WFO in Utrecht, the Netherlands

[Mr. Slowhand]

Hi all, another question, I have 3 layers of blankets around the dome, 4 on top of the dome, is that enough? Should I apply one more layer? Let me know, guys!

[Sixto]

I've read "there is no such thing as too much insulation, but I've also read (I think) that two one-inch layer of blanket plus 1.5" of perlcrete or vermicrete covered by one inch of render/stucco is enough to keep the outside face of render/stucco between 100d to 150d while cooking pizzas at 700d inside... I think this also loses about 200d of retained heat per night for next-day baking, or third day slow roasting??? That was good enough for me, so the 3+ inches you have are already better than that. It's a question of how much are you willing to do to slow the rate of cooling through the dome shell? At some point, other sources of heat loss like how tight, or how insulated your door is, may come into play.

[Mr. Slowhand]

I've read "there is no such thing as too much insulation, but I've also read (I think) that two one-inch layer of blanket plus 1.5" of perlcrete or vermicrete covered by one inch of render/stucco is enough to keep the outside face of render/stucco between 100d to 150d while cooking pizzas at 700d inside... I think this also loses about 200d of retained heat per night for next-day baking, or third day slow roasting??? That was good enough for me, so the 3+ inches you have are already better than that. It's a question of how much are you willing to do to slow the rate of cooling through the dome shell? At some point, other sources of heat loss like how tight, or how insulated your door is, may come into play.

Sixto, thanks, I will be doing the v-crete on Saturday, and I ordered some blanket yesterday. I decided, if it arrives, I will install it, if not, I am putting v-crete with this amount of blanket isolation. Not to waste time, I started with the enclosure, which will be just brick walls.
I have a party sometime on 20th October, would like to have the oven ready and cured, so I need to get going.....

[Sixto]

Sixto, thanks, I will be doing the v-crete on Saturday, and I ordered some blanket yesterday. I decided, if it arrives, I will install it, if not, I am putting v-crete with this amount of blanket isolation. Not to waste time, I started with the enclosure, which will be just brick walls.
I have a party sometime on 20th October, would like to have the oven ready and cured, so I need to get going.....

If you're building an enclosure, you probably don't need to add vermicrete, you can get better results with loose vermiculite on top of the blanket?

[Mr. Slowhand]

If you're building an enclosure, you probably don't need to add vermicrete, you can get better results with loose vermiculite on top of the blanket?

Sixto, you are correct on that, but I have a problem with all that loose material around in the oven. Also, I like that the vermicrete kind of "seals" the blanket around the oven, so it does not move. I like to be quite dedicated to a project, but not very good in being involved in something that needs constant maintenance. Therefore, the vermicrete, would (at least I think so) be a once time operation. Maybe I do a layer of vermicrete, and the rest to fill up with loose vermeculite, don't know, remains to be seen.
The extra roll of the isolation blanket has been delivered, so I am putting the 4th layer, will put 2nd layer around the entry gallery and the flue. Will not put anything around the chimney, as that one is 2-walled, with some sort of isolation in between the walls of the chimney.

I know that the downside to these extra layers of blankets is that my outer oven diameter is larger, so I will probably spend a lot of vermeculite to render the outside.
Plus, I know I will need a couple of months to finish the enclosure (at this tempo). Would be good to be able to use the oven in the meantime. Currently the oven is covered by a temporary roof structure, which will remain in place until we make a large roof. The roof will cover the oven, the BBQ/Fireplace, and the rest of the future outdoor kitchen. This means it might not be over until the spring.
As I am writing this, there is more and more of an idea to just use the loose vermeculite, remains to be seen....
.....I am so easily influenceable. But this is good, that is why I am on this forum...

[Sixto]

I'm planning to start my slow drying fires as soon as I can get the chimney on, which may be in the next few days. You should be able to do the same while you are building your enclosure.

Regarding the question of maintenance of the loose perlite, obviously you have to be comfortable with your level of tweaking after the build, but don't believe you would have to do anything inside the enclosure after you fill it with loose vermiculite. I think a layer of vermicrete that stays in place is fine, any additional loose stuff on top might settle a bit at the deeper sections but that would have a minimal impact on the overall insulation value. Taking that idea further, you can do a thicker layer of vermicrete only the top -more horizontal sections of the dome to make sure that stays in place, (like a knit winter cap on your head) and let the loose stuff settle around the sides like a scarf around your neck... that might achieve just what you are aiming for..

Heat rises, so the top of the dome will be hottest area - and the most prone to heat loss. The fiber blanket is the best insulator we have, so if you take the extra roll and concentrate that on top of the dome, the loose perlite around the sides (between the dome and enclosure) will keep those extra top layers from shifting around and you'll get the best of all scenarios.

[Mr. Slowhand]

I'm planning to start my slow drying fires as soon as I can get the chimney on, which may be in the next few days. You should be able to do the same while you are building your enclosure.

Regarding the question of maintenance of the loose perlite, obviously you have to be comfortable with your level of tweaking after the build, but don't believe you would have to do anything inside the enclosure after you fill it with loose vermiculite. I think a layer of vermicrete that stays in place is fine, any additional loose stuff on top might settle a bit at the deeper sections but that would have a minimal impact on the overall insulation value. Taking that idea further, you can do a thicker layer of vermicrete only the top -more horizontal sections of the dome to make sure that stays in place, (like a knit winter cap on your head) and let the loose stuff settle around the sides like a scarf around your neck... that might achieve just what you are aiming for..

Heat rises, so the top of the dome will be hottest area - and the most prone to heat loss. The fiber blanket is the best insulator we have, so if you take the extra roll and concentrate that on top of the dome, the loose perlite around the sides (between the dome and enclosure) will keep those extra top layers from shifting around and you'll get the best of all scenarios.

Dear Sixto,

Many thanks, that is a good idea. I will start building the enclosure, just from red bricks, and will see how far that goes in the next 2 weeks. Will not start anything with vermicrete until I am done with the enclosure.

[david s]

I've read "there is no such thing as too much insulation, but I've also read (I think) that two one-inch layer of blanket plus 1.5" of perlcrete or vermicrete covered by one inch of render/stucco is enough to keep the outside face of render/stucco between 100d to 150d while cooking pizzas at 700d inside... I think this also loses about 200d of retained heat per night for next-day baking, or third day slow roasting??? That was good enough for me, so the 3+ inches you have are already better than that. It's a question of how much are you willing to do to slow the rate of cooling through the dome shell? At some point, other sources of heat loss like how tight, or how insulated your door is, may come into play.

A couple of observations on this topic.
1. Although CFB is considered the best insulation, loose perlite is actually slightly better, but has a lower temperature resistance than the blanket. Around 1450C for blanket and around 1100C for perlite. I have fired both perlite and vermiculite to destruction to confirm this. As we never go anywhere near these temperatures the higher rated blanket is actually no advantage.
2. Loose vermiculite runs a close second to perlite, although as soon as any cement is added it’s insulation value drops. .A 10:1 vermicrete contains more mass in its cement content than the mass of the vermiculite.( because of this there is no measurable difference in the insulating values of perlcrete or vermicrete for any given equivalent ratios.
3. Powdered clay does a good job of binding the loose aggregate as it imparts way more stickiness and has way less density than cement, so it’s addition in the mix, if not too high a proportion works really well.
4. Cost of making your own perlcrete or vermicrete is far lower than manufactured board or blanket, but does have the disadvantage of containing a large proportion of free water that needs to be eliminated. Just as a pile of sand will remain damp for months inside, even when exposed to sun and wind, while the outer surface appears completely dry, so too does a thick layer of vermicrete or perlcrete.

Regarding the “no such thing as too much insulation”, it complies to the law of diminishing returns, so a doubling of layers, which results in more than doubling costs does not result in a doubling of insulation value. The becomes a pony where additional thickness makes virtually no improvement. It all depends how deep your pockets are. The increase in insulation thickness will also increase the volume of any external render applied or total exterior size of the oven.

[Mr. Slowhand]

Started with the enclosure build last night. After adding the extra CFB layer, actually, not much more room left for the loose vermeculite on the sides, which is good, I thought I would need it much more. The wire mesh that is put to hold the CFB in place is not good enough, it is not holding quite ok.
The other issue I have is that the wire mesh is anchored by screws screwed in the CalSil board. I see that any long term tension will produce tear in the cal sil board. So, I will drill holes in the concrete top, just around the CalSil board. I will screw down the perforated flexible metal band strap to hold the CFB in place. That should outlast screws into the CalSil board.
I understand that this is not an issue for people using the vermicre layer. This layer goes over the mesh, and all is set in place, no tensions that might get loose.
Amazing how every step of the way there is a issue and solution with these WFO, really enjoying the process.

[Mr. Slowhand]

Hoorah!, we started the fires!



Trying to follow the advice of David, in terms of the curing schedule.









Right now building an enclosure from red bricks, while also doing the curing. Should be all done in a couple of days

[Sixto]

Looks great! How hot did you get with three sticks? What is the nice brick structure to the left?

[Mr. Slowhand]

Looks great! How hot did you get with three sticks? What is the nice brick structure to the left?

Apologies, don't understand what the three sticks are?

The construction on the right is just a large fireplace I made. We use it for BBQ, cooking stews, all other fire related outdoor cooking stuff. Some of it will be moved into the WFO, but still a lot of good grilling is done on that fireplace

[Sixto]

Sorry, i meant 3 pieces of wood. When they are small like a thin branch we call them sticks. I was wondering how hot your first fire got since i see what looks like an infrared thermometer

[Mr. Slowhand]

Sorry, i meant 3 pieces of wood. When they are small like a thin branch we call them sticks. I was wondering how hot your first fire got since i see what looks like an infrared thermometer

Of course, yes. Well the first day fire was just the 3 small sticks, and I could not find my IR thermometer.
By the second day, I added some charcoal brikets, that is the photo with a lot of smoke. By that time, the themperature on the surrounding walls (on some places), was 37-40 degrees celsius.
By day 4 I am putting some larger quantities of brikets, and the temp reaches approx 50 degrees. Will keep it this way for some days more, I am still not doing anything with open flame in the oven

[Mr. Slowhand]

Oh, and it took me 2 firings to understand that it is the best to start the brikets outside of the oven, and put them in when they are fully glowing, than to start them in the oven.
But at least, I can confirm, the chimney has a very good draw. And this is with only 50cm, out of 150cm chimney installed, it will even get better