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Foil in the wall interior

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  • Foil in the wall interior

    I had an interesting conversation with our dealer/installer in Oregon, who is building his first Forno Bravo oven, about insulation and different types of heat transfer. I am interesting in hearing everyone's feedback on this. Alf -- let me know what you think.

    The basic theory is that conductive and radiant heat work differently, and that while Insulfrax and Vermiculite do a good job of stopping conductive heat, there is still some radiant heat energy that makes it through the 5" insulating schedule. By adding a layer of foil on the inside of the enclosure walls, around the insulation, you can gain a little insulating efficiency. It's basically cheap and easy to do, and will add some insulating value.

    Brad's theory (I hope I am getting this right) is that radiant heat is able to work its way through the insulation and would be reflect back into the insulation, helping keep the oven chamber warm. The impact wouldn't be huge, but worth it.

    The thermal computer model showed that at equilibrium the the outer face of the vermiculite won't go higher than roughly 170F, using a 1000F oven face as the test surface.

    What does everyone think?
    Pizza Ovens
    Outdoor Fireplaces

  • #2
    (M) James asked, above, "What does everyone think?

    (M) I can't speak for everyone but Brad's theory seems to reinforce my guess that foil, regardless of whether inside or outside the insulation, will reflect back some radiant heat. I chose to have as much heat reflected back before it even gets to the insulation and put my foil right on top of the newly mortared dome bricks.

    (M) I had written, in an earlier post, located at

    a title: "Foil has 4 uses: H20 out, Smoke in, Slip, and heat reflect #06"

    (M) My statement there is reproduced next:

    (M) The 4th reason needs confirmation from a thermal engineer as it is only this novice's theory that it will help reflect radiant, and possibly conduction (not convected) heat back to the dome. If this is not correct, the other 3 reasons are sufficient justification for my use."

    (M) So, Brad, I think you and I are on the same track, but perhaps that's because we both live in Oregon, a GREAT state!

    (M) Here follows some images that were displaced from that earlier posting. I recommend that if you use foil, regardless of whether before or after you insulation, that you work like a roofer, from the bottom up. That way the water, if any, will shed onto the foil layer below. This is difficult because of the vertical surface from which the perlcrete will tend to slide off. But you would encounter that problem even if you layer foil from the top down. Use chicken wire and let the perlcrete dry for a day. If you spread the work over 4 days, using rubber or latex gloves, you will be able to enjoy just as messy a result as shown in the next images:



    "Everything should be made as simple as possible, ...
    but no simpler!" (Albert Einstein)


    • #3
      Ciao Marcello,

      I was thinking about adding a disclaimer. "My apologies if you have posted this theory before."

      My question about adding the foil before the insulating scheme is whether the foil would behave thermally, and not serve the purpose of reflecting heat back into the insulation.

      What do you think?
      Pizza Ovens
      Outdoor Fireplaces


      • #4
        Heat within an oven be it conduction, convection or radiant will migrate through the fabric of the oven, the insulation and the outside enclosure so; any barrier (thermal blanket, vermiculite, charcoal, glass, foil etc) will help to contain the energy within the oven where we want it.

        I have used foil in the past to create a slip between different materials so they can move independently of each other, but not as a barrier against radiant heat for the reason that foil also provides a vapour barrier. This may or may not be applicable to pizza ovens, however, hear are my thoughts and reasons.

        The large low arched bread ovens we build for the commercial trade can consume up to 100 kilos of wood per firing and the ovens are often configured for steam injection or are built so that steam from the baking bread is trapped within the oven. Both the burning wood (during firing) and baking bread contain vast amounts of water that is turned into water vapour or steam. As the firebricks that are used to build the arch or roof of the oven are porous they will absorb some of the vapour or steam within the oven, passing it through the firebrick and into the thermal mass / insulation above. If there is a vapour barrier the moisture can?t escape further.

        We have a cool damp climate here in the Uk and a pizza oven built outside in the garden won?t be used much during the winter and will absorb moisture from the air. Again foil would create a vapour barrier, however, the foil could help keep the elements out and some sort of venting could be arranged to vent moisture from the firebrick / insulation.

        Personally I wouldn?t use foil, however, as we all know in the wood-fired oven world there are lots of different ideas, theories and preferences, and most of them work. So I recon its down to personal preferences.

        How?s that for sitting on the fence!



        • #5
          I agree with Alf in the moisture point.

          Moisture in the oven walls will stop the temperature rise.

          By the way, and thinking in Fio?s and Marcel?s ovens, but valid to everyone, I would like to suggest do not cover entirely the isolated dome oven to decorative purposes, lefting a little open space to allow the moisture to go out.

          One or two little 2 - 3 sqin hole/s in the top of the finishing layer will do the work.



          • #6
            Originally posted by arevalo53anos
            By the way, and thinking in Fio?s and Marcel?s ovens, but valid to everyone, I would like to suggest do not cover entirely the isolated dome oven to decorative purposes, lefting a little open space to allow the moisture to go out.
            I'm going to leave a vent in my enclosure to allow steam pressure to escape.
            There is nothing quite so satisfying as drinking a cold beer, while tending a hot fire, in an oven that you built yourself, and making the best pizza that your friends have ever had.


            • #7
              Re: Foil in the wall interior

              I am new to the forum, but as a Mechanical Engineer I may have something to add to this thread.

              I see two problems with foil in the insulation of the igloo shown above. First the foil is not placed in an area where radiation is occurring, as radiation is a free space phenomenon. The heat of the fire is conveyed to the inner oven wall through radiation and convection. From there heat transfer is purely conductive until you reach the outer wall of the igloo. Here convection and radiation take over again. So, by laying the foil in the conductive regime, it has no insulative value as it is an excellent conductor. If you plastered the exterior of the igloo (sort of a space age look) or placed a loose layer of foil over the dome in a house construction before the insulation, you will reduce radiation losses due to the low emissivity of the foil. This can be significant.

              The house construction with the loose layer of foil also addresses the second problem, corrosion. Foil has a massive surface area to volume ratio and aluminum is very reactive. Yes, a layer of oxide forms on aluminum to self protect, but with foil the entire sheet may turn to oxide. On the upside you'll have a few thousands of an inch of an excellent conductive insulator.

              I'm planning on a layer of foil in my upcoming house type oven enclosure. I will post the calculations when I get around to doing them.

              Note: I have designed refrigeration systems in the past where the addition of radiation insulation doubled the insulation efficiency. This is the same, heat transfer wise, just in the reverse direction.



              • #8
                Re: Foil in the wall interior

                Reading this thread I've got to believe that all ovens will have some cracking, visible and microscopic which will let the moisture out over I wonder if you would actually need to leave a hole to let the moisture out.

                Just thinking that the cracks in my oven might be a good thing!
                sigpicTiempo para guzarlos..... ...enjoy every sandwich!


                • #9
                  Re: Foil in the wall interior

                  I'm looking at these old threads on insulation, vapor barrier and radiant heat loss. My questions are:
                  Why is moisture in the oven bad? I understand that moisture in the insulation layers are bad but why the oven itself. Commercial ovens inject steam. This leads me to suppose that a reflective vapor barrier just outside the fire brick would be advantageous. I'm new here and planning my build, and I'm trying to cover all aspects but of course having no experience with WFOs.
                  I do have a lot of experience building different stuff (from airplanes to cold storage buildings) and I know a bit about vapor travel and insulation.
                  Are there any disadvantages to trapping moisture in the oven while in use. I believe I can devise a simple method of doing so.
                  Does anyone know if a layer of portland cement is waterproof? I know many people say it's so but I don't know that experience bears that out. I think the biggest challenge to a successful barrier would be the expansion and contraction of the dome. How much do these things move?
                  I am contemplating a motarless dome at Les's suggestion and vapor travel into the insulation layers might be trouble. I also think I might cover the dome (after insulation) with diagonally seamed copper.



                  • #10
                    Re: Foil in the wall interior

                    I covered my last oven with foil over the top of the vermiculite layer after I'd cured it in the belief that I might get some improvement in insulation. In fact the reverse occured because it has trapped moisture in the oven and it's going to take a lot of firings to get it out. The moisture will always want to travel away from the heat source. That vermiculite layer takes lots of water. I think the best remedy is to take your time in the build (lots of time to let water evaporate) and do more curing fires than you think is necessary.
                    Kindled with zeal and fired with passion.


                    • #11
                      Re: Foil in the wall interior

                      Thanks for the heads up. I agree that foil on the outside traps vapor in the vermiculite layer as your experience has shown. My thoughts are that a layer right on top of the firebrick, underneath the insufax and vermiculite would trap the moisture where it would help.
                      My guess is that alot of these ovens do have some moisture in the vermiculite, (not alot in dry climates). I suspect it cooks out a bit (with the vapor moving out the through the firebrick and entry arch) after the cooking is done leaving the oven in a fairly dry state until the next firing. Your situation is analogous (loosely) to a house that has been insulated with the vapor barrier to the outside rather than the inside.
                      Another observation is that the little building design has an advantage over a stuccoed igloo. More vapor would be trapped in the igloo, whereas the loose vermiculite poured over the masonry dome would transpire the vapor in a more efficient manner. I would love to have the two side by side and test.
                      I'm not sure I understand all of enz's post but from what I know lining the interior of the "house" with foil would be more moisture trapping with it's attendant insulation effectiveness loss that would outweigh any additional trapped radiation. From enz's post I would suppose a vapor barrier just above the masonry would be effective. Keep the insulation dry and all that.
                      With your build it might be advantageous to drill a 2" hole in the very top, just through to the vermiculite layer and install a small chimney with a cap to encourage a better drying out. If necessary maybe some small covered intake vents around the base at a ratio of 4 to 1 intake to exhaust which is considered the optimal ratio for a passive venting system, (not fan forced).

                      Last edited by MK1; 04-06-2009, 05:14 AM.


                      • #12
                        Re: Foil in the wall interior

                        Foil has a massive surface area to volume ratio and aluminum is very reactive. Yes, a layer of oxide forms on aluminum to self protect, but with foil the entire sheet may turn to oxide.
                        The money quote from an earlier post: Thin reactive aluminum + corrosive portland = dust
                        My geodesic oven project: part 1, part 2


                        • #13
                          Re: Foil in the wall interior

                          This is true and that's why I would use copper foil. Quite common as thruwall flashing for masons in the past. It's a little harder to source these days but it's out there. I love your build and have read every post associated with it. Did anything ever come out of the castable attempt besides a small explosion? I thought it looked promising with the exception of cost.

                          Last edited by MK1; 04-06-2009, 09:54 AM.


                          • #14
                            Re: Foil in the wall interior

                            I put the foil between the brick dome and the vermiculite layer to act as a "slip" plane.

                            Why not ? It's cheap enough.


                            • #15
                              Re: Foil in the wall interior

                              I have tried that technique of leaving a hole at the top and then filling it when all moisture has gone, but find it better to eliminate all moisture from the vermiculite layer before final render.
                              Kindled with zeal and fired with passion.