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-13 HD at
1.0# density
R-19 at 0.55# density
R-21 HD at
0.90# 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;
forced 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
High
Density 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.
Gary
