Currently the Therm-App is only compatible with Android. However, many Therm-App owners with IPhones simply purchase a lesser expensive Android phone such as the Samsung Galaxy S3 (which can be purchased for less than $100 brand new). And make it a dedicated phone for the Therm-App.
As no monthly plan is needed (not internet connection is required for the Therm-App to work). For more details, please contact firstname.lastname@example.org