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-   -   R-30 Fiberglass Thickness Discrepancies (http://www.diychatroom.com/f103/r-30-fiberglass-thickness-discrepancies-162013/)

chronojosh 11-03-2012 05:54 AM

R-30 Fiberglass Thickness Discrepancies
 
Hi all,
I searched the forums and didnt find an answer so I'm hoping someone here knows. I found two fiberglass insulation products with an R-30 value. One is a continuous roll (http://www.lowes.com/pd_177781-1722-...%7C1&facetInfo=) and the other is batts (http://www.lowes.com/pd_323807-1722-...%7C1&facetInfo=). How is it that they have different thicknesses (one in 9.25" and one is 10.25") but still have the same R value. It is only an inch difference, but I still am curious how they can both be the same but are different thicknesses. Confused......

Windows on Wash 11-03-2012 06:52 AM

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

chronojosh 11-03-2012 08:20 AM

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!

Windows on Wash 11-03-2012 04:03 PM

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?

concretemasonry 11-03-2012 04:53 PM

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

Gary in WA 11-03-2012 09:52 PM

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/...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.buildingscienceconsulting...Measure_Up.pdf

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

http://www.greenbuildingadvisor.com/...ort/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/vi...ventilation%22

Help stop air leaks with cellulose or Roxul: http://bct.eco.umass.edu/publication...-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/bigge...ulation-90438/


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