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Gary in WA 12-26-2010 12:32 AM

The "biggest loser" in fiberglass insulation....
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:

The actual test shows at 60-70*F. you can lose 35-50% of your R-value with low density attic insulation:
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.buildingscienceconsulting...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:

After it gets wet, it can continue to grow mold-when you think you got it all:

R-value, density, installation, etc. are important but so is air leakage:

Air barrier (Airtight Drywall Approach), WRB (Water Resistive Barrier) and convective loops with steel studs:

2009 minimum Energy Codes have improved, find your area on the map or the City listing below the map to find your insulation requirements:
Then use the chart and the footnotes for your area category:
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.


jmiller 01-18-2011 07:37 AM


Originally Posted by GBR in WA (Post 556922)
If you have to use fiberglass (least effective) batts, get the High Density- all the rest have convective loops with the low density and low price. If using fiberglass (my last choice) blown-in, install it at more than 0.5# per cubic ft., or choose a better product.


How do you really feel about fiberglass, Gary? :laughing:

As a roofing contractor I'm often appalled at the state of the attic insulation. I'd encourage any capable homeowner to climb up there and check for settled/missing insulation.

Most people in the trades are terrible about fixing the insulation they disturbed when, for example, installing a new bath vent/light or even just making their way from one end of the attic to the other.

gma2rjc 01-20-2015 07:19 PM


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