Thickness of Vermiculite/Concrete Insulation Base

[david s]

The primary goal for floor insulation is simply to isolate the oven structure from heat loss. Having vermicrete (not just vermiculite) below a sheet of Calsil is more of a cost saving method than anything else. Note, using just vermiculite or perlite does not provide adequate compressive strength...that's why using 5:1 (*/cement + a dash of clay) is recommended for base insulation. Also, you need x2 as much of the 5:1 mix to provide roughly equivalent insulation value as offered by Calsil (2.5 cm Calsil offers about the same insulation value as 5 cm insulating concrete mix). Recommended minimum insulation for the base of a WFO is 5 cm (2") of Calsil or 10-12 cm (4") vermicrete. Although you can save some money using a layer of vermicrete, it takes significantly longer to cure and completely dry (which is essential to creating an effective insulation).

Hope that helps, comes down to creating your balance of time & cost in creating the all important adequate insulation base for your WFO.

Thanks Mike, while this is true, the reason for adding cement to the vermiculite or perlite is to provide the adequate strength. This is explained well in the attached table, which shows for instance at 5:1 there is more than double the mass in the cement added to the mix, which in turn reduces the insulation value enormously. However dry perlite or vermiculite actually has a higher insulation value than cal sil, so a dry bed of either over which a cal sil board can sit, will be a superior insulation solution. Some off cuts of calsil board can be used as props if compression of the dry material is still a concern. The caveat is that the dry material must be contained at the sides so it won't ooze out. The added advantage of this method is that there's no moisture to eliminate.
My experiment on drying a vermicrete slab could be instructive.

Drying a 2" thick vermicrete insulating slab

The purpose of this experiment was to calculate the water loss from a vermicrete insulating slab after casting and placement.

Materials Kg Vol (L)

Vermiculite 1.82 27.00

Cement 7.25 5.40

Water 10 80 10.80

Total 17.17 21.60

For the aggregate in this mixture a 50% vermiculite 50% perlite proportion was used.
The mix is generally considered a 5:1 vermicrete and comprised of vermiculite, perlite, Portland cement, water 2.5, 2.5, 1, 1.5 by volume.
. The slab was exposed to approx 3 hrs sun/day top surface only.
Weighing of materials was done using digital scales, but weighing of finished slab was done using bathroom spring scales as the weight was beyond my digital scales capacity.

Although the top of the slab appeared quite dry the bottom was still wet so I lifted it up, stood it on edge to assist drying after Day14
Kg
Day 7 16.0 fine
8 15.0 fine
9 14.5 fine
10 14.0 fine
11 14.0 cloudy
12 13.5 cloudy
13 13.5 cloudy
14 13.5 cloudy
21 12.0 fine
28 11.0



Conclusion
Given the ideal drying conditions, relative thinness of the slab and that it was uncovered, the experiment still demonstrates the large quantity of water present and the slow rate of removal. For a thick covered slab expect the water reduction rate to be way lower. After 3 weeks of drying more than 50% of the water had been removed by the weather from the aprox. 2” thick slab. After 4 weeks approx. 75% of the water added had been removed. Given the 0.4 W/C ratio that consumes the water in the hydration process, that consumes 1.34 litres leaving 6.76 litres of free water. So after 28 days of drying 5 litres lost is pretty much most of it. This excess free water ensures that plenty is available for the hydration process so therefore no wetting of the slab is required during curing.

In hindsight I probably should repeat the experiment and cover it with firebrick, then compare the results. Given that the water under the floor and the base of the dome is the last to be eliminated and the resulting problems and damage it can cause, the drying of a vermicrete slab before building over it is advisable.