Another insulation question !

[Sherlock]

Had another lateral thought.........
I have a cut down celotex board ( 1.2 x 1.3 x 50mm) left over from the house rebuild. How about using it as a base for the oven instead of CF board ?? Know the strength should be ok - rated to 1 tonne per m2 I think !
More research methinks !
Any thoughts appreciated as usual !!
Simon ;D

[Calaf]

I don't know what it is, but would suggest pointing a blowtorch at an offcut and see what happens.

[Sherlock]

Mar 7, 2012 18:25:42 GMT Calaf said:

I don't know what it is, but would suggest pointing a blowtorch at an offcut and see what happens.

does the external base of an oven really get that hot ??
think celotex
www.celotex.co.uk/Applications/Floor-Insulation/Underfloor-heating-insulation
will only take around 100 deg !


seem to have gained a star for some reason !!! ;D

[Calaf]

Insulation would be used to prevent heat leaving the thermal mass of the hearth. So if the hearth reaches 400'C then the face of the insulation will "see" 400'C (given sufficient time for the heat to soak through.).

Ceramic Fibre is rated to 1250'C, CalSil board to 1000'C. (Similarly priced stuff.)

I've heard of folks using ceramic fibre board insulation under a celcon/thermalite base but, in my opinion, from a physics point of view, that makes little sense.

[benjibong]

Mar 7, 2012 19:01:01 GMT Calaf said:

I've heard of folks using ceramic fibre board insulation under a celcon/thermalite base but, in my opinion, from a physics point of view, that makes little sense.

Calaf, why would it not make sense? It ould be the same amount of insulating material either way around. Maybe the advantage of putting the celcon blocks on top would be you use something cheaper than ceramic board... I just ask as I don't understand the science stuff.

[Calaf]

Hi Benjibong,

Insulation is doing two things for you:
1- stopping heat getting to the base
2- keeping heat in the oven

All materials are both insulators and conductors, it's just a matter of degree.

- Firebrick is good thermal mass but poor insulator
- Celcon blocks are mediocre insulators and mediocre thermal mass
- CF board is a good insulator and poor thermal mass

If your objective is purely to protect the base from the heat then it doesn't matter where the high performance insulation goes, above or below the blocks.

If your objective is to keep the temperature of the oven interior as higher for longer then the insulation must be placed above the celcon blocks. Otherwise some of the heat will soak into the thermal mass of the celcon. Much of it will go back into the oven eventually but the effect is to lengthen the heat-up-cool-down cycle.

In theory.

Sherlock's problem is that the proposed insulator is not tolerant of high temperatures, so cannot be placed directly under the firebrick hearth. Firebrick is a good conductor so the temperature differential from the top of the firebrick to the bottom will be small.

However, celcon's thermal properties being 'middle-of-the-road' means it will have a larger temperature differential across its larger thickness, so you could get away with a less expensive insulator under it.

[cannyfradock]

Interesting comments

Here's my thought's.... Celotex could be used between the slab and thermal base layer. As a thermal base layer some people use vermiculite or perlite concrete....I would always opt for thermolite blocks for ease....and value.

So I'm with Calaf on your original question. yes as long as it goes beneath the insulation layer.

Terry

[turkey]

I thought thermalite blocks offered pretty good insulation. I will have to link back to the same blog I think I always link to like its gospel or something lol

fornoeconomico.blogspot.com/2007/09/experiment-2-testing-thermalite-blocks.html

the guy heated the block till yellow hot and could hold his finger 4cm from the heat spot, which to me means its pretty efficient, or at least efficient enough, the down side is the brittleness of the heated area, which is why dedicated material would be better.

[benjibong]

Sounds like blocks are a good solution to insulation if carbon fiber board is expensive.

[turkey]

I bet carbon fiber is expensive but luckily cf is our build is actual ceramic fiber boards :-)

still a little expensive but they have 2 advantages, one they are built to take 1000 degrees c, and they are much thinner in application. You can get these cheaper tho, they are brittle and I notice Liam at kilnlinings often has seconds which for our use would be perfect so if you are planning to order in fire clay and or bricks in bulk then perhaps give him a call and see what he has, these are usually online also. I would expect other stockists to have similar issues and discounts.

I was not ordering bricks so in the end so postage and laziness got the better of me and I just got blocks locally. I am already in the dog house over the size of my oven so what's a few more inches...

Good things do come in small packages apparently. Or so my wife says , sly hint about ring boxes usually, but aparently when it comes to diamonds bigger is better, wish she would make her mind up ;-)

[Calaf]

Turkey, the problem with that blowtorch experiment is he's not using a control to compare it to. You could just as easily conclude that the block is a poor insulator (ie good conductor) as it is wicking the heat away from his finger. What you effectively have there is a huge 10 litre heatsink.

Thinking of it another way, if blocks are good insulators why bother with cavity wall insulation? Or cavities for that matter?

Thermal Conductivity of some building materials

Vacuum -- 0
Aerogel -- 0.003
Perlite -- 0.031
Polystyrene -- 0.03
Rockwool -- 0.04
Softwood -- 0.12
Celcon blocks -- 0.15
Facing bricks -- 0.6
Concrete -- 0.8
Firebrick -- 1.4

But for under an oven it's great value.

[cannyfradock]

"Thinking of it another way, if blocks are good insulators why bother with cavity wall insulation? Or cavities for that matter?"

A cavity creates an air circulation gap. In the 1900's it was 2"....later it was increased to 3"....nowadays it's 4" with a 2" celotex insulation bat cladded to the internal thermolite insulation blocks, leaving a 2" air gap. .....this info and thermal values and newton strength of this and that won't help the members of this forum with their builds.......so I'll concentrate on giving practicable advice....

Terry

[turkey]

do you have the values for vermiculite to see how it compares to pearlite?

can easily see why thermal blankets and pearlite are used form dome insulation.

found an old post where I had found u values for some refractory insulators

ceramic fibre board Thermal conductivity 0.08 ~ 0.18 W/mK
insulation brick 37GD Thermal conductivity 0.2 ~ .032 W/nO



always wondered why they left a gap and if there was a reason why do people fill it? Does filling it ruin its original purpose? Seem to learn as much about random things as ovens here, great stuff.

[Calaf]

Turkey, Vermiculite is 0.06. Half as good a resistor as Perlite. Which is why I've wondered why Vermiculite is the material of choice in oven enclosures. Is it just "cos it's what we've always done" or is there a thermal reason? Have you come across anything?

Air is a good insulator at 0.025. But the reason for filling cavities with Perlite, Rockwool or wotnot is to prevent air circulation, ie Convection of heat across the gap. Air is a superb convector.

Sharing information is always a good thing, imo, as it helps us understand what will work best in our particular application. What happens to all that energy you release burning wood and how to minimise waste.

Disappointing value for those insulation bricks. Better off using celcon blocks then.

[veauifru]

Thank you Turkey and Calef for that information. The thermal values actually DO help a lot when choosing what to use and the list of materials with values creates a clear picture of what is good and what is not. More information please !!!! ;D

Tom