diff --git a/mkosi.conf b/mkosi.conf
index e4c866c9ba..954012ee0f 100644
--- a/mkosi.conf
+++ b/mkosi.conf
@@ -33,7 +33,6 @@ OutputDirectory=build/mkosi.output
 ToolsTree=default
 BuildDirectory=build/mkosi.builddir
 CacheDirectory=build/mkosi.cache
-BuildSourcesEphemeral=yes
 Incremental=yes
 
 [Validation]
diff --git a/mkosi.conf.d/20-build.conf b/mkosi.conf.d/20-build.conf
index 8c16d9b9f8..f9b4a0aaf5 100644
--- a/mkosi.conf.d/20-build.conf
+++ b/mkosi.conf.d/20-build.conf
@@ -1,9 +1,10 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
-# Add a dependency on the build image unless NO_BUILD=1.
+# Add a dependency on the build image and make the build sources ephemeral unless NO_BUILD=1.
 
 [Match]
 Environment=!NO_BUILD=1
 
 [Config]
 Dependencies=build
+BuildSourcesEphemeral=yes