thanks in advnace for any help on this , 
i started to build a small  earth bag building earlier in the year.  unfortunatly i then had to leave the project for a bit with some others left to carry on with it.  
it all went a bit slow after that and consequently  the plaster isnt on and now the temperature has dropped so its belowfreezing most   nights so i didnt to plaster it as im guessing its not good for the plaster to freeze while drying.... 
this leave me with the second problem of how to protect the bags over the winter untill the spring when we can resume  and finish.   
I was thinking a heavy duty plastic sheet might help but obv thats not breathable.... but itwouldnt be air tight either so im sure there would be an element of breathability.....

So your needing to protect already in place earth bags that have been tamped into the barbed wire stabilizers from the elements, Yes?

Plastic can work for this, you need to angle the sides out so there is air space between the completed bag work and the sides of the plastic.
This support can be simple but it will be needed to prevent snow from collapsing the cover tight against the completed work I would think.

Some rocks along the bottom of the sides will help hold it in place in winds and a fairly steep angle to the sides will help snow to slide off instead of simply adding weight to the plastic cover.

Something looking like a pup tent should work quite well and allow air flow too.

Billboard tarps off Craigslist or ask around. Cheap, big, grommets, durable.
Anything else is too expensive for large size, or easily destroyed by UV or winds.

Whitewash?  Dirt cheap.  Will block a lot of the UV.  Mix it with an emulsifier and a bit of linseed oil to make it stick better.  Or add some latex paint.

Or DIY blackwash.  Carbon black and a sticking agent. -- basically india ink.

We used watered down white interior paint, 50/50. It adhered pretty well and greatly slowed the UV degradation. We also tarped it and provided a terrific home for mice. This was in a rural area with a large rodent population. We'd never cover a building again.

I faced this problem myself 2 years ago. I have a small earthbag home in the Upper Peninsula of MI. Alot of advice I recieved was conflicting-so I just went with construction grade plastic not completely tight to the walls but not exactly pulled out pup tent style. Its still not plastered and the walls are good as new!