Fill enclosure with vermiculite??

[rchamlen]

first post - so bear with me if this is in the incorrect location for this question. In a little bit I will post a longer description of "me" in the Introductions forum.

First Question: I am looking at building an oven around the Forno Bravo Permio 40" rafractory dome. The kit comes with ceramic blanket (4") to insulate the dome. The sales people at Forno Bravo state that that blanket insulation (with appropriate stucco on the outside of it) is all that is needed. However, for a number of reasons I am for now planning on building an enclosure and roof around the dome (appearance and weather protection. Also, as part of this "adventure" (wished for for decades, only now really a possibility), I am highly, highly, highly interested in using retained heat to do lower temp cooking activities (both because of "what" I want to do, as well as to be more efficient with time and fuel.

So here is the question: If I have this free air gap between the finished dome and the outer enclosure, what are the implications of filling this "dead space" with loose vermiculite (not vermiculite concrete)?

1) is doing so likely to significantly increase the heat retention, and extend the available retained heat cooking time (in other words, is it worth it, or is the gain in behavior minimal and not really worth it)???
2) Is doing to just asking for problems: like moisture accumulation and mold growth , lack of access to the dome for potential repair, and moisture induced damage to the dome and inner wall of the enclosure?

I have seen a number of postings about filling this space with loose vermiculite, but no serious discussion of problems might be created by doing so....

I have a few other questions as I seriously evaluate doing this build, but lets start with this one....

Thank you in advance for your experience and insights.

[david s]

Loose perlite or vermiculite is excellent insulation and has the advantage of being a better insulator than vermicrete as well as eliminating the problem of having to dry that layer. With 4” of blanket the additional insulation is subject to the law of diminishing returns. Filling the corners of the enclosure with loose perlite or vermiculite around a hemisphere will result in a thicker insulation layer there so you can save material by blocking off those areas, empty plastic bottles work well. The enclosure should be vented, but sealed well enough to prevent animal and insect entry.

[fox]

Yes it is fine to do just that, adding extra insulation is a sure way to retain heat but there are other considerations that can be addressed to retain max heat over longer periods.
Having more mass will hold more heat, most cast ovens are 2” thick where as brick ovens are 4” thick but of course they take longer to saturate that thickness with heat.
Also consider the oven opening where a lot of stored heat can escape and the gallery entrance can suck heat away from the dome.
A well thought out cast oven can take three days to to cool down but you can boost the heat with small fires to keep it going for longer.

[rchamlen]

thanks for the comments. The Forno Bravo Premio kit comes with 3" of inner refractory and 4" blanket all around. I had thought of building from brick .... but am not sure if my back could take it .... and there is all that time savings with a cast kit.... David - good point about only filling the corners and lower areas of the dome with vermiculite - for some reason the image in my mind was of filling the whole volume. venting and protecting from critters is a given!

Fox - the forno bravio kits do come with an insulated door. Im not sure whether when inserted it prevents hot air escape from the chimney - but I think it does? But if so, I think that should address your thoughts about heat loss from the oven opening? (sorry for my ignorance, what is the oven "gallery entrance"?)

Yeah - I know about the heat up / saturation issue. that is exactly part of why I place a high interest in retained heat cooking - it seems such a waste to fire a full oven up just to cook one or two pizzas, or other single dinner dish, then waste all that heat when there is so much more you can do with it if you make sure you have enough energy retained to cook the next day!

[david s]

I don’t think Forno Bravo supply doors with insulating panels. I could be wrong, but you can check out what other builders have done on “show us your door”, thread if they don’t.

Once you mix cement with either vermiculite or perlite you are adding as much mass in cement as vermiculite, at a lean 10:1 ratio and even greater for richer brews. Either of the two will produce the same insulation value when mixed with cement at a given ratio.However, when adding it loose, perlite is a better insulator than vermiculite.

[rchamlen]

I don’t think Forno Bravo supply doors with insulating panels. I could be wrong, but you can check out what other builders have done on “show us your door”, thread if they don’t.

Once you mix cement with either vermiculite or perlite you are adding as much mass in cement as vermiculite, at a lean 10:1 ratio and even greater for richer brews. Either of the two will produce the same insulation value when mixed with cement at a given ratio.However, when adding it loose, perlite is a better insulator than vermiculite.

They state the Premio kits come with "a steel door with thermometer". I could have sworn I recalled them stating they were insulating, but must have mixed that up with something else. That is easily fixed though at any time.

Honestly, I have no intention of using any cement/vermiculite mix. I'll go with the 4" blanket, and anything else I do will be loose. Thanks for the comment re. the relative insulating properties of pearlite versus vermiculite.

Still dont know yet whether the door blocks hot air escape from the chimney. I *suspect* it does, but am not sure......

Believe it or not, I think the biggest stumbling block to my building this this spring is landscaping and underground sprinkler issues. Gotta do more thinking about that.... :-)

[Gulf]

Still dont know yet whether the door blocks hot air escape from the chimney. I *suspect* it does, but am not sure......

If you meant blocking the cooking chamber from the chimney, it does. Here is a the Premo page. Click on the link to the schematics.

[david s]

The door will fit against a rebate in the oven mouth, thus isolating the oven from the flue. When planning your sprinkler system, be sure to avoid any watering of the stand, as water can wick up and into the supporting slab and underfloor insulation. I use an admix in the concrete supporting slab that reduces permeability as well as sealing the top of the stand prior to supporting slab installation to prevent possible wicking issues.

[rchamlen]

If you meant blocking the cooking chamber from the chimney, it does. Here is a the Premo page. Click on the link to the schematics.

Probably bad choice of words on my part, but yes, that is what I meant. I see it in the diagrams - thanks.

[rchamlen]

The door will fit against a rebate in the oven mouth, thus isolating the oven from the flue. When planning your sprinkler system, be sure to avoid any watering of the stand, as water can wick up and into the supporting slab and underfloor insulation. I use an admix in the concrete supporting slab that reduces permeability as well as sealing the top of the stand prior to supporting slab installation to prevent possible wicking issues.

I hear your caution .... *but* ..... it rains. How is there a difference between protecting from water intrusion from rain versus a sprinkler? fwiw - one of the reasons I plan to enclose the dome (versus just leaving it exposed with stucco exposed) is to give protection to t he dome from rain, sleet, and potentially deep winter snows. so .... with an appropriate seal between the "house", slab, base junction, penetration of water from the outside can be protected against, but wicking? the only things I can think of to protect against that is your idea of an additive to the concrete and maybe appropriate sealants on both sides of the cinder block stand..

*but* - that actually brings up a different question (though we are diverging from the title of vermiculite in the house. My comment about the watering system actually dealt more with issues around cutting and rearranging existing watering system piping ..... *one of the options I have* is to locate the oven just off the side of a patio in the back yard. However, this would entail digging in to a slight rise or "berm" in the ground. Hands down in terms of location that would be best .... but it would entail digging, **and eventually having something like 1/3 to 1/2 of the lower portion of the cinder block base "buried" and not above grade. I have worried about that from the perspective of keeping the wood inside dry - but is there an issue if I do that in terms of water damage to the dome or especially the insulation under the floor????

[rchamlen]

It would look remarkably like this (from the gallery) .... but with a streamlined roof structure...

oops .... the forum wont let me post photos yet ..... will do so as soon as I can. but the one in the photo is very, very similar to house/hearth construction I would like, and also how it is backed up into a retaining wall with a rising ground behind it....

[david s]

The difference between water intrusion from rain and that from a sprinkler frequently wetting the stand is that water entry from one comes from above and the other from below. I don’t know about your weather conditions, so you need to be the judge. I’d be thinking hard about not having the stand above ground where ever it’s built. We live in the tropics and during our wet season, even if there hasn’t been rain, with consistent 90% humidity the oven will pick up moisture and require an extended firing to get it back to normal operating conditions. I build igloo style and totally waterproof the dome and supporting slab to prevent rain entry. Drying the oven is not really a problem, most of my ovens up here are out in the open as it’s not really worth the extra labour and expense to construct a purpose built roof to house it.

[rchamlen]

ok .... here is the one that would be very very similar to what I am thinking of building. retaining wall behind, but the front of the base almost flush with the retaining wall. The landing/mantle in front of the oven is also at the same level as the floor of the entrance, which is something else I would like in the build (why dont more people do that???) simpler single pitch roof, brick (or thin brick over cement board) instead of stone (to match our house). Most likely a metal roof (durability)

[rchamlen]

The difference between water intrusion from rain and that from a sprinkler frequently wetting the stand is that water entry from one comes from above and the other from below. I don’t know about your weather conditions, so you need to be the judge. I’d be thinking hard about not having the stand above ground where ever it’s built. We live in the tropics and during our wet season, even if there hasn’t been rain, with consistent 90% humidity the oven will pick up moisture and require an extended firing to get it back to normal operating conditions. I build igloo style and totally waterproof the dome and supporting slab to prevent rain entry. Drying the oven is not really a problem, most of my ovens up here are out in the open as it’s not really worth the extra labour and expense to construct a purpose built roof to house it.

Ahhh - got it. Yeah - I definitely would not have the sprinklers set up so they were directly and frequently watered. Most likely the system would be rearranged so that any sprinkler heads near the thing would spray *away* from the base. It would, though, be *really* nice to build the thing back into the berm........

A few days ago it was 18 below zero (F) (Minnesota) , and in a couple days we are heading for two days of 14 below zero (F) temps. A couple weeks ago I slipped on the ice and took a hard fall on my back (hence my earlier comment about not being sure if my back could take a brick build - though the process looks cool). ***please** do not tempt me with talk about living in the tropics :-) . Why was it I moved here in the first place????? :-)