Experience with std. concrete dome cladding?

[Audionatic]

I know this has been discussed, and I've tried to read all the posts describing the recommended refractory concrete dome cladding. I've priced the KS-4+ castable refractory available locally, and it'll be several hundred dollars to clad the dome. I'd previously decided to invest more heavily in CF blanket and board insulation, which is also expensive. I'm looking to reduce the cost to finish the oven build itself, and am wondering if there's a way to use regular sack mix, and still have an intact, efficient barrel-type oven a couple decades down the line. I'm planning a 2-3" cladding layer, over foil, over the dome firebrick. I'll insulate the clad dome as heavily as I can afford, using a combination of board and CF blanket. Hearth insulation will be at least 4" of FBX1900 board insulation, "rigidized", to reduce any issues with moisture down the road. I'd appreciate opinions from the many of you that've used other than refractory cladding, and what your thoughts are now that you've been running your ovens for a while. Can I use any portland cement product, or after all this, is it foolish to substitute for the refractory castable.
Thanks very much.

[david s]

Re: Experience with std. concrete dome cladding?

There is plenty of thermal mass in the dome bricks so adding an additional 2-3" of cladding is not necessary, unless you are keen on baking multiple batches of bread from one firing. Additional wall thickness means more fuel consumption and longer time to heat those walls.
Many builders, me included, have found the use of foil counterproductive in that it acts as a moisture barrier making water elimination difficult. This can present problems when drying the oven after heavy rain periods. using proprietary castable for a cladding layer is expensive and a waste of money IMO and the home-brew of 3:1:1:1 sand, portland, lime, clay works perfectly adequately for the temps we fire to.
Insulating the dome with board will be difficult because it won't conform to a curve easily unless you cut it, probably better to go with blanket and vermicrete over that.

[Audionatic]

Re: Experience with std. concrete dome cladding?

Thanks for the reply David. I do expect to do some retained heat baking and roasting. I have plenty of room for a cladding layer, and can apply that with the vault "warm" so I can eliminate the foil layer I guess. I may want a little more thermal mass. Would you prefer the propylene fibers or stainless needles in the mix? I rigidized a box of 2" board insulation today, and will either kerf it to form to the cladding, or form the cladding into a doghouse, and use intact boards, over 2-3 layers of 1" blanket. I'm not as concerned with the moisture issue, since the rigidizer reduces or eliminates absorption by the board or blanket material. Anything else you might suggest to help durability?

[david s]

Re: Experience with std. concrete dome cladding?

Thanks for the reply David. I do expect to do some retained heat baking and roasting. I have plenty of room for a cladding layer, and can apply that with the vault "warm" so I can eliminate the foil layer I guess. I may want a little more thermal mass. Would you prefer the propylene fibers or stainless needles in the mix? I rigidized a box of 2" board insulation today, and will either kerf it to form to the cladding, or form the cladding into a doghouse, and use intact boards, over 2-3 layers of 1" blanket. I'm not as concerned with the moisture issue, since the rigidizer reduces or eliminates absorption by the board or blanket material. Anything else you might suggest to help durability?

The polypropylene fibres will increase compression strength but not torsional strength. They are added to allow a degree of moisture to escape reducing steam spalling damage. Stainless steel fibres can be added to increase strength, but check the cost, they are expensive and not worth adding to this layer IMO. My ovens are cast and have walls that are 2" thick, which gives the oven plenty of power to cook a load of bread or the family roast. If you are doing bricks then you oven will have 4" walls that should be more than enough, but everyones needs are different. if building a dog kennel style you can simply add your insulation dry and therefore reduce the problems of having to drive out the water.

[Audionatic]

Re: Experience with std. concrete dome cladding?

Nice to think simpler is better in this case. I'll get this clad with 1"-2" on my dense firebrick, insulate the heck out of that, and call it good. Thanks again.