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extra dome insulation

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  • extra dome insulation

    I currently have between 2-4 inches of inswool on my dome - likely sufficient.

    i am building a frame around it, and figure i have all of that empty space i could fill with further insulation, just to keep it even tighter. the standard approach would be to fill the remaining space with vermiculite, but i estimated i would need around 12-15 bags of it, which would be a bit pricey.

    my question is can anyone think of an issue with just filling the remaining space with standard house insulation? it should not see any heat, as the inswool will stop that, and i believe it has some fire retardant to it.

    it would cost ~$15 to fill the gaps with home insulation, versus ~$150 for vermiculite...


  • #2
    Re: extra dome insulation

    I'm going to give a bizarre hypothetical "What if.." When I was a yound man I lived in Alaska, in the interior just outside of Fairbanks. The winters there are cold and long and something I saw happen during the Spring thaw really amazed me. The walls of homes that were not properly fitted with a vapor barrier literally ran water out the bottom as the temperatures rose. What was happening was water vapor from respiration as well as cooking and some from heating (although most of that was vented out thru the chimney) was moving out thru the insulation until it reached a point in the insulation where it condensed and so lowered the insulation value and it froze. Once it froze the next bit of insulation closer in was the point where the water vapor condensed and so on and on all winter. The exterior walls slowly becoming large sheets of ice over the winter. I know if someone told me this I'd say "Pull the other leg awhile, I'm beginning to walk funny" but it's true.

    So here's the what if: When you fire your oven moisture in the form of water vapor is one of the products of the combustion. The bricks are porous and some of the water vapor certainly moves out thru them. Suppose you have enough insulation that that water vapor reaches insulation that is at the temperature at which it condenses. Not saying it will freeze (temps aren't commonly 50+ below in the lower 48). However, the result might be damp eventually soggy insulation from the inside out. So there might be an argument for leaving some room for airflow around the insulation.

    There's also one more thing to consider about using fiberglass batts as insulation. The filaments are actually very very tiny tubes. That's why when fiberglass gets wet it is impossible to dry out. A portion of the insulating value is due to those tiny tubes of air. Capillary action causes them to suck up any water they come in contact with. That's why one has to throw away wet insulation not "dry" it out, like after a flood. If there was any water vapor that condensed in the batts it would be sucked up and any benefit from the batts would be reduced.

    Perhaps better to spend that $15 on a nice bottle of wine ;-)

    Wiley :-)


    • #3
      Re: extra dome insulation

      If it is not going to see "any" heat, then why insulate at all? Assuming it will see some heat, albeit low temperatures, you could cut down the amount of vermiculite you need by blocking out the corners and such with pieces of masonite or plywood or whatever, to keep the vermiculite just around the oven.

      Or, I would think you could put some kind of vapor barrier in place and then just pack it will fiberglass batts. Or, and I have never heard anybody mention this here so there is probably a reason not to, you could fill it with attic insulation of some type, like gypsum (rock wool) if they still use that, or one of the newer ones. Best had be non-flamable and non-toxic though, since you could concievably crack and oven and have a problem.

      Just speculating ,


      • #4
        Re: extra dome insulation

        I think some people have insulated with rockwool. I think it would be a better choice than regular fiberglass insulation.

        Really, though, what I would do is figure out a way to dam up some of the extra space in the enclosure, so you don't use more vermiculite than you need, and just do loose vermiculite. Ken did this in his enclosure, I believe. You could use hardibacker or screenwire to make a "cage" for it, give yourself 6 inches or so all around, and fill to whatever depth you're looking for.

        Even with two layers of blanket, you'll want extra insulation if you want to retain heat for cooking later in the day or the day after firing.



        • #5
          Re: extra dome insulation

          That's what I did. I blocked off as much of the void as I could inside my enclosure, but it still took 12 bags or so to fill it up. You only have to do it once though, might as well do it right. Sure is nice to have a 1,000 degree fire roaring inside and not a single degree of heat getting through the roof.
          Check out my oven progress here: http://www.fornobravo.com/forum/phot...dex.php?u=4147

          See ALL of my pictures here:


          • #6
            Re: extra dome insulation

            Originally posted by Breven View Post
            That's what I did. I blocked off as much of the void as I could inside my enclosure, but it still took 12 bags or so to fill it up. You only have to do it once though, might as well do it right. Sure is nice to have a 1,000 degree fire roaring inside and not a single degree of heat getting through the roof.
            I agree, fill er up!
            My thread:
            My costs:
            My pics: