Best core fill for cinderblock wall of greenhouse?

Trying to find the sweet spot between thermal mass and insulation.

Design already has a water barrel for thermal mass, and will have foam panels on the outside of the cinderblock.

But how to fill the cinderblock?

1)   Fill with rocks, and then pour cement over, for total thermal mass?
2)   Fill with rocks and gravel, for thermal mass, and some insulation (Air between gravel).
3)   Fill with rocks and then pour Aircrete over?

What's the best tradeoff?

I think it will depend on your climate (local zone cold temperatures in the colder parts of your growing season), but since you are insulating outside the blocks, I would say go with thermal mass.  If you want to be cheap about it, you could fill with rammed earth and stones. The denser the better.  If your insulation is equal to the task of holding the heat of those blocks from creeping outside, then the thermal mass's thermal energy (vibrating particles) hit the insulation and then have to radiate back out into the greenhouse... which is what you want.  If the bottom course is sealed with concrete, then you don't really needed to cap each core with concrete, just keep filling them up with as dense of material as you can.  The only other factor is the thickness (R Value) of your insulation, in relation to your external temperatures.  You want to maximize your insulation, particularly on your Poleward Wall.    

Roberto's post made me wondering how you plan on affixing the insulation.
Foil faced foam boards could be mounted on furring strips,foil side inward,would be great for reflecting radiant heat back to the mass.
Are you going to use rebar as well a packing the cells?
A crushed limestone road base would be a great filling. Paver sand would be nice. I would just use rock and mud, cause I'm cheap...
I wonder if stuffing oil or water filled bottles in the cavities would be worth while?
I think probably not.

Another vote for thermal mass....pack that volume with rammed earth!

I would go with sand. It is easy to pour in, and has incredible thermal mass. Over time, trucks rolling by, tremors in the ground, etc will all cause the sand to compact without work.

I did that when I added radiant floor heat to an existing floor in my home. It was on an insulated concrete pad, but had no pex tubing running through the concrete. So we laid the pex tubing over the slab, then not wanting to mess with mixing concrete, just put sand over and around the pex tubing and put a floor over that. I though that room would be really inefficient, but it is just as good as the rest of the house that has pex running through the concrete.

Sand has incredible thermal mass.

Thanks everyone for replies!  

To answer questions, I', in Zone 3.   Wall will be poleward, but beside a garage/windbreak.

I'm planning to bond the rigid foam insulation to the outside of the cinderblock using pl400.    I'm planning to then cover the insulation with something like structural skin from conproco.

I'm hearing a lot of recommendations to use Sand instead of concrete to pour into the rock filled cores.     Can anyone confirm if sand is superior to concrete as fill?    

I'll leave aside the aircrete idea.    

What kind of confirmation do you need?

Sand will definitely pour more easily than anything else you could get your hands on cheaply. If the voids are ultimately sealed at the bottom, and there's no place for it to go, you don't need concrete. You can tamp the sand down, you can add pebbles (though that would only encourage air spaces, and reduce overall wall density and mass, unlike sand, which would fill every little space), you could even mix in some really fine mineral dust if you had some. The spaces within the wall will be sealed, so there's no place for anything to go.

I don't think that concrete is a good idea at all, actually. Aside from being unnecessary, you're introducing not only whatever is going to offgass out of the concrete as it cures, but moisture into your wall system and building envelope.

As to bonding the rigid foam insulation to the cinderblock, I wouldn't. You will get better insulative performance if there isn't direct contact between the foil layer and the thermal mass. I would just use 2" wood strapping as spacers, leaving an air gap.

Good luck, and keep us posted.

-CK

"Superior" is one of those tricky words.

If you are talking ONLY about heat storage capacity per volume, then concrete is about 50% better than sand.  However, using sand has basically zero carbon footprint, whereas concrete requires enormous amounts of  energy to make and therefor has a relatively large carbon foot print.

Now I'm not against using concrete when it is the best choice, i.e. for a driveway, wall, etc.  But in this case I personally think that concrete is NOT the best choice.
Rocks have roughly the same or better heat capacity as concrete, and you are just wanting to fill the voids around the rocks, so the difference in total thermal mass of the finished wall will be relatively small.
The small improvement in thermal mass is not worth the extra time, money, effort and Co2 footprint, in my opinion.

However, it's your opinion and not mine that matters in this case.

One last point, too much thermal mass can be counter productive.  The more mass you have, the longer it reacts.  If you have too much mass you could be storing heat in the winter that gets released in the summer.  When using concrete walls for thermal mass, only the first 4" or so plays a part in day-to-day heat storage.

I would concentrate on using water as your primary thermal mass.   It has twice the storage capacity per volume as concrete and is more controllable.  I.e you can eliminate the winter storage and summer release by moving the water outside the greenhouse during the summer, where instead you can use it to store summer heat for release during winter.  It's hard to move thermal mass that is inside your walls.

Travis is right that sand is a lot faster than anything else, and is relatively dense.  Compared to concrete, sand is a lot cheaper (possibly free, if it's on your land, or free for the cost of hauling from somewhere else), but even cheap to get a dump truck load.  Sand is also cleaner than most other methods.  Concrete is very resource intensive to produce, and haul, so larger ecological footprint.  Concrete is necessary to bond the bricks, nothing more.  Get your mix right for the bonding.  If you do not have sand on your property, then it will probably be worth your while to simply fill the bricks with whatever you have, rocks and dirt  <-that is on your land.  The cavity is small and ramming earth into it with a blunt stick and rubber mallet will not take much effort if you want to make the dirt around the rocks denser, but this is not necessary.

Do you expect to keep the greenhouse above freezing?  If so, I always wondered about using plastic bags in the cores and filling them with water.  Water is a much better thermal mass than stone/sand/gravel as long as it doesn't freeze and blow up your wall.....

Do a search on thermal mass characteristics of concrete vs sand to make up your own mind.  Sand will pour easier, cost less and be better for the environment.  If there are other advantages to concrete thermally maybe that would make up for it?  Or structural advantages?