I am building a house with icynene in 2x4 walls and have been talking to the Dow people about using 1/2" SIS in place of the OSB and house wrap that i had originally intended to use.  This house is in Memphis, TN.  I haven't really found any advice on this forum about 1/2" SIS and have only seen a few posts about using larger size foam boards.  Will the 1/2" break the thermal bridging and be worthwhile to use in this application?  also if this will have fibercement siding.

thanks,
bob

1/2" SIS will help reduce the thermal bridging... I would prefer 1" of SIS.
If you have a basement or crawlspace, you should have the foam board extend down to cover the rim joist (floor joist area)... spray foam the rim joist too. Tape/seal all seams on the foam board. Extending the foam board down will reduce/eliminate thermal bridging and air infiltration around the joists.
With any kind of siding over foam board, you will have to nail into the studs/framing of the house. If you are using wide trim around windows, doors, etc., plan for this so you have something solid to nail into.