6# vs. 8# insulation

[kebwi]

Does it really matter if you use 6# or 8# insulation board for a given thickness? My shopping often turns up 6#. I realize that mathematically the 8# is more insulative, but does that pan out -- would one actually notice a genuine difference in heat-up times? Should we restrict ourselves to 8# for the most part?

Is it actually mathematically linear? Are three inches of 8# exactly as insulative as four layers of 6# or does it not work that way?

[EDIT: When I started this thread, I myself was fairly new to high-temperature insulation and accidentally combined terminology used to describe the density of insulating blankets (#6 and #8) with references to insulating boards. The mistake does not obviate the question, but may lead to further confusion by other newbies. Good luck.]

[MK1]

Re: 6# vs. 8# insulation

I can't tell you the exact r-values but the #s refer to lbs per cubic foot, density. The lighter stuff (6#) is probably higher in r-value than the 8 as it contains more dead-space.

Mark

[kebwi]

Re: 6# vs. 8# insulation

I realize that 6# means six lbs / cuFt, but I don't think you're right about the intuition that lower density is more insulative. I had the exact same thought you had: more air, better insulation, but everything I have read and otherwise been told tells me otherwise, that 8# is a better insulator than 6#. I have no idea why that's the case, but I think that's what I've been told.

I remain confused on the issue.

[david s]

Re: 6# vs. 8# insulation

Are they both the same thickness ?

[kebwi]

Re: 6# vs. 8# insulation

Thickness has nothing to do with density. You can purchase blankets in 4#, 6#, and 8#, relatively independent of a variety of thicknesses ranging from 1/4" to 2". For a given thickness, I would imagine that one of the various densities is the best, but I'm not sure which. The higher density blankets are more expensive, but have higher thermal conductivies...which I think means they let more heat through...so that means they are a worse option while at the same time costing more. It doesn't make any sense to me. I wish someone would clarify the matter for us.

[david s]

Re: 6# vs. 8# insulation

Thickness has everything to do with density because it is the volume part of the equation. The thicker the insulation of a given insulating material will increase the degree of insulation. But I agree with you that the denser a material is then the better it conducts and worse it insulates. Maybe the lighter weight blanket would be more likely to compress under weight and therefore reduce its thickness and consequently its insulation value.

[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.

[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

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

[JoeyVelderrain]

Re: 6# vs. 8# insulation

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

Pinging James....

[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

[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

[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

[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.

[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.