Insulating properties are rated by a national standard measurement called an R-value.
The higher the R-value, the more resistant the element is to conductively passing heat.
To give a basic reference, a typical double pained window has an R-value of about 7 to 9.
In Colorado climates a typical attic insulation value is recommended to be above R-30,
though in more modern construction R-38 to 48 values are common. Some common varieties are
fiberglass, fiberglass batt, foam, bag wool, rock wool, cellulose, cellulose, vermiculite,
and wood chips, each with a different R-value density.
Insulation is one element in a
tightly knit constructions system intended to improve indoor comfort and reduce energy
consumption. Insulation should never be applied without considering its effect on other
aspects of construction of existing elements of the home. Some additional factors to
consider when evaluating roof and ceiling insulation are density and compressibility,
air leakage, moisture control, and fire safety.
Each type of insulation has a density
at which its R-value per inch is greatest, but reaching this density is not cost-effective.
For 3.5" thick fiberglass batts, an R-13 batt contains 40% more material, and an R-15 batt
contains 180% more material than an 3.5" R-11 batt. Achieving the maximum possible R-value
for a 3.5" batt requires packing in the equivalent of eight R-11 batts.
An example, and
somewhat consistent throughout different insulation types, a popular "pink" loose-fill
fiberglass insulation posts the following R ratings:
19.50 inches = R49
17.50 inches = R44
15.25 inches = R38
12.00 inches = R30
10.25 inches = R26
8.75 inches = R22
7.50 inches = R19
4.25 inches = R11
The reason for the non-linear relationship is partially due to the stratified densities. As more insulation is blown in on top,
the bottom layers become compressed under the increase weight on top.
Most blown-in, or loose fill insulation tends to settle, reducing
its R-value. To achieve a desired overall R-value for blown in, loose fill insulation, specify the R-value or depth as measured after
settlement. The required bag count per net 1,000 square feet to achieve a given settled R-value is listed on the bag, or can be obtained
from the manufacturer. Monitoring installed bag counts is a convenient way to insure a quality installation.
While cathedral, or vaulted
ceilings do not have an attic space, building codes required fiberglass batt installation with very few exceptions. Some older gable type
homes, however, were constructed at a time when natural gas prices were so affordable that in was not considered to be cost effective to
install attic nor wall insulation. It was rather short sighted, but it was considered that dead air space would provide an adequate and
cost effective (at the time) insulating value.
Incomplete insulation at garage firewall into house.