Jeff wrote:

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.

Incorrect. Still air is a better insulator than fiberglass, per inch.
Fiberglass keeps air still and blocks radiation.

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.

Correct.

"mgarvie" wrote:

... I was looking at either the rigid pink foam or styrofoam insulation.

Same stuff, no? R5 per inch for pink or blue or green closed-cell foamboard
with a smooth skin, and R4 per inch for the weaker white open-cell coffee
cup material with a rough skin.

There is a big difference in price for 4x8 sheets but only a difference
in 2 R value.

Perhaps you are looking at 2" pink R10 vs R8 white sheets... 2" foil-faced
polyisocyanurate boards are better insulators, roughly R14 for the board
plus R3 for each foil, if it has air on both sides.

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?

Heatflow is like electrical flow. Ohm's law says a 2 ohm resistor with
12 volts across it will have 6 amps flowing through it. Ohm's law for
heatflow says 1 square foot of R2 wall with a 12 F temperature difference
across it will have 6 Btu/h of heat flowing through it. A 10 square foot
R2 wall will have 60 Btu/h flowing through it. A 20 square foot R2 wall
will have 120, and so on.

An 8'x32' R16 wall has a thermal conductance (U = 1/R) of 8'x32'/R16
= 16 Btu/h-F. If it's 70 F inside and 30 F outside, (70F-30F)x16Btu/h-F
= 640 Btu/h (about 200 watts of heat power) will flow through it.

A 2'x3' U0.25 (R4) window has a conductance of 2'x3'xU0.25 = 1.5 Btu/h-F.
If it's 60 indoors and 40 outdoors, (60-40)1.5 = 30 Btu/h of heat flows
out the window. Cover the inside with 2" R20 foamboard, and the resistance
becomes R4+R20 = R24, so it only loses (60-40)2'x3'/R24 = 5 Btu/h.

A gallon of oil contains about 140,000 Btu... 100 cubic feet of natural
gas has 100,000 Btu, also known as a therm... 1 kWh of electrical energy
is equivalent to 3412 Btu... 1 kW of electrical power is 3412 Btu/h.

Nick