Mongo's 42" CT Build

[Michele Enrico]

Okay, got some uploaded.
Pics of the vent arch and throat.

Aha! This is the detail I have been looking for. Nicely done!
Michele

[mongota]

I did coat the slab under my ceramic fiberboard insulation, bringing the RedGard a few inches outside the footprint of the board. Once the board insulation was in place, I also coated a couple inches of the sides of the 4" thickness of board insulation, lapping that down onto the RedGard already on the slab. I figured that'd minimize water wicking in from the side since I don't have a house over my stone igloo and my hearth slab is exposed to the rain.

It's worked fine for me. I've had no water intrusion since the oven was completed.

The RedGard prevents moisture from coming in through the slab. If it somehow came in from above, the drain holes make it easy for water to get out.

I've not used Foamglas.

[Giovanni Rossi]

Mongo, Thanks for this update. I latched on to your use of RedGard early on in my research. I'm using a layer of Foamglas on the slab then Cal Sil with a smaller diameter on top of that. I want to make sure I have full understanding. I'm planning to coat the slab under the Foamglas with RedGard. I have drain holes. The bottom of the insulation blanket for the dome will rest on the Foamglas. I'm curious what you (or others) think about coating the vertical edge of the Foamglas w/RedGard. I hadn't planned to since Foamglas is hydrophobic. But, maybe it makes sense to have a continuous membrane of Redgard at that seam. I was planning to flash the perimeter of the Foamglas and seal where it meets the slab before applying the perlite/cement render. Appreciate any guidance.

[mongota]

Drain holes in the slab are always welcome. I'm grateful that I've never noted any moisture under mine, even when I taped squares of poly on the bottom of the slab. But you are right, build to prevent water from coming in, but provide a path for it to escape in case it does.

And for anyone new reading this, for clarity by no means am I recommending RedGard on top of the entirety of the board insulation, and setting the OVEN floor firebricks directly on top of the RedGard. I wouldn't want any membrane directly under my oven floor bricks subjected to heat from a high temperature fire built in the center of the oven floor.

[david s]

Thanks for that report, it’s good to know the Redguard stands up to the heat. Apart from keeping the moisture out, it is also important to provide an escape route rather than locking it in. This is of course weather dependant, but after torrential rain or even continuous high humidity the insulation can pick up considerable moisture. Some drain holes in the supporting slab provide an escape route for moisture as it travels away from the fire.
My approach to preventing water wicking up from the stand is a little different in that I use a sealer on the top of the supporting piers and make the cast slab waterproof with both Xypex and Edencrete concrete additives which also increase strength.

[mongota]

RedGard update...

When I built my oven, I painted RedGard on my slab to prevent moisture in the slab from wicking up in to my 4" of floor board insulation. Once the insulation was set, on top of the RedGard, I then painted RedGard on the 4" edge of the insulation, as well as carrying a bit of it on to the top of the insulation at the landing. I'll try to attach a photo from way back when as pictures are always better than words. Especially when I'm writing the words.

All told, the RedGard has performed marvelously. My oven has been in use for several years.

Here in the New England, we've been hammered with rain over the past few weeks. The last storm a couple of nights ago dropped 7.4" in one night. It was pretty amazing.

Before firing the oven up the other day, I pulled a few bricks up off of the landing just to see how the insulation looked underneath. All looked well. There have been a few questions regarding RedGard and how it holds up to temperature. In the photos below, the front row of bricks that make up the floor of the landing sit right on top of this RedGard. The landing is where I start my fires, so it has seen a bit of heat over the past several years.

I'm happy to report that the RedGard looks just fine. It's intact. No tears. No degradation. No delamination from the insulation below. No bubbling. No defects at all that I can ascertain. I can press a thumbnail in it and it's still "rubbery" so to speak. With this being the part of the membrane most exposed to oven heat, this location would have taken the most abuse from high temperatures. And it looks great.

That's all, folks!

[mongota]

You have made a remarkable oven, well done!

My take on oven moisture: If it is fired once or twice a week, the oven heats more quickly, gets hotter, and stays hotter longer than when Moisture builds up from disuse.

Firing the oven, even to an intermediate temperature goes a long way to controlling moisture. You can't get rid of a lot of moisture with just one firing cycle though. The solution is to use it more often. ;-)

Keep in mind that wood contains at least 8-10% moisture, sometimes a lot more. This moisture makes a more moist heat than in your kitchen's oven. There are pluses to baking with that moisture given off by the firewood. JMO

Agree with you completely, Lee.
I've seen the same, with frequent firings the fire itself burns hotter, the oven heats up faster, and the dome clears more quickly. And the outside surface of the oven (in my case the stone dome veneer) never gets warm.
After a period of non-use, all take longer, and the exterior stone veneer is slightly warm to the touch due to moisture drive.

[Lburou]

You have made a remarkable oven, well done!

My take on oven moisture: If it is fired once or twice a week, the oven heats more quickly, gets hotter, and stays hotter longer than when Moisture builds up from disuse.

Firing the oven, even to an intermediate temperature goes a long way to controlling moisture. You can't get rid of a lot of moisture with just one firing cycle though. The solution is to use it more often. ;-)

Keep in mind that wood contains at least 8-10% moisture, sometimes a lot more. This moisture makes a more moist heat than in your kitchen's oven. There are pluses to baking with that moisture given off by the firewood. JMO

[mongota]

Vent Cap update...

I was a bit skeptical regarding the need for a vent at the apex of my dome, but for a couple dollars in materials and 15 minutes of my time I incorporated one. Why not?

I used black pipe with a threaded cap.

After a few months of sitting dormant, Ifired up the oven yesterday and a couple hours in to the firing, I thought "why not?". So i climbed up and unthreaded the cap. Moisture!

The inside of both the cap and pipe were wet.

I left the cap off and when i put it to bed for the night i reinstalled the cap. Came out today and popped the cap off. More moisture


I have 4" of CF board insulation under the floor. I have 4" of insulation over the dome (four 1" layers of CF blanket).


So, I'm sold. It certainly does seem to be a valid escape path for moisture within the dome structure.

[mongota]

Love your work Mongo! Great stuff!

I appreciate those words Mark. Thanks you.

[MarkJerling]

No progress, just an update, I just don't think I ever posted any overall shots of the oven with the stone work done. Here are a couple; front and back.

Love your work Mongo! Great stuff!

[Baza]

Keep the dream alive Mongo ... keep the dream alive!

[mongota]

AH HA!!!

First - if there is one thing you cannot be accused of it's lack of clarity - I mean ... who else takes the time to use one's own pics/drawings to help teach them!?
AWESOME! There is a natural teacher in you!

Second - this makes total sense (though my math was off in my original drawing!).
Yes - I get it now -

Given I would like to follow a similar approach (and apply it to a flared opening - if, that is, flared doesn't mean more cold air to deal with), it still allows the remnants of a cut 9" brick (into 6 1/8" and 2 7/8") to serve as material for the outer arch brick assembly (given your build's 2 3/4" thickness on the outside brick arch). It's the Scot in me that doesnae want to waste!

Wonderful Mongo ... I will take all this learning into the weekend with me!
Fingers crossed!
Barry

I knew I was creating waste...but until my wife didn't want it any more (at least it's on a back-back-back burner), I was going to use the offcuts to build a tandoor oven.

[Baza]

AH HA!!!

First - if there is one thing you cannot be accused of it's lack of clarity - I mean ... who else takes the time to use one's own pics/drawings to help teach them!?
AWESOME! There is a natural teacher in you!

Second - this makes total sense (though my math was off in my original drawing!).
Yes - I get it now -

Given I would like to follow a similar approach (and apply it to a flared opening - if, that is, flared doesn't mean more cold air to deal with), it still allows the remnants of a cut 9" brick (into 6 1/8" and 2 7/8") to serve as material for the outer arch brick assembly (given your build's 2 3/4" thickness on the outside brick arch). It's the Scot in me that doesnae want to waste!

Wonderful Mongo ... I will take all this learning into the weekend with me!
Fingers crossed!
Barry

[mongota]

Mongo - you, again, provide this Forum so much clarity and support - thank you for doing this!!
I am certainly a builder that is wondering the very things you posted!

I am sharing a photo I drew quickly (at work in my office - tell NO ONE!) of your vent build with a question....
You indicated your vent landing is 13.5" ... the length of a full and half brick.
You indicated that you overlapped your oven arch opening by 1.625" to cover the rope heat break

Doesn't the lip of 1 5/8" subtract from the overall length of a full and half brick (13.5") to make it actually 11 7/8" from oven opening to vent opening?
Don't you lose length on the lip at the dome arch?

Sorry to be detail-y ... your build has helped inform mine SO much ... my brain just needs to stop hurting over this!

Thanks Mongo!
Barry

Your welcome Barry Baza , thanks for the kind words.

Regarding your question, I must have explained it poorly in a previous post, but it's the exact opposite. That roughly 1-5/8" lip is added to the 13-1/2" length. So the INSIDE length of my tunnel is exactly 1-1/2 bricks, or 13-1/2" long. The OUTSIDE length of my tunnel is 13-1/2" PLUS the 1-5/8" extension from the "L", or about 15-1/8".