Barrel Vault Design in NC

[david s]

Ron,

the sand/clay levelling mix can be done wet or dry. If left dry it allows some free movement of the floor bricks to expand and contract. Also it enables replacing any floor bricks easier should that need ever occur in the future. It will take more than a week to dry the 3" vermicrete layer. More like 3-4, but it will eventually dry from firing. If good drying weather though, it doesn't hurt to leave it uncovered for longer if possible before building over it. See my experiment attached.

Dave

Vermicrete insulating slab copy.doc.zip

[Ron-NC]

The insulation was placed on the hearth slab today and the 5:1 perlcrete poured around the Roxul insulation. I decided to make the perlcrete 3" thick and will then place 2" of the fiber board on top of the perlcrete (the photos do not show the fiber board yet). The arch bricks and walls will then be placed on the perlcrete / fiberboard area. I mixed the 5:1 perlcrete with 30% water by volume, of all the dry ingredients - something I read from an earlier post from David. This seems to be a very good bench mark with the materials I used - thanks again David, you are truly a wealth of information on this site.

I will let the perlcrete setup for a week before I place the two layers of floor bricks. Next I will pick through the bricks and select the best I have for the floor (the 100 yr old red clay bricks have a lot of variation).

Once I lay the first floor layer of red clay bricks, I am thinking of using 50/50 fine sand / fire clay for the leveling agent. Does this make sense or would it be better to use home brew to better mortar the first brick layer in place. I can use either but would want to use whatever material would best promote heat transfer from the fire brick to the red clay layer - thanks
ron

[Ron-NC]

thanks Utah for the Perlcrete ratios; hopefully I am not being penny-wise and pound foolish about the floor insulation!

[Pizza Julian]

Thanks . I also found sports grade at ACE apparently as marker etc

[UtahBeehiver]

I miss spoke should be 5 to 1 for floor. 8 to 1 is for dome insulation. David brings up a good option of pouring stilts of p-crete to support the floor. We are just wanting you to be successful.

[Ron-NC]

Thanks guys about the wedge advice; I'll cut some access holes to reach some wedge shaped devices

[Toomulla]

Utah, you are totally correct about the compression info. This Roxul board is definitely spongy (I cut both insulations today). So, thinking about the sequence, I am thinking it would be best to layout the insulation and then pour the perlcrete around the insulation to minimize any lateral movement of the Roxul from the bricks. Then, after the perlcrete sets, I would add the bricks which hopefully would compress the Roxul more uniformly. Are you suggesting that an 8:1 mix of perlcrete would still provide the support in this rather unusual application. Time will tell if this less expensive approach works!

David, I have a bunch of 1 1/4" X 1 1/4" X 5 foot long wood that I am planning on putting under the form, which I was thinking I would be able to easily pull out. Do you think this will work or am I missing something? You are right about the absorbancy of the Roxul, I will have to make sure no moisture creeps into this oven.
ron

you need wedge shaped timbers to allow the form to drop or it will not release without wrecking the arch or vault so you need to be able to reach in and tap them loose do not underestimate how tight they will be

[Ron-NC]

Utah, you are totally correct about the compression info. This Roxul board is definitely spongy (I cut both insulations today). So, thinking about the sequence, I am thinking it would be best to layout the insulation and then pour the perlcrete around the insulation to minimize any lateral movement of the Roxul from the bricks. Then, after the perlcrete sets, I would add the bricks which hopefully would compress the Roxul more uniformly. Are you suggesting that an 8:1 mix of perlcrete would still provide the support in this rather unusual application. Time will tell if this less expensive approach works!

David, I have a bunch of 1 1/4" X 1 1/4" X 5 foot long wood that I am planning on putting under the form, which I was thinking I would be able to easily pull out. Do you think this will work or am I missing something? You are right about the absorbancy of the Roxul, I will have to make sure no moisture creeps into this oven.
ron

[david s]

Today the dome form was finished and I started laying out bricks to see how the bricks would lay out on the slab, There is still much confusion about how the chimney arch should be built but I will worry about that later. The dimensions of the floor are finalized so the hearth insulation can be cut to size.

Make sure you put some wedges under the arch form so when you want to remove it the thing will drop a little.you will need to cut some access holes in the front board so you can access the wedges at the rear.

[david s]

Maybe you could cut some strips of the stronger insulation board to place down first, then in fill with the Rockwool. Or if you don'5 have enough you could create some stilts with 5:1 vermicrete. Unfortunately Rockwool insulation has a higher water absorbency rate than pretty much any other insulation type.

[UtahBeehiver]

Hope you are right. The Roxul Comfort board 80 has compressive strength at 10% is 439 PSF which equates to abt 3 PSI. I kind of equate this to putting on home slab on a old land fill that has not been compacted versa building on compacted structural fill. But it is your choice. Why not consider using 8 to 1 pcrete under the floor it almost has the same K value as CF board and inexpensive and probably cheaper than the Roxul board.

[Ron-NC]

Today the dome form was finished and I started laying out bricks to see how the bricks would lay out on the slab, There is still much confusion about how the chimney arch should be built but I will worry about that later. The dimensions of the floor are finalized so the hearth insulation can be cut to size.

[Ron-NC]

Thanks for the specs Utah, I did think the comfort board was 8 psi.
I am hoping the 1" ceramic fiber on top of the Roxul will distribute the weight evenly in the event there are any soft spots on the Roxul. The double brick floor is floating on the insulation, so if the Roxul compresses slightly it should not matter. This is quite hypothetical on my part though, so we'll see what happens. Worse case I can redo the floor since it is floating i.e not attached to any walls at any point.

[UtahBeehiver]

I looked at the cut sheet for the Roxul Comfort Board and it a good thermal ratings. The compressive strength is quite low abt 3 PSI at 10% compression vs 90-100 PSI @5% compression for CaSi board.but since you are ringing the wall/vent sections with 5:1 pcrete the only potential would be possible settling of the brick floor since I recall your original design is red clay bricks topped with fire bricks. Just take a look at the compressive strength not just the density before you make the final decision.No going back one the floor is down.

[Ron-NC]

While I wait for the hearth concrete base to cure, here are some specifics of the floor plan. I am using two types of Roxul rockwool insulation for the most part (comfortboard and batt), but did recently purchase 1" ceramic fiber board as the first layer of insulation. For example, the area under the hearth bricks (excluding the arch walls) will have 4" of Roxul topped with 1" of ceramic fiber board. Since the Roxul board is not as dense as the cermic fiber board, the Roxul hearth area insulation will be surrounded by a 5:1 perlcrete base on which the arches and walls will sit. I have read from many builders that 5:1 perlcrete should be plenty dense to support the arches. The insulation is done in this fashion to bring down the cost of the project as the Roxul product is 1/3 the cost of fiber board.