.
+        Therefore, when calling setup_overlay([x, y, z]), the corresponding
+        mount command will look something like:
+        $ mount [...] -o lowerdir=z:y:x
+        Which means z will be top, y will be middle and x will be bottom.
+        """
         tdir = self.tmpfiles.tdir()
         target = f"{tdir}/mount"
         lowerdir = lowerdir_for(lowers)