Insulation options for steel firepit

[bhh]

Hi all,
Long time lurker and I've learned a lot from you all - thanks. I hope this is the right forum...

We've poured a backyard patio around a 3' x 4' opening where we are placing a firepit. We had the firepit fabricated out of 1/4" Corten (weathering) steel and designed it as a double wall with a 4" cavity to insulate so the outer wall would not heat up too much. Both the inner and outer walls are steel.

I was originally planning on filling the cavity with sand but I'm now rethinking that as sand may be too conductive? Other ideas were pea gravel which would have more air pockets, Roxul (mineral rock insulation). I'm just sort of confused between convection, conduction, radiation, etc.

What do you all think? Whatever it is just needs to drain well as the top is open and whatever it is will have a layer of matching pea gravel sprinkled on top to match the surrounding mulch. Thanks!

 

[kennyp2339]

Fire brick

 

[begreen]

Perlite or vermiculite

 

[bhh]

Thanks guys. I've included a picture to make it a little clearer.

Firebrick is an option but I think I can get better performance out of something that will fill the cavity a lot more evenly and continuous than placing firebricks. I did a bit of research on perlite and vermiculite, those are very interesting materials. Of the two, I think perlite will drain better than vermiculite and probably be a better option. How much better would those perform over sand or gravel which I already have an abundance of on-hand?

 

[begreen]

I suspect Perlite would retain less water and drain better. It's a better insulator. Loose fill it's R= 2.7/inch.

 

[bhh]

I suspect Perlite would retain less water and drain better. It's a better insulator. Loose fill it's R= 2.7/inch.

Wow, R10 for that cavity is great. It will take about 6.5 cubic feet to fill that up so I'll need to find a source of it other than the Miracle Grow stuff from HD as the fertilizer will probably corode the steel too much. Where to people buy the stuff? I'll put a shallow layer of the pea gravel on top for aesthetics but this is exactly what I was looking for! Thanks.

 

[begreen]

Perlite is sold in very large bags in commercial greenhouse and nursery supply houses. Local chimney sweeps may have another supply. I got a large bag from our local lumberyard/nursery.

 

[xman23]

Why are you filling it? It's an open backyard fire pit, correct?

 

[bhh]

Correct but I'm filling it to insulate the outer layer of steel so people don't melt the bottoms of their shoes or burn their knees, etc.

My local Agway has 4 cubic feet bags. Thanks again everyone!

 

[tarzan]

Is there a bottom welded to the outer walls? If so, it will be tough keeping the outer walls cool. If not, I would try a test fire as is. The air gap may be enough.

 

[bhh]

Is there a bottom welded to the outer walls? If so, it will be tough keeping the outer walls cool. If not, I would try a test fire as is. The air gap may be enough.

Hi Tarzan,
No, there isn't a bottom. The bottom was left open so water will drain out and through the gravel bed this sits on. We left a hole in the patio that this drops down into as well so that the rust doesn't run down and stain the concrete. There are a couple of struts/bars you can see in the picture that hold the inner and outer walls together so the spacing stays consistent but we could actually cut those out once the unit is placed if we wanted. The perlite is cheap so I'm going to pick that up this weekend and give it a try since it certainly won't make it worse.

Thanks for all the input everyone. My wife as floored when I told her how many suggestions we got on here in such a short time!

 

[begreen]

Will the top be closed? Perlite will blow away with any strong breeze. It's very light.

Sent from my HTC One using Tapatalk

 

[bholler]

I agree with tarzan I think the air gap may be sufficient. And it will need to be covered in something if you plan on filling it with perlite otherwise it will get full of ash leaves ect and cause problems

 

[bhh]

The original plan was to put a few inches of the same pea gravel we used in the surrounding area on top of the sand so that it blends in with the surrounding hardscape. Will probably make sense to do the same thing with the Perlite as it will look a lot better and address some of the other comments you guys mentioned. We also talked about maybe doing a concrete, bluestone or tile cap on top so there was a place to set drinks, smores, etc. as well which we can always add later if the leaves and things become an issue.