diff --git a/repo2docker/buildpacks/conda/__init__.py b/repo2docker/buildpacks/conda/__init__.py
index 8e818bc4..27446c4d 100644
--- a/repo2docker/buildpacks/conda/__init__.py
+++ b/repo2docker/buildpacks/conda/__init__.py
@@ -178,6 +178,18 @@ class CondaBuildPack(BaseImage):
         """Am I building a Python 2 kernel environment?"""
         return self.python_version and self.python_version.split(".")[0] == "2"
 
+    def get_assemble_files(self):
+        """Specify that assembly only requires environment.yml
+
+        enables caching assembly result even when
+        repo contents change
+        """
+        assemble_files = super().get_assemble_files()
+        environment_yml = self.binder_path('environment.yml')
+        if os.path.exists(environment_yml):
+            assemble_files.append(environment_yml)
+        return assemble_files
+
     def get_assemble_scripts(self):
         """Return series of build-steps specific to this source repository.
         """