32" build in Calgary, AB - pompeii neapolitan

[IH123]

would it be equivalent or better to remove the perclite layer from bottom insulation completely and instead add one more 1" of ceramic fibre board to have total 3"? should I or have to make it 4" for hearth?

[david s]

No, air movement by convection requires it to be open at both ends. If really concerned about this point they could be plugged when not required. There’s far more heat loss from the oven mouth because of its much larger area. In high humidity even a roof won’t hold out moisture. We live in the tropics, oven is out in the weather, but I did have an occasion when after drying and using the oven, two weeks later in a period of very high humidity, but no actual rain, on heating the oven again, the outside shell was hot after an hour of firing, which indicates the presence of moisture in the insulation layers.
Any weep holes should be covered with insect mesh.

[IH123]

how about that:
1)a hole cut straight through the top slab into the open air space beneath CMU block walls will turn into a cold-air intake. When oven is hot, it will draw cold, damp air up through those drainage holes right into the insulation.

2) Freeze-Thaw Ground Moisture: In the spring, melting snow and ground moisture under oven base will evaporate upward. Those open holes in the slab will act like funnels, sucking ground humidity directly into perlite and ceramic fiber layers, causing the exact water-logging issue I am are trying to prevent.


3) heat-loss paths,
4) moisture migration paths
5) insect nesting locations (even using stainless steel mesh, if its not to fine mosquitoes can get in or smaller flies, even crawling insets trying to hide from cold winter. too fine one may clog the channel/hole as debris and dust fills it)
6) collection points for debris.

??

[IH123]

That is something new, will be interested on how this works. Remember, even 5 to1 pcrete seems crumbly until it sets up so don't pull out too soon. Dave brings up a good point on the rebar ridges but maybe the vapor barrier will solve the friction issue of pulling the bars out. What about reversing the direction of the weep hole and rebar then the weep hole is in the back of the oven slab, out of sight out of mind.

reason to use front:

1) I plan to put my feet in the 12" between slab front edge and the front wall, that will make my feet under the slab while my body flush with the slab edge and then my arms can extend toward oven mouth with more reach of the 12"
2) oven mouth will be 12" away from slab front edge that if I stand at slab edge I will be far. reason of leaving the 1ft on front is to allow to rest the pizza on.
3) I plan to store wood in the 12" space at the back under the slab, less space is possible in front where my feet will go. should use external wood storage like a shed for long term (may be not to avoid mice).
4) the top concrete slab is naturally 1-2 degree sloped forward (front lower than back)so its easier to have the dripping water drips forward than backward avoiding additional back sloping

[UtahBeehiver]

That is something new, will be interested on how this works. Remember, even 5 to1 pcrete seems crumbly until it sets up so don't pull out too soon. Dave brings up a good point on the rebar ridges but maybe the vapor barrier will solve the friction issue of pulling the bars out. What about reversing the direction of the weep hole and rebar then the weep hole is in the back of the oven slab, out of sight out of mind.

[david s]

This idea should work quite well. Because rebar contains ridges designed to increase grip and reduce pull out I think some plastic conduit wrapped neatly in plastic would be far easier to pull out, but I’m sure you’d work it out. Also spraying with cooking oil helps release anything from concrete work.

[IH123]

Water expansion on freezing is around 9% and in Calgary is certainly something you need to be wary of , but when it turns to steam can expand more than 1500 times which is 150000% and something oven builders need to consider wherever they are. Water will eventually be eliminated by fire, but its exit is far quicker and simpler if an exit path is provided, just like a fire exit in a crowded hall.
The way you have built your stand will not allow for drilling holes from underneath once the oven is built over it, so plan carefully.

if I lay rebars on top of concrete slab then pour the perlite-cement on them (as shown in above image), after the layer is cured, the rebars can be pulled out leaving a hollow channel in the insulation layer where water can seep out, the channels will meet near the oven mouth area, then concrete saw shallow cut on concrete slab will channel the water further to the front edge of the concrete slab (there is no insulation there, finally a 1/2" hole is drilled at about 10" away from the front edge of the slab allowing any water to weep and drain there. of course, the water channel can be covered with a masonry layer/sheet later to prevent water from back flowing. It might be that half of those channels are needed (like 5 or 3 instead of 9. what do you think? does that work?

[david s]

Water expansion on freezing is around 9% and in Calgary is certainly something you need to be wary of , but when it turns to steam can expand more than 1500 times which is 150000% and something oven builders need to consider wherever they are. Water will eventually be eliminated by fire, but its exit is far quicker and simpler if an exit path is provided, just like a fire exit in a crowded hall.
The way you have built your stand will not allow for drilling holes from underneath once the oven is built over it, so plan carefully.

[IH123]

it all depends on the form of water and where the oven is. In Calgary, water is:

-water=ice
-water=nightmare in -40C, -20C, 0c
-water can turn ice from October to May
-steam water will turn to ice powder in cold weather
- water= damage as it expands and freezes


having weeping holes on slab in winter is not very useful because they will certainly get blocked and ice, only way to get them open is if concrete temperature rise above zero for a while which probably will not happen for hours of firing because of insulation and outdoor temperatures and wind .

A positive thing of living thousands of kilometres away from warm ocean with sun taken vacation for whole winter is outside air is mostly dry so water can fly free when not too cold


Therefore, protecting oven from downpour is the best thing someone like me can do, moisture build up inside oven will be limited to water content inside oven cooking chamber from food and fuel. The thinking of water coming from air vapour and becoming water pooled inside insulation will not apply much as I think dealing with designing an exist for water from a confined place will be unworthy knowing the oven got a mouth and a chimney to vent off. might be best way to adapt is to slow fire each time and master how to get the oven to dry as it warms up

[david s]

I have almost everything I need to build the oven purchased and stored in dry basement over winter, I already purchased perlite, "Ceramic fibre blanket 2300°F 1'' 8LB/FT3 (1 Box 47.3sf)", and 5 count of "Ceramic fibre board 2300°F 2'' X 12'' X 36''" . I even have ThermoBond 915 100lbs refractory mortar purchased about a year ago placed in tight closed plastic pail never opened yet. The only thing I haven't got yet is either refractory castable or a steel plate for the doughnut base. so my design requires using ceramic fibre board under insulation not CaSi board

for weeping holes, I could even cut using my concrete saw shallow channels grid all over the slab; however unfortunately, at this moment, I am still not sure I need to do anything for making a "pooling water" drain from a hole because: I have a 6" thick slab placed mostly on grouted cmu block cores with very little gaps under it and still connected to a bottom slab that is 5.5" thick ... and got lots of fibre on slab. I can't imagine the oven can produce enough moisture to saturate close to 3 tons of concrete between firings . I most likely be a light user of the oven, may be fire it few times a year or so (not good to plan to design it so though). I have a feeling that weeping holes would be more suitable for large open span slabs with wood being stored under the oven that the slab would be bowed in centre creating a gap between the oven base and the slab where it will get filled with no emptiness but moist air. My priority in my oven is to have extremely rigid and stable foundation that will not propagate settlement to oven and water should not get on it. I still have close to a month to research it more and decide , for now just waiting for concrete to cure.

any help about my insulation stack design and if I should use castable refractory for dome wall support or instead use a 1/4" mild steel sheet one? as I am not using tiles under the dome walls, the ceramic fibre board and perlite if they creep or settle could cause settlement on a half brick and that if combined could cause a dome instability, so wanting to put a very solid doughnut under the dome walls ONLY while hearth can be on its own ...

The problem is that water is conductive, air is not. This leads to moist insulation not working as an insulator. This fact combined with water in a liquid state creating more than 1500 times its volume as steam when heated, means that small amounts of water will negate the insulative quality of the insulation. Every oven owner becomes surprised by just how much is generated and how much difference it makes to the efficient operation of their ovens.

Any mild steel used in the build particularly if thin, is prone to corrosion from the combination of heat and moisture, always use stainless.

[IH123]

I have almost everything I need to build the oven purchased and stored in dry basement over winter, I already purchased perlite, "Ceramic fibre blanket 2300°F 1'' 8LB/FT3 (1 Box 47.3sf)", and 5 count of "Ceramic fibre board 2300°F 2'' X 12'' X 36''" . I even have ThermoBond 915 100lbs refractory mortar purchased about a year ago placed in tight closed plastic pail never opened yet. The only thing I haven't got yet is either refractory castable or a steel plate for the doughnut base. so my design requires using ceramic fibre board under insulation not CaSi board

for weeping holes, I could even cut using my concrete saw shallow channels grid all over the slab; however unfortunately, at this moment, I am still not sure I need to do anything for making a "pooling water" drain from a hole because: I have a 6" thick slab placed mostly on grouted cmu block cores with very little gaps under it and still connected to a bottom slab that is 5.5" thick ... and got lots of fibre on slab. I can't imagine the oven can produce enough moisture to saturate close to 3 tons of concrete between firings . I most likely be a light user of the oven, may be fire it few times a year or so (not good to plan to design it so though). I have a feeling that weeping holes would be more suitable for large open span slabs with wood being stored under the oven that the slab would be bowed in centre creating a gap between the oven base and the slab where it will get filled with no emptiness but moist air. My priority in my oven is to have extremely rigid and stable foundation that will not propagate settlement to oven and water should not get on it. I still have close to a month to research it more and decide , for now just waiting for concrete to cure.

any help about my insulation stack design and if I should use castable refractory for dome wall support or instead use a 1/4" mild steel sheet one? as I am not using tiles under the dome walls, the ceramic fibre board and perlite if they creep or settle could cause settlement on a half brick and that if combined could cause a dome instability, so wanting to put a very solid doughnut under the dome walls ONLY while hearth can be on its own ...

[UtahBeehiver]

Our goal is not to harp on the issue but to make you oven the best it can be. With all your hard work of planning and design, it will be a great oven. A 5 to 1 pecrete mix is suggested for under the floor and dome since this ratio has enough compression strength for supporting the dome and floor and adequate insulation value. Roughly 4" 5 to 1 pcrete equals 2" CaSi. So a total of 4" CaSi equivalent is a very good insulated floor.

[david s]

Placing a dry calsil board over the tiles simplifies the need to eliminate water after the build. Casting a vermicrete or perlcrete layer over the tiles requires a layer of weed mat or similar material to hold the lightweight mix away from the channels between the tiles. It is the pressure increase from expanding air or steam as it is heated that forces steam out the bottom.through the channels between the tiles and out the weep holes. If this moisture is locked into the oven, it simply recompenses back to water only to be reconverted to steam the next time the oven is fired, this is what makes its early removal before building over it such a good idea. Sun and wind, if available for drying is the simplest. Fire internally then needs to be done slowly to avoid damage from expanding steam and or huge temperature differences which also mean huge thermal expansion differences between different parts of the oven. This is why it is also important when doing the drying fires, to allow the whole oven to return to an even ambient temperature allowing those hotter parts to contract and allowing some moisture to migrate back to the parts already dried, before the subsequent fire is started. Any moisture contained in wood exits out the flue, but a good proportion of the energy in damp wood goes towards turning that water into steam. Not too many oven owners fire with damp wood.

[IH123]

Ok. 28 days damp curing is the industry standard, so yes, take your time and keep it covered for that long. If you are pouring your own vermicrete slab, be aware that there is a very large volume of water required in the mix, much of it is left as free water after the hydration process has chemically taken up what it requires, so extended drying is important before building over it and locking it in. Take a read of my attached experiment on drying a vermicrete slab.

[ATTACH]n471431[/ATTACH]

I already have 3 bags of perlite, 100 litres each, I plan to use perlite - cement, not vermicrete , while my 2" ceramic fibre board that I have is enough, I will still put a layer of perclite to have the concrete slab stays below 40c, even if oven is fired at 1000F inside. keeping the top concrete slab (6") cool , should help reduce effect of any pooled water I guess, as water wont be warmed enough to steam out and could safely dissipate in concrete while falling downwards and out

[IH123]

ok, david s and UtahBeehiver , I apologize if I appear to have taken this lightly and thank you for pointing this out. my slab workform had accidental 1-2 degree slop forward (front lower than back) and while I sprayed the slab with water this morning I could see the water has tendency to flow forward not backward. This would point out to potential hole location, front. Yes, burning fuel (gas, wood) will generate water vapour as part of combustion and release any water content from wood into air, water will eventually accumulate between cycles and with no guaranteed rate of evaporation in a well sealed oven , the risk of water pooling is possible. however, creating one or two 1/2" holes in slab can only be useful if water can be directed to that hole, for that to happen a porcelain mosaic tile with gaps between need to be placed under the insulation to allow water to flow ; however over time, and heat cycle expanding, contraction, settling: likely those gaps will become not useful because if they were tiny they will be filled with top layer creeping down on from moisture and if they are large gaps then creeping can be even bigger to distort any channels between the tiles. with most moisture is supposed to come from the hearth, not dome walls, and with hearth bricks are not supposed to have any refractory mortar on sides, vapour can go in and also out through chimney and oven mouth , however that will require slow heating and likely each time because no one knows how much moisture is pooling down. I also see it that to have the hole system work, with mosaic tiles, the insulation layer like perlite must be poured on a form and then transferred over the tiles because if pouerd over tiles directly then the cement will coat the gaps and close them, introducing cracking from moving a brittle layer of perclite. yes, building the oven will require a lot of unwanted water , that will be treated by slow curing. for instance, I have a 500000btu propane tiger torch, if needed , I can use it at lowest setting to warm the oven or even better place electric space heater outside at oven mouth blowing inside followed by bigger and bigger fires. I can close the door and place humidity meter in it when cool to tell how moist it is. therefore, treat the building water by curing. for water occurs by combustion, it should go from the chimney as hot gas and evaporation. I just find the simplicity of the design that everything enters from mouth and leave from mouth or chimney is the best, including vapour and water.

Can you tell me please why I should be worried about water pooling after curing? does burning wood produce too much water and does that requires the complicated capillary tile channels to transport that water down ?

I am not sure its worth it, I never seen a single video online doing this, can you please show me who did that?