I know the old myth is "bustd" about compressing FG batt decreasing it's R value. In fact, compressing it increases it's R value PER INCH. Is there any source for what the actuall values might be? For example, if I compress a 3 1/2" R-13 batt down to 2 1/2" what is the R value now (either as R per inch x 2.5" or a total?

Are there any tables on this?

Compressing FG will not decrease the R value per inch, but it will reduce the total R value of a given thickness. A properly installed 3-1/2" batt might be R-11. If we compress it down to 2-1/2", you won't have R-11 anymore, you'll have more like R-7 or 8. Check the attachment for the compression chart

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Posted By Birdman on 04 Apr 2011 01:06 PM
I know the old myth is "bustd" about compressing FG batt decreasing it's R value. In fact, compressing it increases it's R value PER INCH. Is there any source for what the actuall values might be? For example, if I compress a 3 1/2" R-13 batt down to 2 1/2" what is the R value now (either as R per inch x 2.5" or a total?

Are there any tables on this?

An R13 batt in 2x3 framing comes in at R10:

http://www.owenscorning.com/aro...nChart.xls

or if you don't have a compatible spreadsheet software:

http://numsum.com/spreadsheet/show/21111

The density of R19 2x6 batting is lower than R13 2x4 batting- in fact it's the same amount of material(!), so they should cost the same, RIGHT?

The higher the density, the better air-retardency it has, so it loses less R to convection withing the fiberglass at high delta-Ts.  R19 batts are junk- I personally wouldn't install anything less than R21, if forced to use batts rather than blown in a 2x6 framed structure. YMMV.

I was confused in this topic and i think it was lots of them just blow out in cellulose on the top...usually it does cut down the fiber and that is not the best way...
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