Adding thermal mass under thin hearth

[blink]

I have managed to source some very good value vintage firebricks, 18" x 9", which I am picking up at the weekend (so I haven't seen them in the flesh yet).
They are only 1 1/5" thick though, so a bit too thin really. My plan was to make a 4" thick insulated concrete base and then simply pour a 1 inch or so layer of 3:1:1:1 homebrew on top to act as a base for the thin hearth brick, thereby taking the effective thermal mass floor thickness up to 2.5, complementing the dome thickness (blocks made from cut up storage heater bricks).
Does that sound like a sounds plan to boost the effective hearth thickness under the old firebricks?

[Kvanbael]

Do you have enough of them to do 2 layers?

[blink]

No, only enough for one layer, hence my need to beef up the mass.

[SableSprings]

Blink, when you say 4" of insulated concrete for the base...I'm assuming you meant a 4" reinforced, concrete support base with 4" of pcrete or vcrete (or 2" CalSil board) insulation on top. Then you'd be thinking of pouring a 1" thick home brew pad on top of the insulation layer. (Insulating concrete is not structurally adequate by itself to support the weight of the oven.) Cooking floor (thin) bricks would then be laid on top of the home brew.

Don't forget that you will want to drill some weep holes in your concrete support pad with possibly some porcelain tiles laid between the concrete and base insulation (to enhance moisture escape).

Posts #8, 18, & 21 are good illustrating pictures of these concepts in the link below:

https://community.fornobravo.com/for...andpoint-idaho

[blink]

Sorry, I don't mean 'base' really. I should have said that it's being built on an existing retaining wall/patio area so has pretty good foundations already, the insulation layer will be sitting on flagstones which are on solid mortar which is on hardcore which is on compacted earth. So although there is no reinforced concrete there a good stable base that I am pretty confident will cope with the load. Especially as I am building a housed design with loose fill insulation, so it's only a smallish 30" dome with no vermicrete or render layer. My insulating layer is being made with aerated clay which is a bit more robust than vermicrete, although I should probably add and inch to make up for the slightly greater conductivity.

As I'm building a watertight enclosure do I still need to worry about weep holes, how is the moisture getting into the insulation layer, from the oven contents?

[SableSprings]

The weep holes are to give moisture another path out of the oven base. Since you're on a solid base, there's "nowhere to weep" . I'm glad that you're planning on a watertight enclosure (assuming you are building a dog house type structure around or cover roof over the oven). Be aware that you want to keep water from seeping in around the outer base perimeter of the dome (on top of the flag stone). Again the extent of your watertight enclosure is the key.

We have members in high humidity or heavy rainfall areas where rain has been driven into the oven opening under the door or simply condensation problems during long times between firings. This is generally not a serious problem, just takes a few firings to completely dry out the base insulation. Also glad to hear you'll add a bit thicker layer of that aerated clay base insulation.

[blink]

Yes, thanks. I'm planning on a making a pent roof kennel from steel studwork and Hardi backerboards. Slate tiled on the front around the arch and charred & oiled ('shou sugi ban') wood panels on back and sides. I'll put a damp proof membrane between flagstones and insulation - it won't get too hot for plastic through 5" of insulative concrete will it? I'll make a half inch silicone bead around the stud work base and the side panelling will be kept just above the horizontal surface too. Ceramic blanket over the dome and loose filled with the aerated clay.

[SableSprings]

Sounds like a pretty good plan Blink! Do be aware that because the plastic will keep water from migrating up from the flagstones, it will also retain the water in your poured insulating concrete. This is not a problem...it will simply take a few more curing fires to completely dry it out. The longer you can leave the base layer exposed to "air dry", the better. Five inches of insulating concrete should be plenty to protect the plastic from melting ()...make sure you use black plastic because clear plastic can be more prone to deterioration over time.

Looking forward to your build thread.

[blink]

Thanks SableSprings.
I'll use the proper black textured DPM which is pretty hardwearing in all sorts of conditions, there's no UV going anywhere near this stuff as it will all be enclosed so I'm not worried about that.
I think the DPM is HDPE which has a melting point of 130C (266F) which surely I won't get under 5" of insulation.
How long should I let the aerocrete dry before pouring my 1.5" of home-brew on top for the extra thermal mass?
I was think a couple of days would be enough if I then leave the whole combined slab for another 2 weeks before starting firebrick fitting and dome build. If I have to let the insulation fully cure before adding the thin thermal mass layer it's going to be a very slow build.

[MarkJerling]

I have managed to source some very good value vintage firebricks, 18" x 9", which I am picking up at the weekend (so I haven't seen them in the flesh yet).
They are only 1 1/5" thick though, so a bit too thin really. My plan was to make a 4" thick insulated concrete base and then simply pour a 1 inch or so layer of 3:1:1:1 homebrew on top to act as a base for the thin hearth brick, thereby taking the effective thermal mass floor thickness up to 2.5, complementing the dome thickness (blocks made from cut up storage heater bricks).
Does that sound like a sounds plan to boost the effective hearth thickness under the old firebricks?

Another storage heater project! I like it! (I did the same)

[blink]

Mice Mark. Don't you find that huge thermal mass takes a lot of time and fuel to heat up?
I'm planning only a 30" dome so was going to cut each brick into 6! That will give me bricks that are roughly 90 x 70 x 50mm, although with the lip(s) I will probably have to taper each one to eliminate interior mortar joins. This will give me units that are about 3/4 the size of the standard firebrick method which seems appropriate for a dome that 3/4 the diameter of the standard. So my dome should be 70mm thick (my floor will be about 65mm thick). It's a lot of cutting but I'm hoping a 10" diamond blade on a sliding mitre saw will make it a bit easier that freehanding it with a 9" grinder.

[david s]

Mice Mark. Don't you find that huge thermal mass takes a lot of time and fuel to heat up?
I'm planning only a 30" dome so was going to cut each brick into 6! That will give me bricks that are roughly 90 x 70 x 50mm, although with the lip(s) I will probably have to taper each one to eliminate interior mortar joins. This will give me units that are about 3/4 the size of the standard firebrick method which seems appropriate for a dome that 3/4 the diameter of the standard. So my dome should be 70mm thick (my floor will be about 65mm thick). It's a lot of cutting but I'm hoping a 10" diamond blade on a sliding mitre saw will make it a bit easier that freehanding it with a 9" grinder.

Make sure you use a wet saw. The dust from brick cutting is very dangerous to inhale, It also kills the electric motor if it gets into the intake. If you rig up a water feed to the blade you might be in danger of electrocution, be careful.

[blink]

I have a wet 'bridge tile saw' which is excellent but doesn't have the clearance for brick. I am aware that the levels of dust will be crazy and have appropriate breathing protection but I am concerned about killing my saw. I do have vacuum dust extraction built in but I know this will only deal with <50% of it. I thought about presaoking the bricks and seeing if that helps. Too much water will make a slurry that the wet tile saw design can deal with but could be an issue for the chop saw. I have an old saw which I could use. Maybe I will try running a water supply with the chop saw, being careful. The protections built into my house electricity supply are excellent.

[Wickolad]

Hi blink, don't know how far you've got with this, but I used a sliding compound mitre saw for the brick cutting. Bought a diamond saw blade and attached my Henry hoover to the dust extraction port. Presoaked the bricks for a good hour or so, even longer at times and worked fine. Due to density of my bricks there was still dust but much reduced due to vacuum extraction. The brick dust may then be used incorporated into your homebrew.
wearing eye protection and a good dust mask are essential.

Best wishes,
Lance.

[blink]

Thanks Lance - indeed, I put a diamond blade on my chop saw, soaked the bricks and attached vacuum. Plans changed quite a lot since original idea and construction is nearly complete. Many hundred of cuts made to turn storage heater bricks into appropriate oven subunits. I have loads of photos and will create a thorough report of the build as soon as I have time.