On the ranch in SoCal, 3000 elevation, where it gets very cold in the winter and very hot in the summer, the county says we can build anything we like as long as it is 120 sq. ft or less without a building permit or any kind of hassle or paperwork.    Officially these are sheds but I am thinking of using one as a bedroom for when I'm out on the ranch.  I have the following dilemma.   In central California a new firm manufactures and sells 6 inch thick ICF's.   I believe someone mentioned the parent company is from Egypt.   They're actually kind of nifty.   1" thick walls, 4" cores, and if you need more insulation you can use glue to tack it onto the outside.   They only sell straights to keep it simple.  Corners are punched out where needed.  6" thick, 50" long, 10" high blocks cost $6 apiece.   That's cheap compared to the market rate of $16 + crazy shipping costs for regular ICF's.   They don't come with any certifications or standards, but none are required for my pirate project.   Officially the company sells them as a cheaper way to build garden and outdoor walls and what not.   So I'm thinking, should I just haul a small trailer load of these blocks myself down from Modesto and build the 10x12 out of them?   Paste on an extra 1" thick 4x8 sheet of styrofoam to make it 2" total outside, and 1" inside?    On the other hand I could go with old reliable 2x4's which are available for $2 apiece at your local discount hardware and lumber stores and top that off with some wood siding.   I could stuff it with some good R-30 insulation.   That would be very simple!  Another real good thing about 2x4's is that later if I decide to have a window somewhere, I can just cut it out with a sawzall and install it.   With ICF's you are stuck with the design you started with.  Old reliable cheap and simple 2x4's or more energy efficient inexpensive 6" thick ICF's?   My jury is still out as to which method to use.  I want to hear your thoughts this matter.

If you use foam on the exterior, then it should be covered since UV will degrade it over time.

Of course.  That goes without saying. 

forget the lumber. why build with wood. i dont get it.

Yes, it should go without saying but I have seen BlueBoard Styrofoam left exposed on houses for a few years.  Needless to say, the board was no longer blue.  It was more amber than blue from the UV.  Maybe our postings will help someone else that does not know.

Someone asked about the advantages of wood.   The big advantage is one can go down to the local Lowe's or Home Depot and buy quantity one 2x4 for $2.   Remember, the requirements state 120 sq. ft. max can be built without a county permit.  So the focus is a 10'x12' structure.   Now let's compare ICF's.   There's even a factory 2 hours drive away.   They only produce 6 inch core ICF's.   6 inch core is 1 foot wide with stucco and drywall.   So you have a 10'x12' whose insides are 8'x10'.   Wonderful!   The insides of the 2x4 version are 9'4"x11'4".   4 inch core ICF's?   No problem!   The company has plants that produce them in Colorado or Idaho.   In Southern California you're looking at $1000 for delivery costs.   Next consider how much concrete you need.   A barrel truck costs $750 for 10 cubic yards.   You can't order less.   With a pumper and tax that comes out to $1000.   They give you 40 minutes to pour.  Overtime costs $5 per minute.    That's nuts!   With the 2x4 version you avoid the barrel truck madness.   If you make your own concrete sand comes in 12 ton = 8 cubic yard loads.   The yards with the big delivery trucks won't sell you a smaller load.   If you buy sand and cement in smaller quantities from Lowe's or Home Depot it is so heavy that you will have serious problems transporting it and unloading it.  In a nutshell, the advantage of wood is wall thickness, and flexibility and logistics of materials supply.

How is a 1" thick ICF going to hold concrete without total failure ? Stick frame it or make your own SIP units.

Good question/concern.   I have a picture of the blocks close up from the craigslist ad.   Below is a link to their website.   The blocks look real and they look like they are really 1" thick on the sides.   On the website you can see construction pictures.   The walls aren't terribly high, but they seem to be at least one story in height.   So in answer to your question/concern the 1" thickness must somehow hold up and pass the test at least in one story structures.    They probably won't pass building inspectors tests and what not, but for bootleg pirate projects they might be just fine.

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As always, it just depends on what you want. The major(and there are others) advantages of ICF are 1) Insulation/ thermal mass, 2) Sound insulation, 3) permanence, 4)resistance to trauma (earthquakes, tornadoes, cars.)
Insulation you can get with stick-frame.
Permanence is relative. Stick will easily last your lifetime.
ICF stands alone in sound insulation
At 120 sf, you can easily re-build if even totally destroyed.
As others may recall, I'm a HUGE proponent of ICF, but sometimes it's a big gun for a small animal.