REM/Rate version 15.5 has a pretty major bug when it comes to determining compliance for the 2015 IECC. Per NORESCO:
An alert REM user spotted a bug in the v15.5 Performance Path code compliance. It causes the Reference Home to have higher DHW energy consumption and cost than it should -- which makes it appear that homes comply when they should not.We will have a fix for this as part of REM v15.6, due for release prior to the 2018 RESNET Conference. In the meantime, we recommend doing your code compliance calculations in REM v15.4.2. Doing so will avoid nasty surprises when you update to REM v15.6.If you need to use REM v15.6 for reasons other than code compliance, remember that you can always export the building to REM XML under the File menu, and then import it into earlier versions via File>>Import>>XML.
REM/Rate version 15.4.2 is still an approved RESNET version to use. If you are performing code compliance HERS Ratings, you may want to stick with that version until v15.6 is released.