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Community and Q&A

Density of blown-in insulation

Aaron Dent | Posted in Energy Efficiency and Durability on

I was in an attic with blown-in cellulose insulation that was installed in 1987. The insulation is generally uniform (though with some areas lower than others) and is installed at 12″ high, or about R-40 – not bad for 1987, but not to current standards in Northern Ontario. The installer’s form was available from back in 1987 showing a density of 2.35 lbs/ft3 – about 45% higher than the manufacturer’s website recommends. Will this negatively affect the actual R-value performance?

Also, it doesn’t look like proper air sealing was done at tops of walls, around lights, etc… It is a bit hard to justify ripping all this insulation out to do air sealing, but it also seems that adding an additional R-20 won’t help as much as it should w/o air sealing.

FInally, there is an old air exchanger in the attic that hasn’t been used for years. It has two large un-insulated ducts connected through the ceiling space. Seems to me this unit should be removed as it is likely decreasing the effective overall R-value in the attic quite significantly. Thoughts?

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  1. user-945061 | | #1

    Was the installer's form based upon bag count? Seems unlikely that open blown cellulose would hit 2.35 lbs... Not sure what the R value of 2.35 lbs/cf, but National Fiber's tech library puts 3.1 lbs/cf at a higher R value than the typical attic density of 1.4 lbs/cf. So you're unlikely to see a significant drop in R value at your questionable 2.35 lbs/cf value.

    The attic should be properly sealed. Some contractors have special vacuums for efficiently sucking insulation out of attics, usually into a box truck or trailer. But I'd consider having a blower door test done, possibly with testing to specifically measure air leakage across the attic. If the place was built during the era of poly vapor retarders, it might be fairly tight already. I put up tons of plastic in Eastern Ontario houses in the early 90's, and most of those houses would have been fairly tight as a consequence (poly across top plates during framing, at all electrical penetrations, etc.) Not that I'm advocating poly vr's (hope you're reading this Mike Holmes!).

  2. GBA Editor
    Martin Holladay | | #2

    Your estimate for the settled density of attic cellulose sounds about right; my sources say that the settled density is usually about 2.3 pounds per cubic foot. That results in an R-value of about R-3.6 per inch.

    Of course, the installed density was less, because settled density is greater than installed density. But that's normal.

    If it were my house, I'd pull back the insulation in all the usual places to investigate the air sealing. You're right that it's a pain to disrupt 12 inches of cellulose. If your investigations reveal that the air sealing work was done well, you're all set. If not, you may choose to improve the air sealing -- at least at the most critical areas.

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