Hi Guys. I'm new here too, but wanted to put in my two cents.
After doing a lot of research on insulation for my 36x44 building, I learned a shocking thing. The marketing guys have made "R" values relative!
I statered to learn this by reading different manufacturers' own write-ups about the pros of their own product type versus competitive product types (i.e. fiberglass vs cellulose vs poly, etc.).
Some of poly guys were claiming an R value of 10 per inch, and so where some of the cellulose guys. Now, I don't know everything about everything, but I know enough to understand that the key to retaining heat is to not let it migrate through a media either by conduction, convection, or radiation. Since insulation inside a wall isn't really subject to radiation, conduction and convection are the big issues. Heat migration can be controlled by keeping air spaces small enough that convective currents cannot get started, and conduction is controlled by using materials with as low a heat density as possible. That being the case, how could there be that much difference in R value from one material to the next?
I finally got one rep to admit how they were skewing the numbers, and it is based on the non-linear R value of insulation. Without going on ad-nasuem about how fiberglass R values are calculated, they are based on a given static temp of 70 degrees F and a temp differential across the insulation of only a very few degrees. It is also based on CONDUCTIVE heat transfer through the insulation.
It is a known fact in the industry that loose fiberglass (batts) starts to develop convective currents within the fiber when the temp differential across it starts to increase and this allows more heat transfer through the fiberglass and lowers the R value. At temp differentials of 50 degrees (20 degrees outside temp and 70 degrees inside temp), the R value of fiberglass batts is down to about 30% of the R value established by the test conditions at 70 degrees.
Material like closed-cell poly very effectively eliminates convective currents, and so will not transfer the same amount of heat as fiberglass bats at these greater temp differentials. The marketing guys have jumped on this and transformed the data to make their material look like it has a greater R value. The transformation looks like this:
Fiberglass bats at 70 degrees = R value of approx 3 per inch of thickness,
Fiberglass bats at 20 degrees = R value of approx .9 per inch of thickness,
Assumed R value of Fiberglass bats across all temps still equals 3 per inch, even at 20 degrees F, so true R value VS assumed R value = error of 3.3 to 1.
True R value of poly at both 70 degrees and 20 degrees = approx 3.5 per inch so, based on the non-linear R value of batt insulation, poly at 20 degrees F is equivalent to batt insulation with an R value 11.55 per inch!
This is how some of these exceptionally high R values are determined, but they are only representative of performance under significant temp differentials. Under moderate temp differentials, the performance difference between the two materials is not nearly so great.
Cellulose also claims some very high R values per inch, but not normally as high as poly. Cellulose is a bit more "open" in it's structure than poly, so it does have somewhat higher convection rates than poly, but nowhere near as high as fiberglass bats.
All materials have an true R value in the "3+ per inch" range when measured the same way that fiberglass bats are, but their comperative performance across temperature is not uniform.
I mention this for two reasons. The first is that you cannot automatically assume a material with an R value of 8 per inch is better than a material with an R value of 5 per inch or 3 per inch. It all depends on how the value is calculated, and the guys selling a product have a vested interest in using the highest number they can. Closed cell foam is closed cell foam. Door cutouts are not going to have a higher R value per inch than the blue foam, AND the door cutouts are not the same thickness all the way across the panel! I'm not saying to not use them, just understand what you are really getting.
The second reason for bringing this up is that it may have some bearing on what a person selects for insulation in different environments. If you are intending to insulate a place to accomodate a 20 degree temp differential, then all materials are going to perform very much the same, so pick the cheapest material available. If you expect to have to deal with differentials significantly larger than 20 degrees, it is worth spending the money to get cellulose, or even poly (VERY expensive!).
Regarding insulating a slab, 2 inches of blue foam will give you a true R10 all the way across the slab. The door cutouts will NOT give you consistant R8 all the way across the slab.
On 48 sheets no less.
Just call your local entry door salesman and find out who their suppliers are. The supplier(or local manufacturer) buys blank doors from Thermo Tru and others, then they cutout for the windows and hang them in the jamb. The place I'm getting them from is happy to get rid of them for free to me, so they are saving money on waste removal. They even gave me some full door blanks that were damaged or mistakes.
Just saved me $1000.!!!
