So got some 2800 deg fire brick its 3in thick would 1 1/2 work for the top?

for what? speaking in partial sentences is hard to understand, it wont be 2800 degrees if you half it

For a small  9x9 gas forge which is why I  asked in the (gas forge ) section 

There are many configurations of gas forges. We need more details on exactly how that fire brick is to be used and which way are you going to orient or place the fire brick.  Drawings would help.

The floor is two 4 1/2×9 bricks and one on each side my plan was to make the top 1 1/2 thinner on top

I was going to thin down the bricks for the sides and top but opted to keep them full thickness as they are holding some weight on the sides and need to withstand the span for the top.  If a thinner brick cracks it is more likely to collapse.  The 3' will hold itself up better if a crack appears.(well.. after.  a crack is inevitable with IFB)

The only upside to thinning it would be to lessen the thermal mass or back it up with lesser temperature IFB which is what I had planned (I didn't want to go through all that work with my geometry).  Your better off using IFB full thickness for the top than hard brick.. less mass and IFB insulates better.  Hard brick does not insulate so the whole mass wants to get hot.

Take a look at my forge here..  I would suggest just keeping the geometry simple with straight walls like you are.  It took a lot of time to fab mine.  The few pics of it heating there is a casting of Mizzou on the floor under the steel to protect the IFB.   It is 5/8" thick.  Had the forge running for approx 20 minutes before I shut it down.  The underside of that block was glowing. 

 

Thanks for the suggestions I had the hard brick on top just for comparison only using it on the bottom: I did split the 3 in brick in half for the walls to give me more room :thouthought of coating the inside with a refractory cement 

Note: as this is an international site; 9x9 is rather ambiguous: inches, feet, mm, cm meters?  (And yes we have people participating here where 9'x9' would be the small gas forge---they use RR cars to put stuff in them and their tongs have wheels and a steering wheel!)

You know exactly what you wish to do and how you hope to accomplish it.  We don't and after a couple of TBI's my mind reading apparatus is now on the fritz.  Please be gracious enough to give in depth descriptions so we can figure out what you already know. (One of the reasons that we like pictures so much! Thank-you for them!)

Looks like you are going with your own design, so why not try it and report back on success.

If I were building an insulating fire brick gas forge I would be concerned about the following:

1. Efficiency:  I would use a full brick thickness of insulating fire brick (IFB) on all sides, at least.  I might even use a 2600 or 2800 rated brick thickness for the inner wall and a 2300 rated brick for the outer wall.
2. Structure:  In my experience IFB will crack after repeated use.  I would have a steel angle frame that could compress the brick and keep it in place after it cracks as well as supporting the burner.  In my experience refractory cement is not very effective, but YMMV.
3. Flux:  Molten flux will eat IFB like water on cotton candy.  You will need something to protect it if you ever expect to forge weld with flux in your forge.   High alumina refractory materials are a good choice.
4. Flame Contact:  My first gas forge liner was made from some old IFB I had lying around for over 20 years from a glass furnace project that I went a different way on.  I used a full 4" width for the wall thickness, beveled with a band saw to fit into an arch.  It only lasted a couple of firings before the hot jet from the burner fused a hole into the brick opposite the burner.  You need a material that can take that 3,000 deg. F heat from the flame directly opposite the burner
5. Volume:  It is not only important to have the correct configuration for the type of work you want to do, but for the volume to match your burner.  You can have both too much volume to effectively heat and too little volume to allow complete combustion inside the forge.  If you aren't using a tested design you run the risk of making multiple iterations as you learn this.
6. Burner design:  As noted you want a short soft flame to avoid burning out your bricks and to keep the flame inside the forge as long as possible to transfer heat to the walls.  Location of the burner port is important.  It is a lot harder to make holes in hard fire brick than IFB.  Planning ahead here will make a difference.
7. Doors:  So often overlooked or left to last minute but a critical element for forge efficiency, both in heating and usability.  I'm still working on upgrading mine.