Perlite Strength

[mrramsey]

Thinking of a cast oven build but had a question regarding the strength of a Perlite insulation layer for the hearth. I am thinking I would 'pour' a 4" thick layer in a form on the 3.5" hearth slab. Then cast my oven on top of that. Is the perlite mixture strong enough to bear the weight of the oven?

My concerns stem from moisture entering into the hearth slab and getting trapped under the insulation layer. Then add the freeze / thaw cycle of winters here in the northern states. My current design has a recessed area in the concrete slab for the perlite (p-crete). This would create a 'pool' for water to collect. Weep holes may work as far as helping moisture escape during the heating of the oven but I am not sure it will help with freezing conditions.

I should clarify that I was going to build an exposed dome oven (stucco finish no enclosure). The estimated weight of the dome is 1464 lbs. I estimate that the surface contact area bearing the weight to be 456 Sq In. so around 3.2 psi

[david s]

The attached table should give you some idea regarding strength. There is an error for the 5: vermicrete. It should read 175-225, not 175-125.
Most builders find the 5:1 mix has adequate strength.
I suggest you add the weep holes through the supporting slab as well as sloping the slab slightly, away from the insulation to discourage water from running into the basin, as it is common for a crack to form, after a couple of years use, between the base of the outer shell and the supporting slab. This can and should be inspected annually to prevent water entry there. We live in the tropics and our temperatures never get below 8 C, so have no experience re freezing, hope others will chime in.

Vermicrete PHYSICAL PROPERTIES 2 copy.docx.zip

[mrramsey]

The attached table should give you some idea regarding strength. There is an error for the 5: vermicrete. It should read 175-225, not 175-125.
Most builders find the 5:1 mix has adequate strength.
I suggest you add the weep holes through the supporting slab as well as sloping the slab slightly, away from the insulation to discourage water from running into the basin, as it is common for a crack to form, after a couple of years use, between the base of the outer shell and the supporting slab. This can and should be inspected annually to prevent water entry there. We live in the tropics and our temperatures never get below 8 C, so have no experience re freezing, hope others will chime in.

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Thanks David! I did find a similar chart online I do believe weep holes are a requirement to allow any stored water a point of egress when the oven is fired up. I think the only practical way to eliminate 99% is a full enclosure which I do like but my current build location is less than desirable due to the structure being taller and obstructing the view from inside of the house.

[Gretsch]

I’ve also cast a Perlite slab and am thinking of rendering it with a good water tight layer of concrete over the dome and down over the hearth. I think it should prevent water getting under it, but will it crack ?

I’m also concerned over the strength of the Perlite layer, just 2 days of curing and it still looks fairly wet. Hoping it will set nice and hard.