diff --git a/cmake/ssl.cmake b/cmake/ssl.cmake
index a983e98650c9a..2c6a0adcd506a 100644
--- a/cmake/ssl.cmake
+++ b/cmake/ssl.cmake
@@ -12,6 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# The CMakeLists.txt for BoringSSL doesn't propagate include directories
+# transitively so `_gRPC_SSL_INCLUDE_DIR` should be set for gRPC
+# to find header files.
+
 if(gRPC_SSL_PROVIDER STREQUAL "module")
   if(NOT BORINGSSL_ROOT_DIR)
     set(BORINGSSL_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/boringssl)
diff --git a/cmake/zlib.cmake b/cmake/zlib.cmake
index 794e47f7aa549..858d3b42fee7e 100644
--- a/cmake/zlib.cmake
+++ b/cmake/zlib.cmake
@@ -12,6 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# The CMakeLists.txt for zlib doesn't propagate include directories
+# transitively so `_gRPC_ZLIB_INCLUDE_DIR` should be set for gRPC
+# to find header files.
+
 if(gRPC_ZLIB_PROVIDER STREQUAL "module")
   if(NOT ZLIB_ROOT_DIR)
     set(ZLIB_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/zlib)