R-value is a number given to a material to represent its ability to resist heat flow through that material. For instance 3.5” of Fiberglass batts has been rated R-13, also 3.5” of Open Cell is rated at R-13. The problem is that these test performed in a laboratory do not reflect what is taking place in and around our homes and other buildings, leakage.
To illustrate, on a hot summer day, take a Styrofoam Beach Cooler which is rated R-2, and full of drinks and ice; if you leave the lid securely in place you will still have ice and cold drinks tomorrow, but if, however, you leave the lid cracked, the ice will be gone very quickly and your drinks won't be cold for long. The Cooler will still be rated R-2, but if you don’t have an air seal, heat will move freely in and out.
This is the problem with fiberglass insulation is leakage, often from the day of installation. If not installed perfectly, fiberglass batts do not prevent leakage, and over time, they settle and begin to disintegrate resulting in more leakage. If the person building your house were paying your utility bills, they would NOT be usijng fiberlass batts but would spray your house full of foam insulation.
To illustrate what is taking place in our homes. You have set you thermostat on the wall and it just stopped running. A little time goes by and it starts again because all the cooling (or heating) you had just paid for had already leaked out. Keeping the temperature from leaking out is our goal. A house with leaks is our enemy.
Since Fiberglass Batts in the wall are very poor at stopping air flow, it is like leaving your cooler lid cracked open ALL DAY, EVERYDAY!