42” Corner Build in the Shadow of Mount Nittany

[Giovanni Rossi]

The kit comes with enough blanket to do 2 – 1” layers. The 1^st layer rests on the CaSil board and the other is intended to rest on the concrete slab. I wanted to keep the CaSil and blanket away from the slab so that’s why I added the FOAMGLAS with 3” extending beyond the CaSil. I purchased 2 more 50 sqft rolls of blanket with the intent of having 4 layers, 1 resting on the CaSil and 3 on the FOAMGLAS.

Though I had done the math, I wasn’t sure I would have enough to do 4 layers as I had planned. I decided if I was short I would just put as much of the 4^th layer as I could starting at the top of the dome.

The 1^st two layers were wrapped horizontally per the kit directions. I found that difficult to manage with one set of hands. I switched things up and laid the rest by wrapping vertically. I had enough for 4 full layers with about 15 sqft to spare. That was probably roughly the area of the entryway x 4.

Weather at this time of yr in central PA is interesting. It can be over 80^oF during the day and frost warnings at night. I picked a day in the low 60's to do this work and it was uncomfortable wearing all the garb. Didn't finish and started early the next day. By the time I finished it was 86 and I was cooked! So, plan your work accordingly. I wish now I had just set up some lights and worked into the night on the first day.

The kit has 5” nails to use as anchor points for the fine wire that snugs the blanket to the dome. These were useless to me because of the extra layers. I found some 8” Headlock screws in stock at a local big box store. These worked very well. I screwed them through the blanket and into the CaSil, see pic #2. I wasn’t keen on introducing more metal but couldn’t think of an alternative. There was enough wire despite me increasing the surface area.

Lastly, a roll of chicken wire is hooked on those same screws and wrapped over the dome to serve as something for the pericrete render to hold on to. This is one of those additions that’s debated on the Forum. Some advocate its use, others say it’s not necessary to support the pericrete and it adds more conductive material in the insulation layers. I think it also depends on the thickness of the 1^st layer of pericrete as well as the consistency of the mix. I felt I should follow the kit instructions and use it and wasn’t super concerned about the heat conduction with the 4” of blanket.

[Giovanni Rossi]

FINALLY, A QUESTION!

I’m doing 7 days of drying fires now. I’ll post the results when I finish.

I have a question for the group. The kit instructions use 6” alumininum flashing that is to be wrapped around the outer blanket layer. Silicone caulk is used to seal the flashing where it meets the concrete slab. This is to prevent water that may travel under the render from soaking the blanket and then the CaSil layer that is sitting on the slab in their design.

Since I deviated from their design by setting everything on FOAMGLAS, I don’t think I want to flash and seal at the base of the FOAMGLAS. Not doing this would keep a drainage path open under the FOAMGLAS and through the tiles to the weep holes. Also, I don’t see the point in flashing and sealing around the blanket as it sits on top of the FOAMGLAS. If any moisture would get in to the lower part of the blanket or CaSil I would want it to find its way around the FOAMGLAS and into the drainage system.

Your thoughts? Am I missing something?

You can see the blanket in place in the prior post. Here are some views before that was done.

Thanks for your help.

[david s]

I agree, with the combination of weep holes, tiles and foam glass, you have a good set up for removing any moisture that gets in. An entry point can sometimes develop after a couple of years through a crack that sometimes develops where the external render meets the supporting slab. If this happens it will be quite visible and you can then seal it up again. I use a flexible acrylic render product which does the job nicely. Water running off the dome tends to sit in this position so it is important to have a good seal there. In my build the supporting slab is sloped slightly so water does not sit in this position.
When you come to do the rendering job, make sure you get a good bond at that point. Wire brush the surface where the render joins the slab.

[xvandish]

Congrats, very nice work. Best of luck with the drying fires and the soon to come first pizza(s)!

[Giovanni Rossi]

Thanks xvandish, looking forward to your build.

Thanks david s for the wire brush tip. I had read about you sloping the slab before. I gave it about 5 seconds of thought. Then, accepting the reality of my novice skills, I was going to be happy if I got the thing close to level. It's crazy after all this research that I’m still thinking on how I’m going to finish the dome. So, I’m going to have more questions on products for you and the others on the Forum.

It’s been too cool and rainy to think about doing the pericrete (good thing or I would have missed Dave's tip). I have to peel back the tarp covering the site to place the vent pipe while firing so I need only a few hours without rain for each session. Got 6 of the 7 days finished but it looks like we’re going to have a span of rainy weather, so I’ll just post what I have so far…

The kit instructions advocate 7 days of drying fires. I had no idea if this pertained to me because I had done the small fires before winter, it’s been 6 months since the last of the bricks were laid and it was a particularly dry winter.

The instructions state it’s not necessary to do 7 consecutive days so I had the time to work around the weather. The scheme is 2 days of small fires, 2 days med and 2 days of larger. As a size guide, a certain fire footprint is maintained for at least 3 hours and then the oven door is placed until the next firing day. On day 7 the oven is taken up to working temperature. I tracked the temps because I was curious about heat retention even though I have just an 1/8" steel door.

Day	Temp at peak of dome by IR	Next morning temp via door thermometer
1	200oF
2	250oF
3	300oF	80oF (48 hrs after fire and 30o overnight)
4	380oF	120oF
5	500oF	150oF
6	600oF	220oF
7

It was interesting seeing how quickly the dome temp increased each day as the bricks became more saturated. I guess that’s also true as the moisture is driven out; though, it didn’t seem there was much to start with in this case. I laid some plastic over the insulation on day 3 for about an hour and there was barely a hint of dampness to be felt. Maybe it’s just a slower process to work its way through that many layers of blanket??

[Giovanni Rossi]

Using this wet weather pattern to finish my research on rendering the oven. david s, I need to double check your recommendations about AR glass fibers. One post states ~0.4L fibers / 10 L dry mix and another post states 150gm fibers / 10 L wet mix. I suspect both of these are true, but I don't yet have any in my possession to measure.

Also, I can find AR fibers on the big A in 1 pound bags. They are 1/2" (13mm) and claim to be at least 19% zirconium. I'm doing 3 layers about 3/4" each. These should work. Do you agree? Thanks!

If I've done the calculations correctly, the volume of the ~2" layer is about 200L so I'm going to need 6-7 pounds of AR fibers.

[david s]

Did you manage to get the whole inside of the dome white?

As the outer render is just a weather coating to protect your insulation, my belief is that it doesn’t require super strength by making it really thick. Is there another reason you want to go so thick?
I do mine 12-15mm thick and have never had a problem with inadequate strength. After having driven out all the moisture from the inner layers, there won’t be a problem with steam pressure build up, but moisture being drawn away from the render layer from the dry vermicrete is a problem, so I always wrap the rendered layer, in fact the whole oven, to retain the moisture in that outer layer for a week to enhance its strength.

Those fibres sound right. The density of my loose fibres is 250Kg/m3 (250g/L)

[Giovanni Rossi]

Dave,

No, I finished day 6 of 7 of the drying routine last week. It has been raining for 5 days and looks to continue for another 5 days! When I can resume, I’m going to take a whole day and slowly bring the oven up to working temp. I’m not in any hurry to finish so I’ll probably do a few full firings before adding the pericrete.

The kit instructions call for ~2” of pericrete (5:1:1:1 – perlite:cement:hydrated lime:washed sand). As I read the Forum, these proportions are on the strength side of the strength vs. insulation equation. Though, something called Litefill perlite is used which claims to have the perlite particles coated so they don’t retain water which makes the mix a superior insulator compared to using standard perlite. So, maybe, this render is both strong and better insulated?? I don’t have the knowledge or experience to verify that.

Even though I have 4” of blanket, I have so much of this perlite and leftovers of the other components, and I have room on my slab; I may as well do the 2” layer. The only concern is the encroachment toward the front of the oven as the kit plan uses only 2” of blanket. I have room behind the vent pipe. I’ll just have to make sure I tool it well as the sphere “grows” long the entry bricks to make it look good.

I had planned to place 2 layers of the pericrete then fit a custom-made landing that wraps around to the dome and then add the final layer of pericrete to dress up that intersection. I may even tile the top of the slab before that final layer. Once the pericrete layer is completely dry, the kit completes the build with a roll-on acrylic render.

Your comment got me thinking though…

I was adding the AR fibers to decrease slumping during application and improve post-cure strength. Maybe slump isn’t a big concern with adding such thin layers if the mix has the right consistency and I could add the fibers to only the final ¾” layer and save on buying so much of the fibers??

Your thoughts? Thanks.

[david s]

I think you should follow the manufacturer’s instructions. In my build I recommend getting the whole dome white after doing the perlite insulation, but before doing the render because excessive steam pressure in the insulation layers can be very damaging. The black soot on the inside indicates the presence of moisture in the inner dome. Your recorded temperatures are not high enough.

[Giovanni Rossi]

Thanks Dave, I know I'm probably dragging this drying business out longer than necessary, but I have the time.

I'm actually being more cautious than the manufacturer's instructions. I've worked up to the point of firing to clear the dome with the 4 inches of blanket on. I'll probably fire it to clear the dome 2 or 3 times before I add the perlite insulation. I'll install a breather valve at the peak. Once I get the perlite on I'll wrap that and let it cure per your instruction and then do another routine of drying fires. Once everything is dried out, I'll add the acrylic render.

Do you (or anyone) have an opinion about my question about using AR fibers in just the final pericrete layer? Thanks again.

[david s]

Thanks Dave, I know I'm probably dragging this drying business out longer than necessary, but I have the time.

I'm actually being more cautious than the manufacturer's instructions. I've worked up to the point of firing to clear the dome with the 4 inches of blanket on. I'll probably fire it to clear the dome 2 or 3 times before I add the perlite insulation. I'll install a breather valve at the peak. Once I get the perlite on I'll wrap that and let it cure per your instruction and then do another routine of drying fires. Once everything is dried out, I'll add the acrylic render.

Do you (or anyone) have an opinion about my question about using AR fibers in just the final pericrete layer? Thanks again.

I think you should follow Ben's instructions, but if you want to add the fibres in the final layer they do add significant strength, which means you can reduce thickness. As you have also mentioned they reduce/eliminate slump cracking as well as holding the mix together during application. I don't think they are much use in a mix that contains perlite because that makes any mix far weaker and the addition of fibres is unlikely to alter its strength significantly. Perhaps you should ask Ben for his opinion on this.

Regarding the coated perlite in the mix, I can't access the silicone coated stuff and consequently have to deal with the much higher proportion of water in the mix. I'd certainly use it if I could get it.

[Giovanni Rossi]

Ahh, I've been tripped up by the use of the term "render" generically in both my kit instructions and on the Forum.

david s, when you talk adding AR fibers to a render you are referring only to a hard shell layer on whatever insulation layers are already in place !

I'm clear now. Sorry for the confusion and thanks.

[david s]

Ah yes, render is a British/Australian term for what the Americans call stucco, my apologies.

[Giovanni Rossi]

Back at it…

Continued the drying fires with just the blanket insulation. Didn’t push it to full firing temps but managed to get a few pies out.

[Giovanni Rossi]

I’ve already spent too much time thinking about how to finish the dome. The kit instructions call for 2” of ceramic fiber blanket and 2” of pericrete render. As I doubled the blanket to 4”, the larger surface area was going to complicate things a bit. I knew I would run out of the supplied perlite. The product used is Perlite Litefill from an Australian company. A similar coated-bead perlite product avail in the U.S. is from a company called Silbrico. They make a line of perlite products by the name of Ryolex. The correct one for ovens is Ryolex #1 – SKU# 6307. The product is affordable but the shipping is not. It is manufactured in Chicago and available for pick up if you are interested and live in that region.

I could have just picked up regular perlite to supplement, but decided to work with what I have. For novices like me it’s recommended to do 3 layers building up to the 2”. Again, hats off to those of you with the skill to cover the dome in a single layer!