Concerns about insulation underground - advice?

Hi, I'm asking here because I'm sure many of you, particularly those with climate battery setups, use underground around the perimeter.   Are there any concerns with XPS being in contact with the earth regarding chemicals leeching out, or contaminating the soil by the XPS breaking down?  

I am interesting in installing a climate battery system but I am also currently adding XPS around the perimeter foundation of my house.  I hate plastics, but need something more efficient than organic materials.  I.E. I don't have the ability to use hay or anything unsightly.  I would prefer to stick to the question about using XPS and if there are any cautions others have based on experience.  thanks

Can you tell us what XPS means in this context?  All the many meanings of that acronym I know don't seem to apply.  Maybe it's a regional thing.  

Extruded poly styrene.  Insulation board

Buried with zero light and no heat or oxygen is about as inert as you can get it.  Any non plastic options are much more expensive.  The one issue we have is bugs and rodents LOVE to tunnel through it, so you need to make sure you have a solid barrier between it and the dirt.

I’ve never used it in a greenhouse, but have had to use it in home construction. I’m a framer by trade

XPS is eXtruded PolyStyrene. It's the same polymer as EPS - Expanded PolyStyrene, but blown up into a foamy consistency with a different process. XPS uses lots of very-bad-for-the-climate gasses as blowing agents, while EPS uses relatively inert gasses. These gasses will leak out over time, so the harm in using XPS is collective harm to the atmosphere, not a local contamination. It's bad for the climate in the same way as releasing old CFC refrigerants were.

Otherwise the polymer itself is essentially a solid form of petroleum, and, like most plastics, very toxic when burned, so it is treated with flame retardants. Now you've added two different toxic gicks to your soil. If you believe the industry propaganda, it will not break down in contact with the soil (stable for hundreds to thousands of years), but UV will degrade pretty quickly (years to decades), so keep it covered. Reading between the lines, I expect that conventional wisdom to slowly change. Firstly, XPS can be abraded by physical action into micro and nano plastic particles which will become very mobile in soil, water and bugs. Secondly, the long-term stability is mostly because nothing eats it. That will change. This will be a highly calorie rich food for the first bacteria that evolves an enzyme to break those never-before-seen-in-nature chemical bonds that currently make it a stable polymer of a natural food source (petroleum is, of course, a completely natural product that bacterial love to eat!)

Natural alternatives include expanded clay products, perlite, vermiculite, foamed glass pebbles, aerated concrete, biochar, and foamed cement to name the ones at top of mind for me.

I got a massive deal on XPR from facebook marketplace - 100x 4x4 sheets for $200 recycled from a milk barn.  Still in decent shape.   I know it's been used in climate batteries for awhile - at least I thought so, and was wondering if people were changing opinions on these.  

It's def used in building construction like crazy.  And I also know all good things come to an end.  Just wanted to hear your opinions and you didn't let down.  thanks

Reusing/recycling mitigates the climate burden. That cost has already been incurred by the first user. Only purchasing new material adds to the Greenhouse Gas burden.

How thick are these sheet? If its normal 2 inch sheets that is, indeed, a very good price.

R. Scott's advice is sound. Isolate XPS from any exposure to UV light, and especially bugs. Can be hard to achieve 100% protection that lasts a long time underground.

Extruded poly styrene.  Insulation board

Thank you.

I wondered if it was the same stuff I was thinking of.  It's sometimes used in art and terrain and is loved for being pretty inert and ph neutral (somewhat archival), but I've seen reviews where it deteriorates when used with materials that aren't ph neutral.  The crafters stress a ph neutral primer or very slightly alkali.

For use in a structure, check with the local fire code.  This not only lets you know about fire risk, but of additional chemicals that might be required locally to reduce flamibility of synthetic insulation.  These are often more likely to leach than the foam itself.

Touching soil, I would want to put something between the soil and the insulation to protect both soil and extend the life of the insulation.

I've bought some of this from time to time.  Because the gas bothers me, it gets put in the out building until I'm ready.  The rats eat it before I can use it.  So rodents may also be an issue.  

Yes, they are 2".  Some are a bit sloppy, but I've got a good stockpile.

I've already started around the foundation with corrugated metal on top.  I live on a steep slope so in the back of my house the foundation was exposed 4' above the grade.  I didn't both to go below grade there, but it already made a big difference in my basement.  In the front I'm getting about 2' feet underground with the XPS and steel.  I'm guessing the steel will eventually get eaten away (decades?).  

Is there advice on how far down you need to protection from the critters?  I'm hoping to excavate down in the greenhouse so I'll be aware the shielding on the backside facing the soil will be at different heights than the front -- so long as they are tight together to not let the critters between panels.

Why not insulate with something natural like hay or straw?

Do you or anyone else know what people typically use?

I don't know.  It's a material I didn't enjoy working with in small quantities,  so we didn't keep on with the experiments.   Although, if I could find a way to ratproof it, we might try again for a small area of one of the outbuildings

Where I live, last time we needed insulation, fiberglass was considerably more affordable for the r factor.  So this stuff fell off my raidar except for crafting.

Would an experiment be possible?   Three blocks.  One for control, one burried naked, and one burried, wrapped in plastic. Maybe with something written in water-soluble ink to observe how well the plastic works (if ink runs, water got in)?