does compressing r19 fiberglass insulation into a 5 1/2 inch space lower the r-value?

i just a bit confused. insulation companies say that r-19 fiberglass batt insulation is made for 2x6 stud walls. however r19 batts say on them that they are for cavities 6.25 inches deep. 2x6 stud walls are only 5.5 inches deep. so what gives? does anyone know the background on this issue? does it lower the value of the insulation to put it in a 5.5 inch space?

Answers:

From: Joe nolan Electrical Engineer

you run into a concept called "framing factor". this accoutns for the compression of the r19 batts and the fact that you have a specing of 2x6 which does not get insulation in it. in california we are allowed to use a value of r14.5 for r19 in 2x6 walls. joe nolan

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