The RFM12 transceiver has some places which are not isolated. As soon as you use a PCB which is soldered by yourself and has no isolation as well, there has to be an isolation between the OHC device's PCB and the RFM12 module.

You can use a piece of paper or plastic for isolation.

Please note that this may not be optimal if the device is used in moist conditions. I have no long term experience using the paperboard as isolation. But my outdoor temperature sensor did well this winter.

The gold plated connections from the RFM12 are coated with a little tin-solder first.

After gluing the transceiver to the PCB, you can solder the connections to the OHC PCB easily.