The "biggest loser".
When it comes to fiberglass insulation, it used to be R-11. Now R-13 has replaced R-11 because of the DOE minimum Energy Code requirements.
The R-11 was prone to natural convective loops anyway. The R-19 batt is similar (low density 0.55# per cubic ft.) batt 6-1/4” thick which was made for cathedral ceilings, 2x8’s with 1” air space and for use in attics. When it’s installed in a 2x6 wall you lose because of compressing it to R-17. On the other hand, R-21 HD at 0.90 c.f. (5-1/2”), is made for 2x6 walls without the convective loops inside.
Insulation manufacturers have known about convective loops for a long time, at least since 1982 and yet they still sell us low density insulation, so you have to choose wisely: http://infrared-energy.com/files/SpecialReportAtticInsulationProblems.pdf
The actual test shows at 60-70*F. you can lose 35-50% of your R-value with low density attic insulation: http://www.ornl.gov/info/reports/1994/3445603820925.pdf
My local box store carries;
R-19 at 0.55# density
R-30 at 0.57# density
R-38 at 0.53# density
All the low density ones listed will have inherent convective loops in a wall installation.
Fiberglass batt insulation is rated in a laboratory with all six faces covered from air exposure. The batt has to fit the space- exactly
- sides, front and back, top and bottom. On your attic floor and knee walls it is exposed to wind-washing from the soffit venting or the crawl space venting, degrading its R-value.
Using low density batts or blown-in, you not only get inherent
convective loops with the product but with a less-than-perfect installation, it’s a lose-lose situation;
convective loops form around wall batts by leaving rounded corners (inset stapling) or a gap (3/16”-5/8”) at the front, top, bottom or backside which can give you a 25-50% reduction in R-value: pp.43-50: http://www.buildingscienceconsultin...010-03-10_When_R-Value_Doesn t_Measure_Up.pdf
If fiberglass gets wet the R-value drops by 60-70%, due to the temperature differences and moisture content from day-to-night cycling of the wall: http://www.nrc-cnrc.gc.ca/eng/ibp/irc/bsi/90-controlling-heat.html
After it gets wet, it can continue to grow mold-when you think you got it all: http://www.inspectapedia.com/sickhouse/FiberglassMold.htm
R-value, density, installation, etc. are important but so is air leakage: http://docs.google.com/viewer?a=v&q...whJgNT&sig=AHIEtbT0wgPl1yn5T5V8M8l_EeA2F-qvvA
Air barrier (Airtight Drywall Approach), WRB (Water Resistive Barrier) and convective loops with steel studs: http://docs.google.com/viewer?a=v&q...fBrOrJ&sig=AHIEtbSk4qNS_4vQRnyKBiEiG1lHFDd9yA
2009 minimum Energy Codes have improved, find your area on the map or the City listing below the map to find your insulation requirements: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec001_par001.htm
Then use the chart and the footnotes for your area category: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec002.htm
If you have to use fiberglass (least effective) batts, get the H
ensity ones, as the low density ones have convective loops with the low price. If using fiberglass (my last choice) blown-in, install it at more than 0.5# per cubic ft., cover the attic with a housewrap, or choose a better product.
So the new "biggest loser" is R-19
because you compress it to R-18- just to install in a 2x6 wall and with the inherent convective loops, your insulation value will drop tremendously.