Most homeowners can save money on their electrical bill by adding insulation. Properly installed insulation is permanent, maintenance-free, and can return your investment by lowering your fuel bills. You not only help your own bottom line, but reduce carbon emissions as well. The payback period varies with the type of insulation materials, purchase and installation costs, and the price of fuel.
The function of insulation is to resist the flow of heat, which is where the “R” comes in. R-Value is a measure of the ability to retard heat flow rather than the ability to transmit heat. R-value is the numerical reciprocal of "U" or "C," thus R = 1/U or 1/C. Thermal resistance R-value is used in combination with numerals to designate thermal resistance values: R-11 equals 11 resistance units. The higher the "R," the higher the insulating value. This is expressed as an "R" value; the higher the "R" value, the more resistance to heat flow. Insulation materials have different "R" values, so buy insulation according to the "R" value you wish to attain, not by total inches.
As a side note, C-Value (Thermal Conductance) A measure of the rate of heat flow for the actual thickness of a material (either more or less than 1 inch), 1 square foot in area, at a temperature difference of 1° F. If the K-value of a material is known, the C-value can be determined by dividing the K-value by the thickness. The lower the C-value, the higher the insulating value. K-value is a measure of heat conductivity of a particular material, such as the measure of the amount of heat, in BTUs per hour, that will be transmitted through one square foot of material that is one inch thick to cause a temperature change of one degree Fahrenheit from one side of the material to the other. The lower the K-value for a material, the better it insulates. If the K-value of the material is known, the R-value per inch can be determined by dividing 1 by the K-value (R-value per inch = 1/K value), the same as for U and C. Different materials will use different coefficients, although not all of them are insulators per se. Glass is not known as a thermal insulator, but it has a U value. Materilas such as wood, concrete, and brick have K values which are used to calculate the overall R-Value of a particular wall section. This is required by code during the plan review process so determine if buildings meet the relevant energy codes.
Types of Insulation
Most insulation can be installed by a homeowner, while some require special tools and techniques. To assure fire safety, rigid insulation should be covered with 1/2" gypsum wallboard when installed. No matter what kind of insulation you buy, be sure it meets federal standards for fire and vermin resistance. Check for approval codes listed on the packaging.
Insulating material R-value per inch
Batt & Blanket
Mineral wool R—3.15 to 3.85
Fiberglass R—3.15 to 3.85
Loose Fill (Pour in)
Mineral Wool R—2.88 to 3.31
Fiberglass R—2.88 to 3.31
Cellulose Fiber R—3.70
Vermiculite (Expanded) R—2.13 to 2.27
Perlite (Expanded) R—2.70
Rigid Insulation
Expanded Polystyrene Extruded Plain R—5.00
Expanded Polystyrene R—3.57
Expanded Polyurethane (Foam) R—6.25
Fiberglass (rigid board) R—5.00
Where to Insulate
You should periodically inspect your insulation, especially if you have problems with pests such as mice, squirrels, and bats. They can flatten the insulation or remove it from areas to construct nests. Feces and urine can also wet the insulation making it less effective.
Heating Degree Days are defined relative to a base temperature - the outside temperature above which a building needs no heating. Usually the base temperature is 65 degrees F, so a 50 degree day would equal 15 HDD, (65-50 x # of days = 15 x 1 = 15 HDD). There is a similar method for calculating Cooling Degree Days, which uses the outside air temperature above 65 degrees. For a typical New York City winter day with High = 40F and Low = 30F, the Average Temperature = 35F. For that day HDD = (65 - 35) = 30. A month of thirty similar days might accumulate HDD = 900. A year (including summer average temperatures above 70F) might accumulate an annual HDD = 5000.
Here is a comprehensive list of where insulation should be used and the appropriate R-Values.
Ceilings with unconditioned spaces above should have R-38 to 44, although many attics have between R-19 and R-30. Knee walls of a finished or conditioned attic between studs: R-11. Exterior walls or walls between conditioned and unconditioned spaces; R-19 for new construction. Floors over unconditioned or outside spaces; R-19.
Perimeter of a concrete floor slab close to grade level: High R sheathing on the outside
Walls of finished or conditioned basement; R-11 to R-13. Top of foundation or basement wall, rim joist or band joist cavities should be filled with fiberglass batts cut to size. Sloping rafters, such as cathedral ceilings, make sure to leave an air space for ventilation between the insulation and the roof deck: R-11 for 2" x 4" framing and R-19 for 2" x 6". See Below.
Cathedral Ceilings
Cathedral or vaulted ceilings have limited space for insulation and ventilation within the depth of the rafter. Fitting in a 10-inch batt (R-30) and still providing ventilation is impossible with a 2×6 or 2×8 rafter. Usually R-19 is allowed in cathedral ceiling insulation for some house designs, depending on the climatic zone. For most areas, R-25 to R-38 insulation is recommended, if not required. Choose high density R-30 batts, since they are about the same thickness as R-25 batts, and follow the same installation practice.
Additional Notes
• You should also add insulation to your attic door or access panels.
• Wrap insulation around heating and air conditioning ducts which pass through unconditioned spaces.
• Wrap hot water pipes with insulated pipe wrap.
• Add insulation to foundation walls.
Make sure that you do not block soffit vents with insulation. Provide a 3-inch clearance around heat-producing fixtures such as recessed ceiling lights. Building a box out of rigid insulation is best.
Attic storage areas can make it difficult or impossible to install the correct depth of insulating material. If the ceiling joists are shallower than the depth of the insulation, raise the finished floor using 2×4 studs or other spacing lumber. Install the batts before nailing the flooring in place. If mechanical systems are located in the attic, be sure to elevate the equipment to allow for full-height insulation.
Seal all air leaks between the home and the attic, especially duct or wiring chases and bypasses. Remember that most insulation, such as fiberglass and rock wool, does not stop air flow. Use a quality caulk sealant to seal all gaps and holes.
Resources:
Whole Building Design Guide
http://www.wbdg.org/design/midg_glossary.php#R
