What I do is simply enable and configure the easy to use built-in VPN server on my router (Asus RT-N66U). With an encrypted connection between the router and client you are safe and don’t have to worry about encrypting all your home automation (assuming you use security on your home wifi) most all smart phones and computers have a built in PPTP client. Just remember to use encryption.
Otherwise If you use Samir’s mobile web app configured on your RPI using https you should also be better off than going directly to the interval program with http. This way the only http unencrypted traffic will be confined within the localhost. But this is more to configure.
Even if you just use the VPN option above, the mobile app will also make life much nicer because it is formatted well for the mobile environment.