Insulation, Radiant and Vapor Barriers - styrofoam insulation

I have been building a garage attached to my home and have come to the insulating portion of my project the garage is
50x30 and two stories high with 4 bays. I live on a hill so the garage part has 10 foot high and 8 in thick conrete walls on two sides that I have fured out with 2x2s and the other wall is shared with my home. The upstairs is 2x6 walls with 12 in TGIs for rafters with a vaulted ceiling. My question is I was thinking celulose blow in insulation or just plain household FG but a friend at the lumberyard called and said he could get me a great deal on styrofoam sheets in thickness from 1/2 to 4 in, and he says that styrofoam has a R- Value of 4 per 1/2 in so this would by far out insulate the others for even less money from the 2 in I could put in the 2x2 walls to the 12 in rafters and he said the only trick is to make sure it is installed tightly to eliminate cracks is this true? I am a DIYer and have built the whole structure by myself so far from the concrete up but I have looked for this answer and have gotten many diffrent responses if anyone can help I would greatly appericate it. Thanks

Hi BJ, several questions. Clarify what insulation he is referring to, styrofoam is generally considered abour r=4 PER INCH. Polyisocyanurate (generally foil faced) is r=7 per inch, and EPS (pink or blue) is r=5 per inch. There are slightly different applications for each.

What climate are you in?
Any vapor barrier and insulation under the slab?
Upstairs living space or storage? And where is access?
What type of venting are you planning on?

I will attach a couple of links for reading.

http://www.eere.energy.gov/buildings/building_america/pdfs/db/35017.pdf

http://www.buildingscienceconsulting.com/resources/articles/Understanding_Vapor_Barriers_ASHRAE_2004_08.pdf

Bud

Thanks for the links Bud, the styrofoam is white with a paper on both sides and he mentioned an R value of about 4 per inch. I live in Idaho so there is 7 months of winter and very hot summers. I put tar and a 1/2 inch of styrofoam on the out side of the concrete wall before back filling but I have not put anything on the inside but the 2x2s and the garage has a 6 inch concrete slab floor. The upstairs is going to be bedrooms and you get to them from the 3rd floor of the house and there is a door on the opposite end of the building. As for venting I have installed h vac ducts in the floor joists to be hooked to a seperate furnace and central air unit when I get closer to completion and there are three large doormers with 4x5 vinal windows and low e glass

With a sloping roof, the devil is in the detail. Here is another link
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/95/950309.html

Your first post stated r=4 per 1/2 inch so I was looking for a new and better material, shucks. R=4 per inch is normal bead board and works fine. On the inside you will want it to breathe, a permeability rating above one. Another link
http://www.apexhomeinspections.biz/articles/vapor%20barrier%20basics.pdf

GL
Bud