What is the difference between R-19 (for example) and a rating of R-3.44 per inch?
beverlybliesath
| Posted in Energy Efficiency and Durability on
I am going to add insulation to the inside walls of an existing home. I want the best insulation (R-value), bur I don’t know the difference between R-19 vs R-3.44 per inch, for example.
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Replies
Beverly,
Insulation has thickness. For example, an R-19 fiberglass batt is 5 1/2 inches thick. If we want to know the R-value per inch of this product, we divide the total R-value of the insulation (19) by its thickness (5.5 inches). The answer to this division problem: R-3.45 per inch. That's close to your example.
On the other hand, an R-20 piece of extruded polystyrene (XPS) foam is 4 inches thick, so it is R-5 per inch.
Sometimes we care about R-value per inch, and sometimes we don't. If you have to put your insulation in a very narrow cavity, you probably want an insulation with a high R-value per inch. However, if you are piling the insulation on your attic floor, R-value per inch is less important. All you really care about is the total R-value of the insulation layer.
Actually an R19 batt is manufactured and tested at 6.25", not 5.5", even though it is designed to snugly fit into a 5.5" deep cavity. It needs to be slightly oversized in order to guarantee enough loft to fully fill the cavity, which isn't always precisely 5.500". As-tested for labeling purposes it's R/inch is only (R19/6.25"=) R3.04/inch
When compressed to 5.5" in a 2x6 wall cavity it's performance is actually R18, which works out to (R18 / 5.5"=) R3.27 /inch. he higher R/inch due to the higher compressed density, but it's total R is than tested due to being 3/4" thinner than it's manufactured & tested loft where it performs at R19.
Higher density R21 fiberglass or R23 rock wool do not have this funny issue, are manufactured with far less "fluff", and will perform at their rated values in a 2x6 cavity.
When you compress an R19 into a 2x4 cavity, it's performance is R13- it has the same weight per square foot as R13 batts, and is essentially a "fluffed" R13 batt, with the same amount of material, but spun with higher loft.