R value confusion


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?
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I'm no expert in R-value but what I understand is, obviously, the higher the R number the better it insulates. Therefore, I would assume the product with the extra 2 R-value is a slightly better insulator.
Her are some sites I thought were fairly helpful:
http://www.roofhelp.com/Rvalue.htm http://www.ornl.gov/sci/roofs+walls/insulation/ins_16.html http://www.powerhousetv.com/stellent2/groups/public/documents/pub/phtv_se_in_gs_000569.hcsp
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R is resistance to heat transmission. R20 isn't all that much different than R18. R4 is very different than R2.
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mgarvie wrote:

You say you want to "replace" what was "in the exterior walls?" I don't think you want rigid foam if you are replacing it. Now if you are adding to it by making the walls thicker, that is a different matter.
I would avoid Styrofoam. Get that stuff hot as in a fire and it is very very nasty. You also will be required to take certain precautions to meet building code if you use it. I don't know how much better the pink is but it has to be better.
Assuming you are adding it, where are you adding it (interior surface or exterior)? Have you considered the problems associated with making walls thicker? Window trim electrical outlets etc.?
Assuming you are replacing what is there, I would assume you are then removing a wall surface and I would choose professionally applied foam in place or roll fiberglass.
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Joseph Meehan

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The products of combustion are: soot, water, carbon dioxide. No worse that what an oil burner puts out. As for special precautions, it only has to be covered with sheetrock. The foam is a modified material that will not burn unless there is another source of ignition.
Houses are being built with eps foam on both the outside and inside. Take a look at www.polysteel.com or www.integraspec.com Very energy efficient, very quit, extremely strong.
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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.

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

Correct.
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.

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.

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
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