Found this at Lowes today.  It is extremely dense and hard.  Very light.  I weighed the piece I bought (24 x 24 x 1 $6.00) and it was 1.7lb/cf and carvable just like a balsa block.  With a 4-5lb 1/32 skin molded you could make just about anything.  It sands better than any foam I have seen.  I am going to see if it fills using modpoge.  If it does, I am going to use it for blocks.  Already making molds from it.

Anyone else tried it?

ken

Priced a 6lb 1 x 3 x 24 balsa block today $16.  They are now calling 6lb super light.   This stuff - 75 cents.

Looks very promising. Thanks for the info.

Looks like Kingspan makes GreenGuard LG insulation board in at least three densities. They list a 25 lb/ft^2 compressive strength, 40 psi, and 60 psi. Their data sheet says the 60 psi material is nominally 2.2 lb/ft^3. The others aren't listed.

The label on Ken's piece doesn't have all the part number info, which may be a local-market kind of thing. So we don't know if Ken's piece is the lightest version. But if it does the job, then it must be the right stuff! Says they make it in 1-1/2", 2, and 3" thickni. And 2' and 4' widths by 8' in longness.

In addition, since your turtledeck will be derived from insulation material, you have a better chance of keeping your pushrod cool!

Material: extruded polystyrene, closed cell,

It is "plastic" so I don't have any idea what will or won't melt it.

Ken

Well, I would use either epoxy (scraped on very thin), wood glue/titebond, or maybe the original gorilla glue, as we know those 3 are foam safe.

Steve

Is it different than the blue or pink polystyrene we all know? Some years ago I ran across densities for the various extruded foams available, must have failed to file it in a way I could find it. Density definitely related to compressive strength. The 15lb boards tend to be mushy, but the 25lb sound similar to this. 15 and 25lb 4x8 up to 2" are readily available up here where it snows.

I saw this xps foam weight info over on RCgroups.  Perhaps others have manufacturer derived weight info.
https://www.rcgroups.com/forums/showthread.php?2300174-Weight-of-15-psi-extruded-foam

The weight of Dow and Owen-Corning extruded foams vary according to compressive strength.

The 15PSI foams such as Owens-Corning Foamular 150 (pink) and its Dow (blue) counterpart are about 1.3 Lb./Cu.Ft.
The 25PSI foams, Foamular 250 and Dow equivalent are 1.5 Lb./Cu.Ft.
The "white" styrofoams vary from .5 to 1.2 Lb./Cu.Ft. depending on formulation.

Visiting the factory datasheet, this is absolute close relative of the blue and pink foams.
(as you measured, its density fits to the lower third of extruded polystyrene foams. )
About complete technology, covering, etc. materials, see:
"The bluefoam model, or how to..."  topic here.
I described the most usual traps/failures working with XPS foams.
Istvan

Spent the afternoon cutting some foam stabs using this Kingspan GreenGuard foam.  It took a much higher wire temperature vs Dow foam. 
Produced several nice stab sets for vacuum bag experimentation.
8" root, 6" tip, 13" panel width. 
Each panel of the stab weigh in at 13g for the Dow grey 25psi XPS foam and 18g for the Kingspan GreenGuard.
So, that is not a deal breaker weight, but it is something to consider.

Dear Brent,
Magnifying your photos, it looks to be cut at too high temperature, regarding the pretty big waves on the surface.
I suggest to adjust the minimal current on the laboratory supply unit*, which is enough to cut very slowly the foam. Be patient!
 And longitudinal, "sawing" movements** are a "must" to get waveless surface.

*no precise result can be obtained without it
**yes, the surface will be hairy, but smooth, without grooves and ridges... Hairs can be easily "harvested" from surface with well known tricks/procedures. My friends in our club built computerized cutting rig, and I observed the basic differences between manual, "sawing", and the digitally feed techiques. Manual technique melts the foam by transferring the heat by contact between the string and the foam, and "sawing" is needed to enlighten the feed, resulting "hairs" in 20 or 30° direction. The gap is absolutely equal as string's diameter, not bigger.
Computerized rig naturally cannot do "sawing". It needs significantly higher temperature, and the melting heat's transfer is by RADIATING, not by contact. Will not pull "hairs", hurray!! But...
... the melted channel/gap is some 1-2 millimeters big, and the surface -however isn't hairy- not smooth enough, it is some printed picture of melting "appetite" of actual spot. It can be sanded to smooth (even with wet paper), but the surface is far from the original path, unlike the "sawed" one.

Istvan