# R-30 Fiberglass Thickness Discrepancies



## Windows on Wash (Aug 30, 2011)

3.25 is what most fiberglass is estimated at. The other one could be mislabeled or a low density vs. high density batt.


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## chronojosh (Feb 11, 2011)

Does the density make a difference in its performance? I looked at both in the store and they are at least labeled the same in store and online. I bought the rolls and I am planning to use them over the R-30 batts that are already in my attic. Thanks again!


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## Windows on Wash (Aug 30, 2011)

The higher density batts perform better in wall application because of their increased resistance to convection. Not sure it is as much of a difference maker as it applies to the attic applications.

Why not loose blow the attic?


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## concretemasonry (Oct 10, 2006)

When you use batts to get an R30, you do really do not have a R30 ceiling because R30 is the theoretical/mythical insulating value of the insulation itself in a short term test in a laboratory of the insulation you install. The real value in place will be lower or much lower depending on the thermal "short circuiting" of the studs or joists (spacing and materials - wood or steel). - Also the installation of the batts does have a bearing on the effectiveness/ineffectiveness.

Dick


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## Gary in WA (Mar 11, 2009)

your R-30 has R-2.93 per inch: http://www.jm.com/insulation/faqs/1024.htm

Notice the different comparisons when compressing it from 1-2"; http://www.greenbuildingadvisor.com/sites/default/files/Compressing fiberglass.JPG

The high and medium densities are made by compressing their standard low density ones, that way they don't need to completely change the manufacturing machine set-up; Page 41; http://www.huduser.org/Publications/pdf/walls.pdf

Convective looping is inherent in low density wall insulation, pp 45: http://www.buildingscienceconsultin...010-03-10_When_R-Value_Doesn t_Measure_Up.pdf 

The density is very important, especially in loose-fill f.g.: http://www.homeenergy.org/show/article/year/1993/magazine/77/id/910

http://www.greenbuildingadvisor.com/book/export/html/12484

You have convective loops in a vented attic space, cover the air-permeable fiberglass with a housewrap to stop feeding them; fig.7b, d, f, and h: http://digitalcommons.unl.edu/cgi/v...y rising` attic air from passive ventilation"

Help stop air leaks with cellulose or Roxul: http://bct.eco.umass.edu/publications/by-title/cellulose-insulation-a-smart-choice/

Air seal the attic first by looking for existing dirty f.g.; http://www.finehomebuilding.com/PDF/Free/021105092.pdf

Gary
P.S. further reading, if interested: http://www.diychatroom.com/f98/biggest-loser-fiberglass-insulation-90438/


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