Firebrick confusion

[icebecker]

I am just about to start a 42" Pompeii type oven. I have seen some threads mention that low duty firebrick works well. The Pompeii kit from FB includes medium duty firebrick. I have access to some 1700 deg F yellow brick that weighs about 7.5lbs for $1.60 per brick here in Albuquerque. I can get brick that is 2000 or 2600 deg rated for $2.80. The dealers aren't sure about the alumina content. Which brick should I go for?
Also, I've seen differing opinions regarding the insulating board under the oven floor. Is insulating concrete as good? Should the oven floor bricks set into the insulating concrete?
Thanks for helping a neophyte.

[Fairview WFO]

See if you can find out the manufacture of the brick as they may have mix design info online. If you have the Pompeii plans you can read the section discussing heat absorption and the benifits of the low duty and the problems with the others.

As far as insulation, you don't need the fiberboard, it just gives you a better insulating value per thickness. I just used 4" of insulated concrete mix over the 4" structural slab and it works fine. I did use 4" of ceramic and about 4 large bags of perlite on the top however. If you want overkill, use both, many do. My oven holds cooking temps for 2 days, so I'm not sure you really need more, unless you just want it.

[Tscarborough]

Unless you plan on smelting, use the cheaper firebrick, you will never hit 1700 without a bellows anyway.

[icebecker]

Thanks for the helpful replies. That clarifies the situation greatly. I have decided to forego the firebrick. After looking at the helpful info in the forum, I have decided to go with a modular oven. I am having someone experienced in adobe ovens build the stand but he doesn't have any experience in building a brick oven. I won't have the time to supervise the building of a pompeii oven. It looks like a great project but I think the modular casa 2G 100 will work better in this situation.
I look forward to many years of great wfo pizza, flat breads, and rustic breads. The slab is being poured tomorrow and the oven has just been shipped. I have recently been using Peter Reinhart's books and can't wait to get the wild yeast whole grain breads out of the wfo to feed the twins.