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Perlite vs. Blow in Insulation?? - Forno Bravo Forum: The Wood-Fired Oven Community

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Perlite vs. Blow in Insulation??

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  • Perlite vs. Blow in Insulation??

    So here is my situation and my question. My dome is built, I have 4" of FB blanket insulation wrapped around the dome and the walls around the oven are almost complete. I am building a gable roof structure over my oven so I am needing to fill the open cavity with something??

    Everything I have read says to use Perlite, but why couldn't I use standard blow in insulation? The insulation is much cheaper but I am worried about the flammability of that material. The package says fire resistant but doesn't give any sort of temperature range.

    If anyone has thought related to this dilemma they would be greatly appreciated.

  • #2
    Re: Perlite vs. Blow in Insulation??

    With 4" of CF blanket, I don't think you need any additional insulation in the cavity. It's not going to add any R-value, it's just overkill.

    Check out the data sheet on CF blanket - with just two inches of insulation, a 1200 degree hot face drops to a 149 degree cool face. You then have a 149 degree hot face on your second 2" layer that will mean the cool face is basically the ambient temperature. Your money, but you can't really lose heat that is not there to lose.

    http://www.fiberfrax.com/files/Fiber...lanket-Mat.pdf
    Last edited by deejayoh; 03-04-2013, 02:59 PM.
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    • #3
      Re: Perlite vs. Blow in Insulation??

      Originally posted by deejayoh View Post
      With 4" of FB blanket, I don't think you need any additional insulation in the cavity. It's not going to add any R-value, it's just overkill
      I disagree, the Perlite tends to fill in small voids in the blanket that are almost impossible to avoid, The blow in insulation that is fire resistant is most likely celulose Think shreded newspaper, that has been treated with a flame retarding chemical. Blow in fiberglas if that is what you are considering has chemical binders that will stink when hot and that is not even considering what it could be doing to your food.

      You are spending a great deal of time, energy and money to build your oven, why not spend another 60-100 bucks to make it right. Here that money would buy 4-6 4Cu Ft bags of perlite.

      I put 4 bags over my 3 inches of blanket and it holds heat for 4-5 days in winter and 6-7 days in summer,

      I put sheet metal to round off my corners to reduce the amount needed there as the perlite would have been over a ft thick at the 3 corners if I had not done so and used up another 1-2 bags of perlite.

      My two cents,

      Chip
      Last edited by mrchipster; 03-04-2013, 03:07 PM.
      Chip

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      • #4
        Re: Perlite vs. Blow in Insulation??

        The void argument seems a spurious one. Assuming no gross ineptitude in installation, there will be little chance of voids that go clear through all the insulation layers. At most, there might be a void between one layer and the next - but the only voids that could be filled with perlite are between the outer two layers. If a person leaves voids when installing between 2 and 4 layers of CF blanket that can somehow be penetrated with Perlite, then that person should leave installing CF blanket to someone else.

        The argument about heat affecting a secondary insulation layer doesn't ring true to me either. If your CF insulation is hot enough outside of 4" to make any material hot enough to stink or burn, then you probably got something other than CF insulation. The outer layer will be ambient temperature.

        My view, sometimes more is better - but sometimes more is just more.

        Would you fill your entire attic with insulation?
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        • #5
          Re: Perlite vs. Blow in Insulation??

          One item that I should clarify. My dome is a Casa 90 and not the true Pompeii brick oven. Not exactly sure if that changes any of these responses. I wouldn't think it should matter but I wanted to clarify.

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          • #6
            Re: Perlite vs. Blow in Insulation??

            Originally posted by deejayoh View Post

            My view, sometimes more is better - but sometimes more is just more.

            Would you fill your entire attic with insulation?
            I agree that the perlite would probably only fill a void in a seam at the first layer but it would also fill in any gaps that appeared at the bottom of the dome and it is still a very inexpensive solution.

            If there was no heat loss based on what I have done then I should see exactly the same performance winter or summer and I clearly do not....

            On this site it has been recommended to not even have wood as part of your wall or ceiling structure and those items would in most if not all cases be well outside the 4 inches you propose. Here in Minnesota there are two types of blow in insulation as I indicated in my prrevious post. cellulose is made from recycled newsprint and corregated box recycles. I think it would be a serious safety issue if there was an oven crack that allowed hot gasses to escape into the ceramic at the same place that there was a small void in the coverage.

            I am using a double wall stainless Flue and the documentation that came with the sections requires that it be 12 inches from combustables. There is no allowance for non flamable insulation as a gap. I am sure when saturated with heat the outsides of my bricks next to the insulation are at least 700F. I have never recorded a temp over 400 on the outside of my flue yet it requires the 12 inch gap.

            I also have 24 inches of insulation in my attic and my neighbors; one house to the south of me with the exact same style house with the extact same color shingles have 12 inches of insulation have no snow on their roof and ice dams in the gutters. I still have 8 inches of snow on my roof and no problems with ice dams.

            Question. How long does your oven stay above 200 when it is averaging 10F for the week? Daily high 20F nightly low -5F

            Chip
            Last edited by mrchipster; 03-04-2013, 06:38 PM.
            Chip

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            • #7
              Re: Perlite vs. Blow in Insulation??

              Originally posted by deejayoh View Post

              My view, sometimes more is better - but sometimes more is just more.

              Would you fill your entire attic with insulation?
              If that were true I should see exactly the same performance winter or summer and I clearly do not....

              On this site it has been recommended to not even have wood as part of your wall or ceiling structure and those items would in most if not all cases be well outside the 4 inches you propose. Here in Minnesota there are two types of blow in insulation as I indicated in my prrevious post. cellulose is made from recycled newsprint and corregated box recycles. I think it would be a serious safety issue if there was an oven crack that allowed hot gasses to escape into the ceramic at the same place that there was a small void in the coverage.

              I am using a double wall stainless Flue and the documentation that came with the sections requires that it be 12 inches from combustables. There is no allowance for non flamable insulation as a gap. I am sure when saturated with heat the outsides of my bricks next to the insulation are at least 700F. I have never recorded a temp over 400 on the outside of my flue yet it requires the 12 inch gap.

              I also have 24 inches of insulation in my attic and my neighbors; one house to the south of me with the exact same style house with the extact same color shingles have 12 inches of insulation have no snow on their roof and ice dams in the gutters. I still have 8 inches of snow on my roof and no problems with ice dams.

              Question. How long does your oven stay above 200 when it is averaging 10F for the week? Daily high 20F nightly low -5F

              Chip
              Chip

              Comment


              • #8
                Re: Perlite vs. Blow in Insulation??

                Originally posted by mrchipster View Post
                If there was no heat loss based on what I have done then I should see exactly the same performance winter or summer and I clearly do not....
                I would think that in extreme temps, the door and landing probably account for most of the difference you see in heat loss. There is plenty of exposed brick at the landing, and the doors leak tons of heat even with insulation. The surface of my door with two inches of insulation sandwiched in it is too hot to touch the morning after I fire. But the 4" of insulation on my dome was never warm to the touch even at full pizza temps. Where is the heat loss? (edit - btw, I just checked my door two days after firing. It's ~95 degrees on the surface while inside the oven is ~260. The temperature of my housing is mid 40's, same as outside. and we don't get 10F temps here )


                IMHO, there's a lot of recommendations for things that I think are overkill. But that's just my opinion, I know others differ. There are people on the forum who dump insulation into their oven houses but don't bother with heat breaks for their vent. To each his own. By the time I got to the "insulation in the housing question", I was tired of spending money and really couldn't see the benefit. If my oven is going to be 150 instead of 165 on the fourth day after firing, so be it. I doubt the difference is much more than that. My druthers would be to take the $100 and invest instead in a good door. It'll serve you better in the long run.
                Last edited by deejayoh; 03-04-2013, 07:19 PM.
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                • #9
                  Re: Perlite vs. Blow in Insulation??

                  Originally posted by deejayoh View Post
                  I would think that in extreme temps, the door and landing probably account for most of the difference you see in heat loss. There is plenty of exposed brick at the landing, and the doors leak tons of heat even with insulation. The surface of my door with two inches of insulation sandwiched in it is too hot to touch the morning after I fire. But the 4" of insulation on my dome was never warm to the touch even at full pizza temps. Where is the heat loss? (edit - btw, I just checked my door two days after firing. It's ~95 degrees on the surface while inside the oven is ~260. The temperature of my housing is mid 40's, same as outside. and we don't get 10F temps here )


                  IMHO, there's a lot of recommendations for things that I think are overkill. But that's just my opinion, I know others differ. There are people on the forum who dump insulation into their oven houses but don't bother with heat breaks for their vent. To each his own. By the time I got to the "insulation in the housing question", I was tired of spending money and really couldn't see the benefit. If my oven is going to be 150 instead of 165 on the fourth day after firing, so be it. I doubt the difference is much more than that. My druthers would be to take the $100 and invest instead in a good door. It'll serve you better in the long run.
                  Yes I do have heat breaks at both the inner and decorative arches, and 4" ceramic FB board inside of my door,

                  The landing is joined at the oven floor with insulated brick and I do have 2 inches of FB board under the oven in addition to 2.5 inches of 7/1 Vermicrete under that.

                  An 11/1 perlcrete buttress surrounds the vent arch so I have taken great care to save heat and beieve the $60 in extra perlite was a good investment. To each their to their own designs and tolerance for cost, asthetics and design.

                  Sometimes your environment can dictate the need for more in certain areas and less in others. For example it would have been difficult for me to justify a dome finish in my area due to harsh weather here even though I like the look of that style better.

                  I still wish I had more insulation under my floor.

                  Chip
                  Chip

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                  • #10
                    Re: Perlite vs. Blow in Insulation??

                    Originally posted by tferin View Post
                    One item that I should clarify. My dome is a Casa 90 and not the true Pompeii brick oven. Not exactly sure if that changes any of these responses. I wouldn't think it should matter but I wanted to clarify.
                    The type of refractory Brick or cast should make no difference in the type of insulation other than the fact that a cast oven is thinner and would reach higher temps on the outside of the refractory quicker due to a thiner cross section.

                    Chip
                    Chip

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