Longmont, CO new WFO build - casted over sand

[cnegrelli]

After 4.5 years of lurking, I'm now about to build the WFO. I've worked out most of the materials and found pretty much everything I need. Some useful info on where to get materials came from DrakeRemoray and Coloradoredusa. In the end, Russel's advise of EJ Bartells (now Distribution International) for insulation products and Threewit (firebrick, castable, etc), along with our host site, have been the best sources.

I will post plans soon, have been accumulating a lot of knowledge, especially from david s UtahBeehiver Gulf and others. My basic plan is to build a standard stand. I plan to cast a dome that will be a 28-32" I.D, over sand mold. Insulating base layer of 2" of FoamGlas and 2" of CaSi board (as seen UtahBeehiver), to alleviate moisture concern.

Firebrick floor (cheapest source found is $1.95 USD per brick, after much searching. Gave up on pursuit of used, as they were all in questionable condition and heritage.
Casted dome over sand mold using a commercial castable (likely Heatcast 40 - again not cheap here). With SS needles if I can find them. Will also cast an entry/flue area. Thermal break likely between inner arch and casted entry.

Insulated with Ceramic blanket. 48" high 6" Inner Diameter duravent chimney. Plan is to stick with a stucco igloo design, uncovered. I plan to vent the shell with david s chimney vent designs.

Before I begin, I have the first question for the board. I am hoping to re-use the concrete pad that held a 4000 lb hot tub for 10+ years. It's reinforced concrete that is 4" deep. It held the hot tub for 10+ years (fully loaded) and this past winter without a load, with no heaving or cracking. I plan to dry stack cinder block, filling every other hole or so with concrete and rebar. No plan to tie the rebar into the pad.

Any concerns with this pad?

[cnegrelli]

Bumping - anyone care to join in and comment on the existing pad reuse? Please?

[david s]

It sounds plenty strong enough. I usually pin the stand to the foundation slab. You can do this by drilling into it and hammering in a short length of bar.

[cnegrelli]

I can do that! Every block or just 4 corners or so?

[david s]

I just do the corners.

[cnegrelli]

I've been searching for the best castable source and I finally found one that is both stocked locally and at a more reasonable price. Would this be the right stuff here?

The "foundry guy" suggested this product for troweling on over a sand mold and for casting a flue. It's 145 lb/cu.ft. density.
Contents is:
Al2O3 45.0% SiO2 48.0% Fe2O3 <1.0%

http://www.pryorgiggey.com/TDS/Castm...28%20HR-td.pdf

[david s]

Yes, that'll be good. The stuff sets fast, particularly in hot weather so don't mix up too much in one go. I use chilled water in hot weather which gives you extended working time.

[cnegrelli]

I know I've seen discussion here on this top, but for planning purposes, assuming my igloo has 3" of insulation and a minimal amount of Perlcrete/Vermicrete to smooth that out and reshape, and then Stucco (with some water resistance additive in final coat), how much thickness should I plan for from the insulation layer out?

[david s]

You will need to decide how thick you want to make the inner dome first. Most cast ovens are 2" thick. The Primevera 60 is 1.5" thick. You then need to decide how much thickness of blanket you want. Two layers of 1" blanket and a 1.5" layer of10:1 vermicrete is about normal, then the thickness of the outer shell. I do mine around 12 mm which is probably about the min. Add up all your numbers to get the outside diameter.

[cnegrelli]

Thanks for the feedback of Vermicrete and Stucco. I'm going to make sure I have some wiggle-room of a few inches either way, as I may change my mind on the final finish (Stucco igloo vs. enclosure).

I'm doing a 2" inner dome, but also still was considering a little thicker for extra mass. But I think I will go with 2".

My insulation is the FireMaster FastWrap+, which is 1.5" thick. I will do 2 layers minimum, but would like feedback of doing a third.

My use model:
Mainly Pizza - heat up time, fuel usage minimized. Based on experience, I will make them one at a time. So a huge oven is not necessary.
However, I am trying to balance that to allow for some cooking roasts and bread. So I'd like the size to accommodate this use model.
I currently believe a 32" Internal Diameter (ID) is sufficient. Comments?


My other assumption is that while the 2" casted dome (with firebrick floor) will not provide maximum thermal mass, if I can super-insulate it, I should have good results in retaining heat for bread, etc.
I plan on a thermal break right where the dome meets the gallery (inner arch). I will build a very good thermally resistive door. I have the blanket to go 4.5" thick, plus vermicrete. I plan on 2" of FoamGlas under 2" of CaSi board under the oven floor.

Comments and discussion requested!

[david s]

You will have plenty of thermal mass with 2" ofinner dome thickness. With a small oven the heat loss is greater at the door because the door is a bigger proportion of the oven surface. My feeling is that 4.5" of blanket won't give you any advantage over 3" as the insulation is subject to the law of diminishing returns. Remember also that a layer of 10:1 vermicrete over the blanket also provides good insulation.

[cnegrelli]

So I've been a bit on hold for awhile while the HOA endlessly debates my build. I got approval today, but cannot do a stucco finish. They were not clear about the oven itself, and didn't really ask about the oven appearance, so I'll worry about that later. I'm told they will never come back to inspect it and my neighbor behind me who can see it isn't going to complain as long as I feed him pizzas. But I needed approval as the lady across the street would freak-out when the concrete and block arrives, if I didn't have HOA approval.

Got 7" of very heavy and wet snow Thursday/Friday, so spent most of today cutting up branches which I've stowed away for drying (mostly Maple). I've got some Apple and Cherry, plus Aspen. All will be useful at least in curing (plus I have a bunch of Lump Charcoal I use for my homemade Big Green Egg).

I've started to dig out for a small 40"x40" pad that will hold the Santa Maria Grill I've decided to do. Rather than go go real deep to assure I'm below frost, my architect buddy told me to build it exactly as that of the pad that will hold the WFO. If there's some heave, it's best to have them both floating. He told me today that while my hottub was indeed very heavy, it was a uniform load. So I'm rethinking a bit how I orient the oven, but plan to make it very well supported and well distributed on the pad.

[cnegrelli]

Long story, but the Santa Maria Grill has been put on hold. It may come back in the picture later, but there was a complication with having to do a 2nd slab, and how that would fit. Made decision last night, to get moving on the WFO. CMUs and 60# concrete bags expected tomorrow. Ready to start the dry stack and drilling in rebar.

[Damer419]

Bummer about the grill, but the WFO is the main attraction............Enjoy the build

[cnegrelli]

It has begun.... And so have the rains, almost every day right now.