diff --git a/.gitignore b/.gitignore
index ff5a080f..caf970cf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,6 +12,7 @@ Web/fluid_properties/fluids/molecule_sdf/
 .version
 
 /bld*
+/.cpm_cache/
 
 /dev/VS2010
 
diff --git a/cmake/dependencies.cmake b/cmake/dependencies.cmake
index 5c1afee9..8e060b72 100644
--- a/cmake/dependencies.cmake
+++ b/cmake/dependencies.cmake
@@ -1,6 +1,11 @@
 # Dependencies for CoolProp managed via CPM.cmake
-# Set CPM_SOURCE_CACHE in your environment (e.g. ~/.cache/CPM) to share the
-# download cache across git worktrees and build directories.
+# CPM_SOURCE_CACHE can be overridden via environment variable (e.g. ~/.cache/CPM)
+# to share the download cache across git worktrees and build directories.
+# Without a stable cache location, FetchContent re-runs on every cmake configure,
+# touching header timestamps and forcing a complete C++ rebuild each time.
+if(NOT DEFINED CPM_SOURCE_CACHE AND "$ENV{CPM_SOURCE_CACHE}" STREQUAL "")
+  set(CPM_SOURCE_CACHE "${CMAKE_CURRENT_LIST_DIR}/../.cpm_cache" CACHE PATH "CPM source cache")
+endif()
 
 include("${CMAKE_CURRENT_LIST_DIR}/CPM.cmake")