Why not insulating firebricks for hearth insulation?

[kebwi]

I've never seen any suggestion of using insulating firebricks for the hearth insulation, only various "boards" (superisol) and aerated concretes (vermiculite & perlite). Perhaps I'm misunderstanding something, but isn't this exactly what insulating firebricks are designed for?

What am I missing here? Are they really expensive or not very insulative or difficult to work with? I'm sure there's a perfectly good reason for why they aren't recommended, much less barely discussed, but I'm not aware of what that reason is yet.

Final thought: even if they shouldn't be used for the entire floor, would they make good standoffs or "columns" embedded into the vermcrete/perlcrete at, say 12"-18" spacing (half bricks perhaps) to provide compression resistance to the weaker concrete?

...of course that idea is only necessary if compression is a serious issue in the first place and I get a very strong impression from this forum that compression is virtually never a problem, even at 7:1, but would they help in such a case?

Just brainstorming.

Thanks.

[david s]

Re: Why not insulating firebricks for hearth insulation?

I agree with you. Why not use them ? Trouble is they're expensive. They are also designed to withstand really high temps (1200 C+), which in our application we don't need. Aerated concrete (Hebel) is another alternative, but it is also dearer than vermiculite, for me anyway. If you can access insulating firebricks at a good price then go for it.

[david s]

Re: Why not insulating firebricks for hearth insulation?

Kebwi,
Forgot to add that insulating firebricks are not that strong because of all that air in them. Probably not much stronger, if at all, than vermiculite. There's a job for you, run some tests.

[John K]

Re: Why not insulating firebricks for hearth insulation?

Hi Guys

I am in the process of laying Insulation bricks on my hearth and will cost me just under $250.00 (75 Bricks).

The supplier advised that I will get better insulation properties using these and I won't have an issue of these crumbling under the weight of the dome.


I guess I will know shortly!!

[ThisOldGarageNJ]

Re: Why not insulating firebricks for hearth insulation?

Hi John K

Are these the very white bricks they use in kilns and furnaces ? I want to build an outdoor furnace and Im thinking about using a product like that.
Thanks
Mark

[John K]

Re: Why not insulating firebricks for hearth insulation?

Hi Mark

Yes these are the white bricks and are very light!! see attached photo

I also jumped on the suppliers website and have found this spill listed under Insulation Bricks


INSULATION BRICK

Field Furnace Refractories is an agent for ISOLITE Insulating Firebrick.
The Insulating Firebricks have densities ranging from 0.50 -1.8 grams per cubic centimetre and temperature ratings from 900oC to 1800oC.

Isolite Insulating Firebricks have low thermal conductivity good cold crushing strength; low reheat shrinkage and excellent dimensional tolerances.

We can supply the insulating firebrick required for any furnace design to give optimum thermal insulation. Isolite IFBs are used as back up and as a hot face in a myriad of furnaces and kilns.

hope it helps

John


You can follow my build at http://www.fornobravo.com/forum/f8/j...-wfo-7911.html

[david s]

Re: Why not insulating firebricks for hearth insulation?

Yes they are very white and very light, also very refractory.

[dmun]

Re: Why not insulating firebricks for hearth insulation?

They work fine: they have the same insulation value as vermiculite concrete. The problem is price. At least around here they are far more expensive than regular firebrick, and an order of magnitude more than vermiculite concrete. If you're spending that kind of money, you should spring for the modern insulation boards, and get the advantage of higher insulation values.

And just for the record: there's no compressive strength problem with vermiculite concrete. It has no tensile strength, but you'd be hard pressed to crush it.

[ThisOldGarageNJ]

Re: Why not insulating firebricks for hearth insulation?

I think I will stick with the vermicrete,, I love to work with things that have a known factor,,, I have used it with good success, and to date it amazes me how much heat it can hold...

Mark

[kebwi]

Re: Why not insulating firebricks for hearth insulation?

So, let's say, theoretically ;-) that I have access to 50 new insulating firebricks for $75. Nudge, nudge, wink, wink. Clearly, that's a good price. It won't really complete the floor, even for a 36" oven. Could I make a workable design with this by filling the center with bricks and then expanding the circumference with perlcrete, or vs/va, or something like that?

I'm thinking this might work...but just to clarify, as was stated above in this thread, it is only as insulating as perlcrete, not as insulating as board, so I either need a few inches worth, or I need a layer of perlcrete above or below it...or some board...I'm thinking of putting 1" to 2" Insblock 19 under the hearth anyway, regardless of whether I use perlcrete or insulating firebricks.

Thoughts?

Thanks, as always.

[fxpose]

Re: Why not insulating firebricks for hearth insulation?

Personally, I would stick with one type of insulation just for the sake of simplicity.

[GianniFocaccia]

Re: Why not insulating firebricks for hearth insulation?

kebwi,

My original plan was to go a little (overbuild, as usually) heavy on the vermicrete but the local ANH guy made me great deal on a case of 2" Insboard 19. I ended up topping 3" of vermicrete with the board which was really easy and fun to do. I'm pleased with the way it turned out and the fact that I have a primary insulating layer and a backup layer, not to mention that the whole thing cost around $135 to install.

Depending on the insulating firebrick coverage you have, you could always place the bricks under the dome and vermicrete under the landing and entry, OR, work your way from the outer circumference of your dome inwards with the bricks and fill any remaining space with vermicrete. Either way, I'd guess the insulating value of either material is pretty close. After my vermicrete cured I POUNDED on the insulating board to see if I could get it to budge and hurt my hand.

[kebwi]

Re: Why not insulating firebricks for hearth insulation?

Thanks. I already passed up the insulating firebricks, and at the same time got my hands on a rather large quantity of leftover vermiculite (that I hope doesn't have too much rockwool mixed into it).

I have been thinking about Insblock 19 too. I haven't decided if I want to just do 4" of vermicrete or if I want to do 2"-3" of vermicrete with 1" of Insblock 19. The latter option is more expensive but I kind of like the idea of having a better insulator directly under the bricks, and also thought that board might be a little more even than direct vermicrete...of course the boards might wobble on the vermicrete for the same reason.

I can't decide. Not sure the board is worth the cost. I can get the 36"x12"x1" for $10.65 locally or a pack of 16 for $92 (or a pack of 8 2" for the same price), from Northwest Iron Works :: Home Page, but which more than I need. I think five or six boards would about do it.

What do you think? Is the Insblock layer worth an extra $70 to $100?

[egalecki]

Re: Why not insulating firebricks for hearth insulation?

I have insblock on top of vermicrete. My board never moved at all. If you get the top of your vermicrete nice and smooth, (well, maybe flat is a better word) the board goes on well and the bricks on top of that even better.

[Bajapizza]

Re: Why not insulating firebricks for hearth insulation?

Does anyone know if you are supposed to water the insulating concrete during the curing process. I just finished pouring a 5 1/2 inch perlite hearth for my 54" wfo.