Strengthening Rockwool underfloor insulation ?

[Passaro]

Hello all and a huge thank you (especially to david s for cast ovens) for the wonderful wealth of information on this forum, it’s really great.
My friends and I are building a 70cm (28”) mobile cast oven. And for the hearth insulation, we are planning to use rockwool (we have it for free).

I know this material is not ideal, especially for underfloor insulation (moisture / compression issues).
I’m also aware we could just cast the entire insulation in vermicrete instead as it’s cheap. If needed, we'll do it.
But I’m still curious to see if we could make it work with rockwool, as a design challenge.

So I’d love to have your feedback on the following ideas :

As for moisture problems, they will be addressed by weep holes in the slab + raised ceramic tile channels.
For compression issues, it's more delicate.
I’ve calculated the oven will be around 150kg. We don’t have the data sheet for our rockwool, but I know this material's compressive strength is limited in general.
So I don’t want the hearth to shift over time if the rockwool compresses, especially in a mobile oven.

To counter this, I see four possible methods :

1) Stacking three slabs on top of one another (each slab is about 5 cm – 2”) so that more overall strength is achieved.
=> I’m not sure this actually makes sense.

2) Topping the insulation stack with a more solid layer (IE Vermicrete 5:1 or cement backer board), so that it is the one to take the direct load. (We would probably do this in any case to have a smooth surface to apply sand + clay on for brick levelling.)
=> I wonder if this will have any use in terms of compression. The rockwool will still take the same weight from the oven.

3) Using the concrete hearth slab itself for support.
We could cast it with raised edges so that it encloses the insulation tightly => essentially turning the slab into a frame. I see two ways to accomplish this :

- Either by simply encasing the insulation in the concrete – see option A on the drawings below.
- Or by having the solid top layer of the insulation partially rest on that frame (outside of the cooking floor area) to spread part of the oven load on that frame – see option B on the drawings.



However, I’m wary of this idea, because :

• This would add a lot of weight, as we would have a ring of 75mm concrete in supplement to the original slab.
• Concrete being porous, and as the slab / frame is cast in one piece, moisture could maybe travel from the base slab up the raised edges into the insulation ? does that seem possible to you ?
• I’m not even sure it would actually be much of an improvement in terms of compression strength ? what do you think ?

4) Having a rockwool core encased in a vermicrete container.
(The pieces of the container would be cast and dried separately from the rockwool so that vermicrete moisture would not get trapped in the rockwool.)



This method seems the most promising to me, but I still wonder :

• If it would be enough in terms of improving compression ?
• If yes, how thick to actually make the walls of the container ?
• david s has mentioned angling the edges of the slab slightly downward to prevent rain from pooling. Am I understanding the idea correctly ?

I’d love to hear your thoughts on this ; and if you have other ideas I didn't see, I’d be glad to hear them.

Thanks !

[david s]

I've used a lot of different materials for insulation and I'm a great believer in experimenting, contrary to the opinions of others who advise the unsuitability of certain materials. But, of all the insulating materials I've tried I would consider Rockwool bats to be the most water absorbent material I've ever seen. This of course presents problems. Not only is it very absorbent, it also compresses very easily and if its thickness is reduced by compression then its insulating value will be reduced by the equivalent amount. Compared to ceramic fibre blanket the Rockwool is far more compressible, a lot harder to cut and fit, while being a bit cheaper and for the equivalent thickness a slightly poorer insulator. For these reasons I use the CFB for over the dome insulation, having used the Rockwool for two ovens. For underfloor insulation the same problems apply. However if you got it free then that is a major consideration.

A mobile oven is subjected to quite a lot of road trauma. Make sure you fit shocks on the trailer. I think your plans might result in collapse of the floor in the centre.

For my first oven, back in 2007, I scored some free foil backed fibreglass insulation and after advice from forum members not to use it because the binder used in the fibreglass wouldn't stand up to the heat, I decided to go ahead anyhow and overcame the compression issue by cutting some insulating firebrick to act as props to support the dense firebrick floor bricks, which were pretty big. After a couple of years use, with the oven performing nicely, I decided to rebuild it as it was too small and of an updraft design (flames out the top of the oven) that wasn't as efficient as a front flue cross draft design (present oven in signature pic is around 14 years old now). On dismantling the oven both the underfloor and over dome fibreglass insulation was still in excellent condition with no sign of deterioration. Fitting the overdome fibreglass insulation and trying to avoid compression was a real bitch.

I think you may be able to use the Rockwool using a similar system as illustrated. Its big advantage over a cast vermicrete slab is that it's already dry so you are not adding more .water which requires removal.

The slope on the perimeter of the slab discourages water wanting to pool at the dome to supporting slab joint and enter under the insulation, but it doesn't have to be as pronounced as you've illustrated. Any moisture under the floor will want to run away from the fire, but it does need an escape route provided by the weep holes.

[Pizzarotic]

To counter this, I see four possible methods :

1) Stacking three slabs on top of one another (each slab is about 5 cm – 2”) so that more overall strength is achieved.
=> I’m not sure this actually makes sense.

No, stacking more layers will not add any strength and your floor bricks will collapse.

2) Topping the insulation stack with a more solid layer (IE Vermicrete 5:1 or cement backer board), so that it is the one to take the direct load. (We would probably do this in any case to have a smooth surface to apply sand + clay on for brick levelling.)
=> I wonder if this will have any use in terms of compression. The rockwool will still take the same weight from the oven.

You still have a flexible and unstable substrate and your floor bricks will collapse.

3) Using the concrete hearth slab itself for support.
We could cast it with raised edges so that it encloses the insulation tightly => essentially turning the slab into a frame. I see two ways to accomplish this :
- Either by simply encasing the insulation in the concrete – see option A on the drawings below.
- Or by having the solid top layer of the insulation partially rest on that frame (outside of the cooking floor area) to spread part of the oven load on that frame – see option B on the drawing.

Option A, your floor bricks are still unsupported
and will collapse.

Option B, the solid insulation layer is not strong enough to provide support in the centre of the dome and your floor bricks will sag.
.

4) Having a rockwool core encased in a vermicrete container. (The pieces of the container would be cast and dried separately from the rockwool so that vermicrete moisture would not get trapped in the rockwool.)

The vermiculite box provides nil strength to support the weight and your floor bricks will collapse.

what do you think ?

I don't think either of these options will end well for a mobile oven. Use the rockwool for dome insulation and choose a more suitable material for the hearth.

[Passaro]

Alright ! Thanks a lot to the both of you.

[Pizzarotic]

If you are dead set keen on using the rockwool for underfloor insulation then you could possibly use a thick cement board on top, at least 1/2" but 3/4" would be better. Don't use any support around the outside edges, just have the whole board sitting on the rockwool so it is evenly supported. Otherwise, maybe a steel plate instead of the cement board?

[Passaro]

Yes, I also saw this suggestion on other topics in the forum.
But seeing as one of our objectives is to minimize cost, I think we'll rather go full vermicrete as it is cheap