R values and their metric equivalent, RSI values, are a way of labeling the effectiveness of insulating materials.
R-value is a measure of resistance to heat flow through a given thickness of insulation material. Theoretically, the higher the R-Value, the better the insulation.
Insulation is meant to stop the movement of heat and R-Value is a popular way of comparing insulation, however heat can move in and out of your house in different ways: Convection – Conduction – Radiation – Air infiltration. R-Value does not measure air leakage.
The advertised R-value of conventional insulation is determined by multiplying its thickness (in inches) by its R-value/per inch.
Example: R-Value per inch (3.7) x 10 inches of insulation = R-37.
To get the most payback from your investment, consider products that can provide the R-Value required by your building code as well as stop random air leakage.
The R-Values of insulation materials are measured in a lab. That would work great – if your home were inside a lab! But your home was built outdoors, and that means there are other factors like wind, humidity, and temperature changes in play. These factors create pressure differences between the interior and the exterior of the building due to things like hot air rising, wind, and mechanical systems forcing air through every tiny little opening and making its way to the interior or exterior, or to unconditioned areas of the building like attics, basements and crawl spaces.
Your home or commercial building may look solid, but there are thousands of tiny gaps, cracks and penetrations between building materials. For example, if you apply the air pressure of a 20 MPH wind on a 20 degree F day to a building, the typical R-19, fiberglass insulated wall often performs no better than the wood studs (R-6) because of air infiltration, with heat being transported around (bypassing) the fiberglass batts through convection. In very low density materials like loose blown fiberglass, heat will actually radiate right through the insulation, and this, along with convection, significantly reduces fiberglass’ installed performance and your comfort.
A superior insulation system will have good R-Value (prevent heat loss via conduction), will be pneumatically or spray applied, fully filling the building cavity (prevent heat loss via convection), and will be densely packed (prevent heat loss via air infiltration and radiation). Fiberglass meets the first criteria, but not the other three. Spray foam meets all four of these critical performance criteria!
In addition, you want your insulation to do more than just insulate. Besides insulating, spray foam blocks the transmission of sound much more effectively than fiberglass. The insulation in your walls, ceilings, attic, etc., has a lot of jobs to do besides insulating – and spray foam is up to all those jobs!