Insulating the underground perimeter of greenhouse and with what materials?

In efforts to retain heat in the earth under a greenhouse, especially one using a climate battery or subterranean heating and cooling system, I’ve heard the importance of insulating the perimeter. What materials would one use to do this?  

Is there concern with burying a closed cell foam board?  Either for toxic reasons or for fear of it falling apart if you need to remove it.

There could be interesting alternatives.

To insulate the fieldstone basement of my Tiny Home, I use hay. It is cheap, and at the end of the year I can finish composting it (which it has a great start on)

Assuming you mean below-ground level. I used EPS- expanded polystyrene (it goes down 3ft below the ground level). I couldn't find anything else that would survive being constantly wet and still be insulative, and that I could afford. So I used recycled/reused polystyrene, I figured I was giving it another life for a period of time.

Other options could be:
- Vermiculite board. The board structure would start to crumble in the damp, but wouldn't break down because of it- once held in place by the surrounding soil the vermiculite would be fine.
- Leca/leica (expanded clay pebbles)- wouldn't be insulative when full of water, but wouldn't deteriorate in damp. Used in external insulation inside french drains, around houses.
- Foam-glass- drains and isn't damaged by damp, non-capillary so shouldn't fill up with water when wet, therefore staying insulative

I'd avoid anything that would potentially fill up with water and not drain, so isocyanate/PIR insulation.

I don't think rockwool would work (but I could be wrong)- it shouldn't be damaged by the damp but is wicking so I think it would constantly soak damp up from the ground. And it wouldn't be insulative when wet.

Charli Wilson wrote: I used EPS- expanded polystyrene (it goes down 3ft below the ground level). I couldn't find anything else that would survive being constantly wet and still be insulative, and that I could afford.  

Thanks, not knowing much about this product I always assumed it was a “green” choice. But this website makes it sound very environmentally friendly. https://insulationcorp.com/eps/

Would the normal closed cell foam option not be as suitable or environmentally friendly?  I’m concerned about it breaking down in the soil, do you use something to back against it?

I was intrigued about the use of the expanded clay to “insulate” inside French drains. I had only heard of people using rocks for this.

Thanks

Sheep manure will not break down.

I use this around my pipes buried underground so they do not freeze. Sheep manure by its nature has a lot of hay mixed in with those convenient-to-work-with pellets!

Seth Marshall wrote:

Charli Wilson wrote: I used EPS- expanded polystyrene (it goes down 3ft below the ground level). I couldn't find anything else that would survive being constantly wet and still be insulative, and that I could afford.  

Thanks, not knowing much about this product I always assumed it was a “green” choice. But this website makes it sound very environmentally friendly. https://insulationcorp.com/eps/

Would the normal closed cell foam option not be as suitable or environmentally friendly?  I’m concerned about it breaking down in the soil, do you use something to back against it?

I was intrigued about the use of the expanded clay to “insulate” inside French drains. I had only heard of people using rocks for this.

Thanks

I wouldn't think EPS is environmentally friendly, but if you're taking it from the waste stream where it would otherwise be landfilled- then the choice is up to you (the decision I made was to count this as a resource!). XPS- Extruded polystyrene would work just as well (probably better, more insulating)- I used EPS because it was easily available second-hand in the form of used packaging. It is recommended if you're insulating externally on a house, below the dpc- that you use XPS. EPS was really hard to work with- I had to keep hoovering up the 'little beads' as I cut it and shaped it, I was afraid the birds might try to eat them!

I had the EPS for my greenhouse in situ for the buried portion of the SCHS- buried a metre down and sticking up about 30 cm, for over two years- no sign of breakdown! However I can't comment on if the plastic is likely to leach or anything. Since then it has been covered up- it has recycled plastic cladding on both sides (to stop me putting spades and things through it)- soil in my greenhosue raised beds is up against this plastic board. The use of plastic nearby your food crops is a decision only you can make- it isn't something that bothers me (I live on an old coal mine, there's much worse stuff in my soils), but it depends on your preferences. I only really know about 'standard' building, not more green/natural building practices- of which there may be many.

Leca in french drains is mostly used because it drains easily, but it does also claim to have insulating properties: https://www.leca.co.uk/drainage-properties-of-lecar/

I used extruded polystyrene.  I think I heard that expanded will let water through and isn't as good for below grade.  Hopefully I didn't get that backwards.  I looked for a while for a greener alternative.

I did find a Rockwool board that may work.  When I called them one person said it would work and the other said it wouldn't.

Another good option would be a fiberglass product.  It's a rigid board that's made for foundation insulation.  The problem for me is that it's only recommended for vertical foundation walls.  I wanted to do a Swedish Skirt which it wasn't intended for.

Mike Jay wrote:I used extruded polystyrene.  I think I heard that expanded will let water through and isn't as good for below grade.  Hopefully I didn't get that backwards.  I looked for a while for a greener alternative.

In external insulation for houses in the UK, below grade they recommend extruded polystyrene- so you are right! The expanded stuff is meant to let water through (which is what I used, I'm on a massive hill so this didn't matter to me) and isn't quite as insulative.

Sandwiching the foam boards between a layer of pond liner may work.  I would use a single piece and fold it over at the top, so the opening is at the bottom, and then maybe a french drain or something to ensure that water can't wick back up into the foam.  I would also put down felt to protect the pond liner from rocks, especially when laying the foam boards with a horizontal tilt to increase the size of the skirt.  Maybe it would be prudent to call both manufacturers to ensure that the rubber liner and the EPS won't react with one another and cause degradation while in contact underground.

Charli Wilson wrote:The expanded stuff is meant to let water through (which is what I used, I'm on a massive hill so this didn't matter to me) and isn't quite as insulative.

Hi, could you please explain why this doesn’t matter to you because you’re on a hill?  Thanks

Seth Marshall wrote:Hi, could you please explain why this doesn’t matter to you because you’re on a hill?  Thanks

When I was starting to build I looked at my area- I'm on a huge hill and the ground water is some 27m below me. So I didn't think water moving around would be an issue to me- dead flat so the water doesn't really go sideways, only down. Since I was only affecting the top 1m of earth I figured it wouldn't matter. I put more effort into finding material from a waste stream, because I though the differences between the two materials wouldn't make a difference in my circumstances.  I'm obviously no engineer or anything, so this thinking was made up from random internet-reading, anecdotes and my experience of insulating standard-built houses.

As my own greenhouse thread shows it turns out perhaps I should have thought more of it- because the clay here gets saturated and very soggy, despite the actual water table being so far below me. So perhaps I should have paid more attention to it!

Having said all of that- I'd do the same again! The climate here is not very cold (East Midlands, UK), we get ground frosts but nothing more so I don't really have to protect from extreme weather.

Does that make any sense? I'm not convinced my thinking necessarily does make sense.. apart from to me.. but the reasoning inside my head, without having to explain it to anyone, seemed sound at the time!

Charli Wilson wrote:

When I was starting to build I looked at my area- I'm on a huge hill and the ground water is some 27m below me. So I didn't think water moving around would be an issue to me- dead flat so the water doesn't really go sideways, only down. Since I was only affecting the top 1m of earth I figured it wouldn't matter.

.....Does that make any sense?

Yes!  And thank you for explaining, I was genuinely confused but didn’t think about the water table. Good to know you’ve still encountered some wet soil as something to consider. Thanks!

Seth, it's very good that you're thinking about this.  I just checked the temp in my greenhouse.  It's a brisk -23F outside right now and the insulated (and unfortunately unheated) greenhouse is at +16F.  So I'm maintaining a 39 degree F difference due primarily to insulation.  I have R24 in the E/W/N walls, 4" of expanded polystyrene on the block stem walls and 2" of expanded polystyrene underground in a skirt (down a foot and then out three feet).  The south face is two layers of greenhouse poly.  My greenhouse is fairly big so I think that helps as well.

Mike Jay wrote: I have R24 in the E/W/N walls, 4" of expanded polystyrene on the block stem walls and 2" of expanded polystyrene underground in a skirt (down a foot and then out three feet).  

Thanks!  It sounds like you went all out with the R24!  What are block stem walls— I feel like I should know this...  

Do you wish you went down more then one foot?  For the skirt, do you mean you laid it horizontally flat up against the base of the underground polystyrene, and then buried them?  Do you wish you did it differently or any other learned experience from doing this?  

Have you had any concerns about using polystyrene?  Did you back them with another material before you buried them?  Thanks so much for sharing!

Hi Seth, yes I went for it with R24.  Might as well insulate well if I'm going for passive solar awesomeness

I might be using the wrong word...  I have shallow footings that are just barely below grade (in sandy soil).  I have a cinder block wall that is 4 courses high sitting on the footing.  Then the greenhouse sits on that cinderblock.

We put 4" of polystyrene against the cinderblocks down to the footing.  Then we decreased to 2" as we went down the footing a ways.  Once we were about a foot below the exterior grade, we excavated out 3' at a slight declining slope and laid down 2" pink styrofoam.  We didn't protect it or back it with anything.  We raked out the excavation so the styrofoam was pretty flat before laying it down.

And by "we" I mean the missus.  She did a lot of digging in October while I was framing in the walls and other stuff.