6# vs. 8# insulation

[lwood]

Re: 6# vs. 8# insulation

Just as a side note, R is the reciprocal of K. That is R = 1/K, and K = 1/R. I work with products that require thermal insulation protection and the useful value we use is K.

I agree, I can think about it easier in terms of K, thermal conductivity. Thermal conductivity is how well the material conducts heat. The smaller the K the better the insulator.

By the way, here is what wikp has to say about it:
* thermal conductance = k/A, measured in W?K−1?m−2;
* thermal resistance (R value) = t/k, measured in K?m??W−1;
* thermal transmittance (U value) = 1/(Σ(t/k)) + convection + radiation, measured in W?K−1?m−2.

[vintagemx0]

Re: 6# vs. 8# insulation

Just as a side note, R is the reciprocal of K. That is R = 1/K, and K = 1/R. I work with products that require thermal insulation protection and the useful value we use is K.

[lwood]

Re: 6# vs. 8# insulation

That's great advice Mark, insulating to the nth degree is not the only design parameter to be considered. Since i haven't even reached this step in my build, i have become an expert on the subject. Ha ha. But Mark makes an excellent point that maybe you want to give up a little insulating R value for the thermal mass that you gain especially in a confined space. What i have found out in my quest for alternatives to the rock wool you all talk about.......is that perlcrete is an excellent insulator. As a matter of fact, even unexpanded perlcrete is an excellent insulator. Out of necessity, I will be doing a solid unexpaned perlcrete shell, probably 5-6 inches thick, because the rock wool is just not available here in the provincial Philippines. The difference in the thermal conductivity (K) of the the expanded vs unexpanded perlite is very small and orders of magnitude smaller than most other solid rock materials because of it's honeycomb-like physical structure.

I think you all should be using K rather than R-value. Have you ever seen units on an R-value? Never, I don't have my CRC sitting next to me but K definitely has units.

Since you are compressing the the rock wool, it seems to be approaching the the pure thermal conductivity K of the ceramics in the wool and the perlcrete. The wool just provides an airspace. So, you are exchanging thermal mass for insulating value. Depending on how you want to use your oven and how important wood conservation is to you should determine how much insulation to use. If you really are concerned about wood consumption you should make the biggest airspace you can with the rock wool. Actually here, firewood is not a problem, typhoons provide more firewood than I could use in a lifetime. And roasting a whole pig is something I want to do in my oven. So, my thinking is that my 5-6 inches of perlcrete should be ok. We shall see when I fire it up.

[ThisOldGarageNJ]

Re: 6# vs. 8# insulation

My thoughts, The amount of insulation is based on final plans for your oven,,
If your only cooking pizza, minimal insulation is enough...
If you want to roast a pig,,,,You want more heat retention and holding temps and mass.
If you want to bake bread,,,, You want a lot of thermal mass
I can only add that I thought I only wanted to cook pizza, then I found out I could cook bread, chicken, steak, etc.... My point is dont cut yourself short now,,, Your not talking about theat much more expense and in the long run you will be able to leave a pork loin in the oven slow roasting overnight after you finish cooking your pizza's

Mark

[kebwi]

Re: 6# vs. 8# insulation

I'm a newbie here, so my advice is worth only the degree of pompousness I convey through my writing. However, the wisdom I am gathering from the FB community is that the "right" amount of insulation is the amount you are willing to pay for, have physical space for within your hearth bounds, and have the patience to go to the trouble of installing.

I do believe 2 to 4 inches of 8# blanket is considered acceptable by the FB community. I'm sure someone else will chime in.

[philiph4@ameritech.net]

Re: 6# vs. 8# insulation

I am planning an igloo style oven using 8# Kaowool blanket only with Chicken wire and stucco on top. How many inches of insulation do I need? The pompei oven ebook says 2 to 3 inches. Is this enough?

[jpar]

Re: 6# vs. 8# insulation

8# blanket insulates better then 6#, 6# insulates better then 4#. This holds true until you get to about 12 pcf, then insulation value starts to go down.
This being said, the temperature differences that you will actually see in an oven using 6# blanket instead of 8# blanket will be very small. Use whichever blanket is easier to obtain.

[kebwi]

Re: 6# vs. 8# insulation

Agreed. 6# (and 4# even more so) is both cheaper and more insulating at the same time, so it really is the better option *unless* it will compress under a few inches of perlcrete.

Although I am doing an igloo, I am considering the possibility of a design which will take all the weight off the dome by building a rebar/chicken-wire mesh dome several inches away from the brick dome, stuccoing it, and filling the cavity with loose perlite. I haven't decided yet, but if I go that route then I won't have any more weight pressing on the blanket than one would have in a conventional "house" enclosure.

We may not get a definitive answer on this issue in the forum however. The question of whether 6# blanket compresses under vermcrete/perlcrete seems like a fairly esoteric thing to notice or keep track of. It's possibly no one really knows.

[RTflorida]

Re: 6# vs. 8# insulation

John's comments sound logical. To me, it sounds as if the #8 resists compression the most and would be best suited for an igloo style. If you are building an enclosure, save a little money and go with the #6.
I used the #8 Insulfrax, covered with wire mesh then perlcrete and Type N mortar. I remember the blanket seeming pretty dense. When I first read the plans, I thought anything being refered to as a "blanket" would compress and lose much of its insulating value - not the case. Once you get to that step you will see that it actually takes quite a bit to actually compress the #8. The perlcrete just seems to stick there and does not seem to have much effect on the blanket.
Any users of the #6 who have built an igloo need to chime in with their opinion.

RT

[JAG]

Re: 6# vs. 8# insulation

All,

When I was shopping for my blanket I was talking to my refractory supplier and I asked the question of "differendce between #4, #6, and #8. He stated the the heavier weight blanket would be used in areas where insulation spacing is very tight and the #4 and #6 would be used where the spacing is not so tight, and thaat the insulating difference between the 4,6, and 8 are minimal.
I have a tech data sheet and MSDS that he gave me with my purchase that explains the insulating properties of each weight. If anybody would be interested in a copy I could send it out. Also, if there is a place on the forum that it could be posted that might be handy also.

Another thought. Is there a place on the forum where technical data is located? If there isn't, maybe there can be a location where members can submit any tech data that they have on materials that they used or researched.

John

[ThisOldGarageNJ]

Re: 6# vs. 8# insulation

the only advice i can offer, I used th 8#, but I bought the water soluble,at that time they didnt have it in 6# which means, IF you happen to inhale any it will dissolve in your throat and lungs,and not cause permanent damage.... I knew exposure would be minimal, but figured every little bit helps

Mark

[JoeyVelderrain]

Re: 6# vs. 8# insulation

Found this thread just in time, I am about to purchase some insulating blankets...

Pinging James....

[david s]

Re: 6# vs. 8# insulation

Ask James. He's the one who's presumably recommended #8. My guess is that it resists compression better.

[kebwi]

Re: 6# vs. 8# insulation

...which is odd since FB, which is targeted specifically at oven makers, sells 8# blanket instead of 4# or 6#. By the logic presented in this thread, 8# is the most expensive yet worst option of the three common densities available.

I still get the impression that most of the responses in this thread are speculative as opposed to truly knowledgable. I agree that less dense materials *seem* better since they have more air and I agree on the intuitive interpretation of the thermal conductivity specs, which are lower for 6# than 8#, but my original question remains unanswered:

Is there a *qualitative* difference or for our purposes are they functionally equivalent? Does anyone's oven *behave* differently for having been built with one or the other? That's the question I'm trying to get an answer to in this thread.

...I am also confused why FB sells 8# for the reasons stated in this post.

[david s]

Re: 6# vs. 8# insulation

The technical data does not mention the insulation values, only thermal conductivity, which is like the opposite of R values. The stuff is extremely fireproof with a rating about double the temps we fire to, so presumably the denser material would suit a higher temp application. I use vermiculite for insulation and have not had experience with blanket, but it sounds like the low density blanket would be better.