chmod -R only files or only directories?

This question started when I wasn't able to tar up a directory because the permissions where set in such a way that tar wasn't able to read some things, even as root,

brute force chmod -R +x on the directory and everything under it helps, but might open up some vulnerabilities.

Can you tell chmod -R +x to only effect directories or conversely only to effect 'actual' files?

Many thanks in advance.