Straw Bale vs Dirt Berm for heat retention

I have an old barn that is begging to be converted into a green house. The size of the barn is such that if I am going to have any hope of heating it through the winter I am going to have to get a lot of stuff right in the build and still yet do a lot more stuff once the passive measures are in place.

The walls of the old barn are eight inches thick and have 3 one inch steel cables through them. They stand 8 feet tall measured from the slab and seven and a half to nine depending on the slope on the outer walls. I am considering wrapping them in straw bales in the same fashion one would build a straw home. A second more complicated and more expensive option would be to hire in dirt work to wrap the north east n west sides of the building in dirt. My desire is to approach the function of the an under ground greenhouse.

Given that most of my heat loss is going to be up out of the roof....

Anyone know if or think I will even notice the benefit of either straw or dirt with an already 8" thick concrete wall? The straw gives up mass but traps more air. The dirt has more mass but it will be biased to the bottom of the wall where the straw will be the same width top to bottom.

Any advice on or off topic is appreciated

Thank you permies I have learned so much already and feel like I will be able to apply so many ideas I discovered here.
Thanks again

Russ

Mostly off topic but I've thought about this a bit with straw bales. I think one could use straw bales to set up basically sun trap/greenhouses. Inoculate straw walls and make use of that space for food too. I haven't actually done this yet but I've sure as hell talked about it a whole bunch.

I'm not sure if adding soil to the outside would come without down side. Seems like soil that is not insulated or protected from water is would be more heat suck in heat sink. The straw bales have a great advantage in that they are cheap , easy to do and add insulation to existing thermal mass.
Maybe you could double glaze the roof?
I look forward to seeing your project!

Thanks for the thoughts. I will post pics as I go

A well compacted earth berm that is also well insulated (dirt berm covered by straw bales?) could, in theory, act as a thermal flywheel (Annualized Thermal Inertia). You could maintain a thick cross-section by using gabion lined with geotextile.