Cellulose vs. Fiberglass- R-Value

Cellulose
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
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 lose 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.

Cellulose
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 settlement.

Fiberglass
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.