diff --git a/README.md b/README.md
index 931fe261..72e35b2f 100644
--- a/README.md
+++ b/README.md
@@ -98,6 +98,13 @@ of the element directory describing what it is for.
 Writing an element
 -----------------
 
+Conform to the following conventions:
+
+* Use the environment for overridable defaults, prefixing environment variable
+  names with "DIB\_". For example: DIB\_MYDEFAULT=${DIB\_MYDEFAULT:-default}
+  If you do not use the DIB\_ prefix you may find that your overrides are
+  discarded as the build environment is sanitised.
+
 Make as many of the following subdirectories as you need, depending on what
 part of the process you need to customise:
 
diff --git a/bin/disk-image-create b/bin/disk-image-create
index 50456566..f45903e1 100755
--- a/bin/disk-image-create
+++ b/bin/disk-image-create
@@ -94,9 +94,9 @@ echo "If prompted for sudo, install sudoers.d/img-build-sudoers into /etc/sudoer
 ensure_nbd
 
 mk_build_dir
-eval_run_d block-device-size "IMAGE_SIZE="
+eval_run_d block-device-size "DIB_IMAGE_SIZE="
 
-qemu-img create -f qcow2 -o preallocation=metadata $TMP_IMAGE_PATH ${IMAGE_SIZE}G
+qemu-img create -f qcow2 -o preallocation=metadata $TMP_IMAGE_PATH ${DIB_IMAGE_SIZE}G
 
 # Should have a grab-next-dev helper ?
 NBD_DEV=/dev/nbd0
diff --git a/lib/img-defaults b/lib/img-defaults
index a1350f00..0f707cc5 100644
--- a/lib/img-defaults
+++ b/lib/img-defaults
@@ -19,7 +19,7 @@ FS_TYPE=${FS_TYPE:-ext4}
 # Used to set the file extension only at this stage.
 IMAGE_TYPE=${IMAGE_TYPE:-qcow2}
 IMAGE_NAME=${IMAGE_NAME:-image}
-export IMAGE_SIZE=${IMAGE_SIZE:-2} # N.B. This size is in GB
+export DIB_IMAGE_SIZE=${DIB_IMAGE_SIZE:-2} # N.B. This size is in GB
 # Set via the CLI normally.
 # IMAGE_ELEMENT=
 _BASE_ELEMENT_DIR=$(dirname $0)/../elements