Insulating an attic in Florida. Trying to figure out if this would work or not or any drawbacks. Roof is simple gable approximately 3/12 pitch. Vents at eaves, ridge and gable ends. Was thinking of laying fiberglass batt between joists and then putting four inches of foil faced polyiso board over that with radiant barrier facing upwards. Any potential issues with this idea? I’ve never seen it done this way. 

see attached image for detail. 

thanks!