FB Board Thickness

[rbigante]

Anyone know if there are any benefits with laying 4 inch thick FB board beneath the oven floor (2-2" layers) or just one 2 inch layer? Goint to be hopefully starting the floor by the end of the week and I have enough FB board to do two layers but wasn't sure if there was any benefit to it??????

[egalecki]

2 inches should be enough. I have 2 inches on top of several inches of vermiculite concrete- so it's overkill. If you want a little more insulation without having to buy more board, use both.

[SteveP]

I only used 2" of FB board on top of my regular concrete hearth slab (no vermiculite layer). When my cooking floor temp is >1000 F, the bottom of my hearth slab is around 110F. I think 2" is more than sufficient.

[rbigante]

Elizabeth and Steve, Thanks for the responses.

I, like Steve, haven't used vermiculite concrete anywhere. It sounds like the 2 inches of FB board is going to be plenty of insulation based on the #'s given by Steve (great information BTW). Sounds like the 2 inch of FB board is keeping the heat in the bricks and oven.

I'm also going to be installing 2 inches of FB blanket and a couple inches of loose vermiculite over the dome, so it sounds like there should be no problems with insulation.

[arriflex]

Does the insulation board have any structural value? Is structure the sole purpose of the thick concrete hearths when ample insulation has been provided by the board?

arri

[dmun]

Does the insulation board have any structural value?

No. It has a consistency slightly denser than the old acoustic ceiling tiles.

Is structure the sole purpose of the thick concrete hearths when ample insulation has been provided by the board?

Yes. Brick ovens are heavy and fragile and need support. The support slab has no insulation value.

[arriflex]

So, theoretically, I could construct a structural base out of steel to support the insulating boards upon which the firebrick floor of the oven could rest?
How much heat would be transferring to the base with a single thickness board?

In perusing the technical characteristics, one might expect the conductivity to be around 0.1 at 1000 deg F, meaning to answer my second question I could expect the bottom of the board to be at approximately 100 deg?

In SteveP's example above where he suggests the bottom of his concrete hearth slab is right around that number, I am surprised his slab is offering basically 100% thermal conductivity. Unless his thermometer doesn't read higher than 1000 def F and the actual floor temp is much higher?

[rbigante]

thought about it a little bit. i'm going with 2 layers of 2" fb board. my wife's cousin had an wfo installed but seems to loose way too much heat in the floor. It goes good for about 5-6 pizzas then it really looses heat in the floor. i don't want that to happen to me. the seams will not overlap other than where they cross at 90 degree intersections. i have enough fb board to do it, so what the heck.

i have my base complete (finished in fall), I've cut a bunch of brick with my new 10" wet saw and it looks like the weather will cooperate for the next 4 days in the pittsburgh area.

[arriflex]

It would seem as though the issue there is more likely the retention value of the floor rather than the energy loss through it... Remember that the Pompeii design would have you place the floor bricks flat (2.5" thick), thereby leaving nearly half the 'thickness' of the walls that are half bricks at 4.5".
From an insulation standpoint, remember that a single 2" board only conducts one tenth the heat put into it, which means normally your hearth isn't likely to be more than a few degrees above ambient unless it is below freezing!

I'd save the board for another oven or use it around the oven. If you want more energy to be held in the floor, you'll be better served by increasing the thermal mass there.

[MK1]

I'd be careful about adding mass to the floor. I've investigated this extensively on the forum, and the experts here have tried the variations you contemplate. If I remember right, you would be altering the heat exchange balance between the dome and the floor. The way the Pompeii is designed the dome and the floor gain temp at the same rate. I can't say I understand the whole dynamic but this design is fairly proven.

Mark