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Moisture Problem - Forno Bravo Forum: The Wood-Fired Oven Community


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Moisture Problem

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  • Moisture Problem

    I built a Pompeii Oven in 2009 with a stucco finish over vermiculite insulation. I've been having a chronic moisture problem, particularly when firing it up after winter. The heat causes water to drip from the base. It takes several firings to dry it out. Even then it doesn't retain heat well.

    The oven was well-insulated when built using Forno Bravo insulation covered with vermiculite concrete. Water effloresces over the winter, leaving heavy white deposits. The stucco has a number of very narrow cracks over the surface. I repainted the stucco with Zinssner's oil-based Watertite paint, but that was ineffective. The cracks have reappeared, and moisture remains a problem.
    I've just found an effective chimney cap. Until now I didn't have one, at least an effective one. I'm considering have a cover made by a guy who makes boat covers, but would rather avoid the expense and the hassle of putting it on and off.

  • #2
    Re: Moisture Problem


    I used a product by Dryvit. I have had no problems with water. It's easy to apply and comes in any color you want.
    Check out my pictures here:

    If at first you don't succeed... Skydiving isn't for you.


    • #3
      Re: Moisture Problem

      It certainly sounds like you have water getting in. Cracks on the dome are entry points as is water down the chimney. Sometimes cracks develop around the base of the dome where the outer shell meets the base. Rain can also enter through the entry as well as wicking up the stand, particularly if there is no moisture barrier between the stand and the supporting slab. Some oven owners find a tarp is a reasonable solution, but it also holds moisture in. A permanent roof over the oven is a better solution but requires effort and expense. We live in the tropics and experience very heavy rain during the wet season and the oven gets quite wet every year. We just give the oven around three really long and slow burns and it's normal function is restored.it does not need the really slow and tiny fires that you require for the initial curing. A dog house is probably a better solution than an igloo in keeping out rain.
      Last edited by david s; 07-11-2013, 09:59 AM.
      Kindled with zeal and fired with passion.


      • #4
        Re: Moisture Problem

        Thanks for the insights. It hadn't occurred to me that a tarp would serve to keep moisture in as well as out. I'll fire it several more times, then see how much my new chimney cap helps, perhaps add another coat of paint.



        • #5
          Re: Moisture Problem

          Maybe the water is coming in at the base, where the dome meets the slab? Where is the moisture inside the oven? do you see efflorescence in the dome?
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          • #6
            Re: Moisture Problem

            The base is now well sealed from rain. The oven seems damp generally, sometimes if I've neglected to clean out ashes, they are pretty wet. I don't see efflorescence anywhere but running down the legs, mostly just on one. If I can figure out how to post the photos I have, I'll show you.

            The more I reflect on the problem, the more likely it seems that rain is simply coming down the chimney, entering the oven and then seeping between the dry-laid bricks that make up the floor. Those bricks extend beyond the front of the oven, so could soak up all the rain that fell on the. I've now tiled over them so they should now be waterproof.

            The oven seems to be drying pretty well on its second firing for the season. For the moment, I'm inverting a plastic bucket over the chimney, so that won't be a pathway any longer. I'll probably wait a year before doing anything major.




            • #7
              Re: Moisture Problem

              Hi Ely,

              I am also in Ohio, Youngstown, and have noticed the same mositure problem. I followed the forno bravo instructions for construction and also have decorative stone over the entire oven. I see no way moisture getting in other than through the chimney and possiblly the front from rain or moisture in the air.

              I also have noticed that if I don't clean out the dry ash after a burn that the ash is like mud the next day. I'm thinkng this may just be from humidity in the air. In fact I just discovered today that if I leave the door to the oven tightly closed, mold will begin to develop. I'm not sure how to prevent this. Here are my questions.

              1. Is it better to leave the opening to the oven open, partialy open, or closed when not in use?
              2. Would it be a good idea to keep a natural drying agent like rice in the oven when not in use?

              I'm going to do a burn to clean it out, but not sure what else to do to prevent this or if this is normal.

              Hope to hear from anyone on this topic.



              • #8
                Re: Moisture Problem

                If you are getting wet ash inside your oven, I doubt the cause is humidity. A dome covered with masonry or decorative stone is not going to be watertight as anything with grout or mortar is going to wick water through to the other side. Unless you put a waterproof membrane underneath your stone layer - and a way for water to escape once it penetrates the masonry, it's likely the water is coming right through your igloo and getting into your insulation/dome. There is a reason that standard practice for stucco calls for a membrane layer.

                I've now tiled over them so they should now be waterproof
                Tile is not waterproof either.
                My build progress
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                My dome spreadsheet calculator