R value is the fraction of heat that would be transferred compared to air.
So R-19 (6" of fiberglass) would transmit 1/19 as much heat as 6" of air. R
values are used since they can be added to calculate the R value when
insulation is combined. Example put R-19 fiberglass on top of R-11
fiberglass in an attic would give R-30.

"mgarvie" wrote in message
m...
I'm getting ready to remodel my bathroom and figured I would replace what
was placed in the exterior walls and called insullation back in 1957 (one
inch of fiberglass mat). I was looking at either the rigid pink foam or
styrofoam insulation. There is a big difference in price for 4x8 sheets
but only a difference in 2 R value. I've read what I could and still
don't understand what a difference of 2 R would make. Can someone explain
or point me to a site where in REALLY simple terms I can find out what an
"R" really is?