Cellulose vs. Fiberglass- R-Value
The R-factor of cellulose insulation is approximately 3.8 per inch and it does
not vary significantly over a wide range of densities. In an attic, 10 inches
of cellulose insulation will have an R-Value of about R-38, regardless of the
density of the material.
"Fluffing' cellulose by adding excessive air during installation does not
change the R-factor of the insulation, so it is easy for an inspector or home
owner to check the R-value of the installation. As long as the insulation
maintains the required thickness it will have the specified R-value (Refer to
the section on Settlement for more information.)
In addition to maintaining R-value over a wide density range, cellulose
insulation also maintains R-value under cold conditions. At an attic
temperature of 20° F below zero the R-value of cellulose insulation is higher
than at 70° above zero.
It's important to emphasize that while R-value is important, many other factors
determine the real world thermal performance of buildings. Just because the
R-value between two insulation types is equal does not guarantee that they will
perform the same.
Fiberglass R-value changes with density. Light, fluffy blown-in fiberglass may
have an R-factor of 2.2 per inch, or less; The dishonest practice of "fluffing"
blown-in fiberglass both reduces the R-factor of the material and increases the
amount of settlement that will occur. Depending on the density of the material,
12 inches of blown fiberglass may have an R-value anywhere between R-26 and
R-38. Under winter conditions the R-value of fiberglass is further reduced. The
actual R-value of blown-in fiberglass in an extremely cold attic may be 30,40,
or even 50 percent lower than the purchaser thought they were buying. The more
you need it the less insulating performance it delivers.
The stated R-value of fiberglass batts is based on full thickness. Batts
stuffed into wall cavities are often compressed to less than full thickness and
Settling and Loss of R-Value
All loose-fill insulation settles after installation. Cellulose insulation
should be specified and sold at settled density, so compensation for settling
is built into the bag count and material weight columns of cellulose coverage
charts. Other loose-fill insulation materials may or may not compensate for
expected settlement in coverage chart data.
The Federal Trade Commission R-Value rule and accepted industry standards all
require cellulose insulation to be specified and sold at settled density.
The bag count and weight of material columns on cellulose coverage charts give
precise and accurate information on the amount of material that must be
installed to produce the specified R-value. It is not necessary to install more
material than is indicated by these columns to compensate for settling.
Compensation for settlement is built into the coverage chart. In fact, it is
commonly believed the test specified results in overcompensation for
There are no industry or government standards that address settlement of
fiberglass insulation, in spite of the fact that several studies indicate blown
fiberglass settles. The FTC R-value Rule acknowledges that fiberglass settles,
but contains no provisions requiring manufacturers to account for loss of
thickness after installation.