Natural insulation for a quonset shaped roof

I'm building a creatively designed greenhouse with curved wooden trusses (approximating a quonset hut shape).  The best options for insulation I've come across so far is either rockwool batts/sheets or pole barn fiberglass rolls.  Is there a hempcrete/straw/etc method that I should consider?

The curved trusses are 8.5" tall and will be 4' on center.  I'll have 1" thick pine boards attached to the trusses to support the insulation and roofing.  They can be either on the inside or the outside of the trusses if it helps the insulation out.  

My current plan is to put the pine boards outside the trusses, cover them with visqueen, lay batts or sheets of rockwool insulation over that, lay purlins across the insulation and put metal roofing over that.  Possibly with a housewrap between the insulation and the roofing.  

Am I overlooking a more natural insulation option?  Hopefully one that isn't too expensive?  Could I put the pine boards on the inside and pack or form a natural insulation to the shape?  Then put purlins across the tops of the trusses and attach roofing to that?

I know exceedingly little about hempcrete, slip straw and some other methods that sound like they involve a wet bulk that is formed in place to give you a wall and insulation at the same time.  Thanks and sorry for the ramble

The purloins attop the insulation?
That will pinch the insulation.
If your metal roofing is corrugated lengthwise,like most are, you could set 2x4s perpendicular to the arches,and lay your insulation in between the 2x4.
Then a layer of visqueen followed by the metal roofing.

Alternatively, we use the space between the trusses.
Visqueen on the inside, held in place by the 1" pine.
Fill the space between the trusses with insulation.
Visqueen, then metal roof.
I've had good luck protecting visqueen from Sun and other damage by covering it with carpet,but metal is tougher and mostly water proof.

I love the cast insulation, but I only have direct experience with perlite concrete. It must be kept dry, otherwise freeze/the cycles wreak havoc on it.
I've recently tried soil cement with good results,but it wasn't insulative at all.

If you wanted to try casting in place maybe try  styrocrete or perlite concrete.
I would run some screws through the rafters,to give the cementious mix something to hold onto
Visqueen and 1" wood along the bottom.

Mix in ratios of 1 cement to 6 insulation to 3.5 liquid by volume.
Pour it on, pushing it into the corners,scrape off level with the top of the rafters.
I would top this with mesh of some kind stapled in place,followed by a coat of 1 cement to 2 sand to 1.5 liquid.
I keep saying liquid because various admixture add waterproofing and help newer and older cement meld,but I've not tried any of them yet, but intend to try strait latex paint, because it's cheap (free).
Other water proofing layers can be waxes, oils,soaps strait cement, waterstop cement, commercial cement water proofing products, simple exterior paint,
One recipe for waterproof whitewash includes oil which is said to react with the lime and form soap.
A visqueen layer would have to be held in place by something that will either be iffy (a glue) or penetrate the visqueen itself.
It also needs protection in its own right, and that layer will need a way to connect.
Of course metal is still an option.
Other than expense, the only drawback I see is  water maybe gathering in the corrugated channels.

While Ive never built anything really signifigant with these techniques, I have used water stop cement over mesh, stappled to a wooden surface to create a kitchen counter. In use for years with no sign of rot.
I have used regular cement/mesh/visqueen/wood as the roof and floor of a chicken coop with similar results.
My rocket stove cores are all cracking due to being wet/frozen/thawed. Oops, I got dustracted, but it's no reflection on the material itself.

I'm not sure perlitecrete is better insulation than rockwool, in fact I doubt it is.
More natural insulation is out there, but their main benefit(they will eventually returns to the earth) is also their main flaw.

Thanks William!  I am planning on a metal roof over top of the insulation so at least we have the simplicity of that "standard" building material to rely on.  While I could run the corrugations horizontally I was planning on running them vertically like in a "normal" roof.  My curved trusses are 15' long and I don't think I can bend the metal roofing that way.  So my plan is to cut the roofing into 5' pieces and have some purlins on the flat and some on edge so the roofing pieces will create a facet that approximates the curve.  If that makes any sense.  

I realize there is a chance of pinching the insulation with the purlins but my insulation guy thought it wouldn't be a problem with rockwool bats.  If I go with pole barn insulation rolls they would definitely get pinched so I'd have to use standoffs or something to keep that from happening.

If we put a castable/pourable insulation between the trusses, it would be contained on the inside by the pine boards.  Would it be stiff enough to not slump out or would I also need a containment material on the outside as well?  In addition to the purlins every 24-30" that is.

With the perlite concrete, would water be the liquid if we were relying on housewrap/visqueen and metal to keep it dry?  Sorry for the dumb question, but what function does the cement provide?  Keeping the insulation from settling?  Keeping critters from living in it?  And where do you get perlite in gigantic quantities?

Ok, I went to the Midwest Renewable Energy Association fair last weekend (awesome event) and talked to some natural builders about my project.  They suggested that the best options for my situation were either expanded polystyrene sheets on the exterior of the pine boards or blown in cellulose into the truss space.  They were interested in saving the world from the ills of styrofoam but these were the options they thought were best.

One presentation was on "papercrete" which was cellulose or shredded paper mixed with some cement or possibly clay.  Their main use was to make bricks to then build walls with.  The cement/clay was just to help hold the cellulose in shape.  If I was just in-filling a wall it sounds like the papercrete process would just be overkill compared to blowing the same cellulose into the truss space.

An interesting option would be to make some 4x8 forms and make papercrete sheets an inch or two thick.  Then install it like styrofoam.  I'm not sure if I want to try that experiment but it's a thought.