Hearth insulation

[CvC]

Hi there,

I was looking for a cheaper alternative for ceramic fiber board for the insulation of the hearth and have heard of ovens that are insulated with gas concrete (the brand name in Europe is Ytong).
Has anybody here tried this and can report of his/her experiences?

Christian

[Les]

Re: Hearth insulation

If money is an issue why not use vermiculite? It's proven to work.

[CvC]

Re: Hearth insulation

Money is not really an issue, but on the other hand I?m not willing to pay 300? (aprox. 350$) only for the insulation of the hearth, so I?m looking for alternatives.
Vermicrete is an alternative, I just have to check if it?s insulating properties are as good as that of Ytong. (Is the name "Ytong" common in the states for gas concrete?)

[Tscarborough]

Re: Hearth insulation

It is better.

[CvC]

Re: Hearth insulation

It is better.

O.k.,

do you have any facts?

[Tscarborough]

Re: Hearth insulation

I do, I have the same facts that you can find. Perlcrete can range up to 800 psi depending upon the mix, and a simple search of this site should reveal the posts where I researched it. AAC is brittle and fails under moderate to high heat, although that is not much of a factor for this use.

If you want advise, I gave it, if you want a report, I charge for that.

[Tscarborough]

Re: Hearth insulation

(I don't mean to be snotty, but the question is asked frequently).

[Les]

Re: Hearth insulation

I have read posts on this site that say vermicrete has a rating of 100 psi. ]

Even at this low rating it is more than adequate. The foot print of one brick can support over two tons. Multiply that by the amount of bricks in the circumference of the oven and you are talking an incredible amount of weight.

[Tscarborough]

Re: Hearth insulation

Yes, that is one of the most expensive things about building generic plans as FB has to do. For most people, the foundation, stand, and hearth slab are over engineered by magnitudes of degree. Of course some places need a seismic design and others need a 4 foot frost footing, but if the calculations are done individually for the specific location the specifications (and cost) can drop a lot, like more than 75%. The actual footprint PSI requirement for a normal oven foundation is roughly equivalent to ordinary compacted dirt.

[CvC]

Re: Hearth insulation

Perlcrete can range up to 800 psi depending upon the mix

That?s a fact, maybe.
But this doesn?t solve the problem.Vermicrete and Ytong are good enough at "normal" temperatures, no doubt about it. Complete houses are built using Ytong/Hebel, the problem is, is it still good enough to carry the oven dome after some fires?

By the way, in the FB plans, vermicrete is mixed with ordinary portlandcement, which is (also) not meant for high temperatures.

In fact, this mixture is proven to work, but what does this say abut the ability of Ytong?
Are the temperatures at this point (under the hearth and dome) not that high, or can portlandcement take more temperature than thought?

[david s]

Re: Hearth insulation

I think you are correct in your assumption that there would be little difference in refractory properties of the two materials as they both use portland cement.
From a practical standpoint I have used Hebel (manufactured by CSR) as floor insulation in a few ovens, although short of demolishing them can't tell how the stuff is standing up. I also used Hebel (75mm thick reinforced with 5mm steel cast into the centre, they call this stuff "Power Panel") as both the supporting slab and insulating layer under the floor of my mobile oven. I did it this way in an effort to reduce weight.Because this is visible from underneath where it sits in a steel cradle supported by 4mm flat bar with spacings of 1 ft. It is evident that there are some large cracks.I do not know if the cracks were caused by the Hebel being weakened by the heat or whether it was going over a speed bump at speed after I forgot that I was towing an oven (oops, no shocks)
End result though is that I don't believe the stuff is as strong or heat resistant as they claim. Vermicrete for me and it's cheaper.

[cobblerdave]

Re: Hearth insulation

Gudday
I've used "hebel' as hearth insulation in my oven. Mainly it was cheap ( I as given 1/2 of it) and was a lot more avaliable than pearlite or FB board at the time. Its been about 18 months now with the hearth still being straight and true and yes I have put a straight edge across it.
I have played with the hebel before and made ugly "stone" garden carvings carved wall panels and the like with a friend. You can saw it, rasp it, use power tools dremels etc but the one thing you have to be carefull with is if you take to it with a hammer and chisel. The wrong angle a bit to big a hit and you can crack it so easy. You have to watch it when transporting it and wrap it up carefully otherwise it will break in the boot of you car even on a small trip. No wonder it cracked on your trailer model Davids.
I can't see however that FB board or pearlite would put up with oven bouncing on them as there pretty soft and you can dent them with a finger nail.
I'm reluctant to pull up my hearth to check the condition of the Hebel so I can tell you its worked well for 18 months with no indication that is failling in any way. Ive also used it in an oven door which got cracked from dropping from the first couple of uses but it lasted till a month ago when I replaced it with a secound door also made of hebel.

Regards Dave

[CvC]

Re: Hearth insulation

Thanks Dave,

that was a really encouraging answer.
I?ve heard from a lot of people here in Germany that they?ve been using Hebel/Ytong for the insulation of the hearth, but I couldn?t get any information if it remained stable over a longer time.
What I was concerned about was the combined thermic/mechanical stress.

But in fact, the mechanical stress seems to be at least a dozen times smaller than Ytong/Hebel can take and I?ve found some measurements here on the forum that the temperature under the hearth is by far not as high as inside the oven, so I will give Ytong/Hebel a go.

[CvC]

Re: Hearth insulation

Hi there,

I found another variant of insulating material on the www, this is sold in Germany under the brand name "Poroton", I don?t know the brand name in the US, but this is often used here for insulating the hearth.

As it is made of clay, just like ordinary bricks, it should be able to cope with the temperature.

The insulating properties are pretty much the same as Hebel/Ytong (which are excellent) and it is cheap.

So it is proven to work, it is cheap and it is sold in every DIY-store here.
My stand is completed by now, tomorrow I will start building the hearth, I think will use Poroton.