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.
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.
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.
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.
"effi" < email@example.com> wrote in message
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
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