Under Floor Insulation Qld Aus

[Will4740]

Hi Guys

I am in the stages of planning my Pompeii build to start in early January but am still wrestling with what to use for the underfloor insulation.

at the moment my plan is to have 100mm hebel then a 40mm calsil board and then 75mm fire bricks on top of that I am just not sure if the hebel will be able to withstand the force of the oven on it? I am sure someone has given it a go and i would love to hear how it went.

Cheers Will

[david s]

Re: Under Floor Insulation Qld Aus

Under compression it's fine. Not so good for tensile strength. It al depends on what's under the Hebel. It's about a third the strength of standard concrete. Do you have anything structural under it? If not you could reduce the span by cantilevering which would increase the strength.

[wotavidone]

Re: Under Floor Insulation Qld Aus

Couldn't find the compressive strength of Aussie Hebel, but the US Hebel Technical Manual lists compressive strengths from 290 to 580 PSI.

That's 290 pounds of weight per square inch.
Thinking on this, say your dome and floor weigh, I dunno, 500kg/1100 pounds. And say that weight is spread over a square metre. It works out to less than 1 pound per square inch.
In theory, the Hebel should be well capable of supporting the weight of an oven.
Several people have used it, and they all say it works. I've built a couple of ovens for people who have insisted on using it, one of which has done nearly two years under what I would call high use conditions for a domestic oven. The floor bricks are laid straight on top of the Hebel in these two ovens.
Cobblerdave says his oven has given at least 4 years (I think) of service now. The floor is laid straight on the Hebel, but he recommends that you use a layer of calsil between Hebel and brick to be on the safe side. I agree with that recommendation.

I recommend the Hebel is laid over a structural slab of regular concrete. Compressive strength is one thing, bending resistance quite another.
I'm assuming you propose to use the Hebel for extra insulation, I'd be interested to see if other forum members think there is any need for it, given you have 40mm calsil board.
I'm tipping that calsil might be enough insulation on its own, and that you could save a significant amount of money by simply laying a normal 4 inch reinforced slab, putting the 40mm calsil on that and laying the bricks.

[TropicalCoasting]

Re: Under Floor Insulation Qld Aus

If you wanted to have an alternative to a slab or some support for under your hebel you could add some hardie panel
James Hardie 2400 x 1200 x 15mm 2.88sqm HardiePanel Compressed Fibre Cement Sheet I/N 0715463 | Bunnings Warehouse
Its a bit exxy though
My veranda is made out of it with just waterproofing and tiles so it should be fine with the weight load.
It will still need insulation though if you are going to use it without the hebel

You might be able to get an off cut from some builders.

[Will4740]

Re: Under Floor Insulation Qld Aus

Cheers for the quick replies

I am not using is as the structural layer, in my situation I have decided due to cost and availability to weld a steel frame together of 50mm or 2'' square tubing which is directly under the bricks, (building a 36'' pompeii so the frame roughly 900mm square, this will load the frame directly in compression), then reinforcing with cross braces etc. with 15mm compressed cement sheeting on top and then building on that.
This is why I am thinking about adding the hebel level, 1st to add extra insulation as i don't have a concrete slab as mentioned and 2nd to try and lift the calsil board off the floor as I have read that it is easily ruined by moisture.

[wotavidone]

Re: Under Floor Insulation Qld Aus

Cheers for the quick replies

I am not using is as the structural layer, in my situation I have decided due to cost and availability to weld a steel frame together of 50mm or 2'' square tubing which is directly under the bricks, (building a 36'' pompeii so the frame roughly 900mm square, this will load the frame directly in compression), then reinforcing with cross braces etc. with 15mm compressed cement sheeting on top and then building on that.
This is why I am thinking about adding the hebel level, 1st to add extra insulation as i don't have a concrete slab as mentioned and 2nd to try and lift the calsil board off the floor as I have read that it is easily ruined by moisture.

The basic idea is sound, but I should mention a 36 inch pompeii will not fit on a 900 x 900 area. 36 inches inside diameter is about 915 mm, then you need to allow say 100 mm each side for wall thickness, plus 100 mm each side for insulation, plus about 20 mm each side for render or what ever outside treatment you plan. So a 36 inch pompeii needs a slab about 1340 wide, minimum. Maybe that's what you mean? 900 by 900 frame topped by a 1340 platform?
I've been refreshing my memory on the insualtion properties of the materials. You are going to have a seriously well insulated floor.

[Will4740]

Re: Under Floor Insulation Qld Aus

Haha yeah when i say 900mm i mean that is where the steel frame sits so it will be directly loading the frame and not loading it at a distance causing a larger load. The cement sheeting will be larger about the size that you said i believe would have to check CAD. And yeah I would be interested to see if people think the hebel is even necessary. I just didnt want to under insulate the floor and then be regretting it down the track when it doesn't hold heat.

Cheers will

[david s]

Re: Under Floor Insulation Qld Aus

If I were in your situation I'd be skipping the Hebel, but doubling the cal sil so you have 80mm of it. Regarding the cal sil dissolving when wet, I tried soaking some of mine in water for a few days, it did not dissolve and dried out to normal again. The stuff may vary from manufacturer to manufacturer so can't say some stuff would be without problems. Someone else may chime in.