compressed fiberglass insulation

if 4" thick R-15 fiberglass insulation 23" wide is compressed to only 2" thick what does the R value become of the 2" thick compressed fiberglass insulation?
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effi wrote:

Don't really know precisely. But do you know that the material used in insulation has no effect on the R value? It is the amount of stagnent air trapped in the material that has any effect on the true R value. If you crush 4" thick insulation down to 2", you can expect the R value to decrease. Perhaps even by half.
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Would you also get half of the R value if you cut the 4" thick fiberglass in half to 2" thick?
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effi wrote:

There would be a decrease in the R value. Whether it would be exactly half is best left to the insulation experts. The R value of fibreglass insulation can be from 3.0 to 3.7 per inch of thickness. Therefore, the thicker the uncompressed insulation, the more R value.
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...so 2" of fiberglass has about the same insulating value as 4" of fiberglass compressed to 2"...
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replying to willshak, Don Forcash, Mechanical Engineer wrote:

The R-Value per inch goes up within a band of approx 25% compression, but the R-value of the purchased thickness goes down. Once you pass this critical compression point, the R-value per inch begins to go down again and the R-Value of the purchased thickness continues to go down. The science has to do with the trade from compression of the increased resistance to airflow (convection heat transfer) versus the increased heat conductivity. A practical way to visualize this is a limit analysis at the extreme end of the total compression vs. heat flow curve. At the extreme end of the curve, fiberglass is compressed to the point of being glass, with essentially no ability to stop heat flow (maybe R=1). To save money, fiberglass insulation manufacturers go a little light on the compression - Just shy of the optimal density. Compressing to approx 75% of purchased thickness achieves the optimum R per inch, which will yield approx 6% is R-value per inch improvement. So putting a 6" batt in a 4" stud cavity can get you to R-15. Interestingly, some manufactures sell a high-density more expensive 3.5" fiberglass insulation that will actually give you an R15 at purchased thickness. If you were to further compress this insulation you would see a reduced R per inch and a reduced total R-value because they have already compressed it to the optimal point.
This is real science - Not opinion. If one wants to spend the time, there is empirical and analytical data on the web that explains this in both mathematical and measurement terms. Hope this helps.
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Fiberglass insulation is based on thickness and density. 1" fiberglass (uncompressed) is R-4.2 . 2" doesn't necessarily mean 'double'. A 3.5" (uncompressed) is R-11, 4" thick insulation is R-19, and 8" thick is R-30. These ratings are based on the correct installation of a vapor barrier as well. The density of the different types of fiberglass does play a role in the R-value. To what extent I'm not 100% sure. But do notice that the thicker the fiberglass is, the less and less the insulating quality (Q) is. That first inch is the highest per inch. The last inch is the least. Compressing the fiberglass will have an effect on the Quality. To what extent I'm sure isn't good. It depends (like the last posting) on the fact that air is a poor transmitter of heat energy and trapping the air creates an excellent insulator.
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Most manufacturers will publish R values when their insulation is compressed by certain percentages as well as when installed with air gaps, etc.. Call the manufacturer to get this info. When you compress insulation, you drastically decrease the effectiveness of the insulation...

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