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Effect of adding cellulose insulation on top of existing R-19 fibergass batts

My builder has installed R-19 fiberglass insulation (with vapor barrior) in the attic of the 2000 square foot one-story house he is building for me in Maine. He now plans to add 13-14 inches of blown Green Fiber cellulose insulation on top of this. Cellulose insulation being as heavy as it is, my concern is that it will compress the fiberglass, diminishing its loft and hence diminishing its R-value. Does anyone have any data on the relationship between compression and loss of R-value with fiberglass insulation? Or on the question of how much such a layer of cellulose would compress the underlying fiberglass batts? If this could be known, the cellulose layer could be increased in depth to compensate for the R-value loss.

Asked by Robert King
Posted Sun, 03/23/2014 - 11:14


3 Answers

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In new construction I would think one would just use cellulose. What your builder is doing is how we add insulation when a home is already built and needs more.

As to your question, compressed fiberglass has a higher R value per inch but now is less inches thick. Not to worry. The cellulose is a benefit. Keep writing checks to your builder.

Answered by aj builder, Upstate NY Zone 6a
Posted Sun, 03/23/2014 - 12:03
Edited Sun, 03/23/2014 - 12:05.

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The only mystery here is why your builder ever installed the fiberglass batts. If he is smart enough to suggest cellulose insulation, he should have just installed 100% cellulose, without the fiberglass, as A.J. noted.

Once the fiberglass batts were installed -- whether deliberately or mistakenly -- the decision to cover the fiberglass batts with cellulose was a good one. Fiberglass batts have several disadvantages, and covering such attic batts with cellulose is a standard solution.

I hope, however, that your builder remembered to do a careful air sealing job at the ceiling plane before any insulation was installed.

The cellulose may compress the fiberglass batts a little, but the potential downsides to this compression are far outweighed by the advantages of the added cellulose. When a fiberglass batt is compressed, the R-value of the batt decreases slightly, but the R-value per inch increases. I will attach a chart that shows the relationship between compression and R-value for fiberglass batts. [Click on the image to enlarge it.]


Compressing fiberglass.JPG
Answered by Martin Holladay, GBA Advisor
Posted Mon, 03/24/2014 - 06:27
Edited Mon, 03/24/2014 - 06:29.

Helpful? 0

An R19 batt is the same amount of material as an R13 batt, just fluffed up. Compressing it to 3.5" from 6.25" (it's manufactured loft) would only be a "loss" of R5, but the improved air-retardency of the now denser fiber makes it work better at the temperature extremes.

Focusing on the total post-blown depth of cellulose + fiberglass is the right way to approach it, since the slightly compressed R19s and cellulose will have about the same R/inch.

Note that in the compression charts an R19 batt only performs to R18 when installed in a 2x6 wall cavity, and performs at R13 when compressed into a 2x4 cavity. Labeling it R19 is somewhat deceptive since the most common application is in 2x6 framed walls, and is why R20 wall cavity insulation is specified in newer versions of the IRC, not R19.

Answered by Dana Dorsett
Posted Mon, 03/24/2014 - 12:01

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