3/4" = R2. Two layers with a 3/4" airgap pumps it up to a R6.
insulation
There are many types of styrofoam. The white, bubbly expanded sheets have a very low R rating. The denser extruded styrofoam comes as polystyrene or polyurethane based, in blue or pink sheets. I believe they are 5 and 7 R rated, but all the sheets are marked very prominently with their respective ratings and uses.
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As strange as it may sound dead or still air is one of the best insulators. Insulation such as fiberglass or rock wool and styrofoam work because they are fluffy with trapped air. If you have a sheet of four inch thick R40 fiberglass and squeeze it into a wall made of 2X3 studs you will have less R value than than a 3 inch piece of fiberglass insulation. So the highest R value goes to the insulation that traps the most dead air per inch of thickness and is installed propperly.
While drywall, or gypsum board, does give insulation value it is not a high amount, a 1/2" piece of drywall will only raise the r-value by .45, if you are looking to improve the insulation of a wall you are much better off using rigid foam insulation. An extruded polystyrene board, that's Styrofoam, of 1" thickness gives you an R-value of 5.00, which is over 5 times greater per inch than that of drywall. You can further improve the insulation value of a wall by add some kind of batt insulation in-between the studs.
The R-value is a valuation of how well-insulated the garage door is. The higher the R-value, the better insulated it is, but when comparing it is wise to learn if the given rating is based on the entire door or only certain portions.
Floors, attics and walls all require insulation of different R-Values which are measured by the thickness of the insulation. The R-Value determines how well the insulation will resist heat flow. The higher the R-Value the more resistance it has to heat and cold. If you are adding to existing insulation there is another factor to consider. You will have to determine the R-Value of the existing to know how much additional insulation to add
it is a good thing i think R Value is the thermal resistance of a given material. The higher the R Value the more thermal resistance and the better the insulation. So, an R48 roof system is warmer than an R32 roof system. All insulation materials are rated by their R Value. An inch of rigid insulation has a higher R Value than an inch of fiberglass insulation. Check with the insulation manufacturer for the R Value.
Either - the R number refers to the ability to block heat transfer, not absorb it.
R-38
It is the measure of insulation against heat/cold afforded by varying thicknesses and composition of construction insulation materials. The higher the "R" Value the better the insulation characteristics.
The U value is the inverse of the R value. For R value 19 insulation the U value is 1/19, or 0.0526.