How many of you have heard the statement, "compressing fiberglass insulation reduces it's r-value"? I'm going to assume everyone has, myself included. Well, the statement is correct, but the interpretation or application of it often leads to a wrong conclusion, for example: Stuffing extra fiberglass insulation into a wall cavity will reduce the walls r-value. That statement is most often wrong, here's why.
I found it confusing that compressing fiberglass insulation would reduce it's r-value, yet we could buy high density fiberglass at a higher r-value. So what is the difference between high density fiberglass and regular density that we compress into a tighter space? It turns out to be, not as much as we thought. The distinction is between the r-value of the fiberglass batt and its r-value per inch. Take for example, r-19 batts. They are nominally 6 1/4 " in thickness, thus an r-value of 3.04 per inch. Now, that's not the r-value our books give us, but it is what the numbers say and what the pink panther says. Now, install it in the normal 5 1/2" cavity and what do you get, 3.04 X 5 1/2" = 16.72, no. The panther says we get r-18, so what happened to the r per inch. It went up to 3.27.
So compressing the batts increased their r per inch, while reducing the total number of inches, resulting in a lower total r-value, and although the total went down, we can see that a limited amount of compression is not necessarily a bad thing. Where the turning point is, I haven't determined and obviously we can't squash the fiberglass flat and expect it to continue to increase its r per inch. But a little stuffing here and a little there can be a good thing.
Here is the panther's web page with the numbers I used and it does show that their high density for a a 5.5 inch cavity is actually R-21. Whether that's accomplished because they know how to increase the density better or because we need to add more insulation than the 6.25 in the example I started with, I don't know.http://www.idimn.com/pdf/insulation/fiberglass/high-density/21149-D.pdf
I however do know I now understand the original statement better, I just hope my explanation helped you as well. Corrections welcome if I am wrong.