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  1. #1
    Join Date
    Nov 2012
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    Default R values for foil-backed board

    I am using some Tuff-R 2 inch double foil-backed foam board. It is rated R-13. Does this include the effect of the foil? I understand that the foil is not technically R-valued, but what is the effective value? What about dead air space behind the board? Also, I was told that the foam will eventually break down. What will happen to it and in how long?

  2. #2
    Join Date
    Dec 2010
    Location
    Houston Texas
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    2,360

    Default Re: R values for foil-backed board

    We like the board here in the south as a radiant barrier. We have sheets installed for almost 15 years now and are still fine. They do decompose when the foam is exposed to sunlight, but it still takes quite a while.

  3. #3
    Join Date
    Dec 2007
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    Tennessee
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    Default Re: R values for foil-backed board

    Heat move three ways, conduction, convection and radiation. Radiation is determined by the emissivity factor. Emissivity is also governed under the Law of Reciprocity.

    Emissivity is the property of being able to accept or emit radiation. For thermal radiation, a flat black surface would have an emissivity factor of 1. A typical mirror would be around 0.01 although there are mirrors used in lasers that are as high a 0.0001. A foil surface would be around 0.03 to 0.1. A bright white surface runs about 0.2 and most other colors run around 0.8.

    For the foil to really have any affect, there needs to be a gap between it and any adjoining surface. The absolute best gap would be a vacuum, hence the thermos bottle. A vacuum would block any conduction or convection of heat. Air is a pretty good medium, although there are some gasses that work a little better such as argon.

    Foil is highly conductive so anything that touches the foil will readily transfer heat. Without the gap, the foil can be pretty worthless.

    The law of reciprocity means that the characteristics for absorption and radiation will be the same, that makes it as effective in winter as in summer.

    I think the R-13 for the 2" think board is not even taking the foil into consideration. An air gap on one side would add something like an additional r-1 or more. If it faced another foil surface that is 3/4" away, it would add a total of an R-3. Any more than 3/4", the R value actually goes down, but not too rapidly, like R-2 with a 3.5" gap from what I recall.
    As for the degrading of the foam, once protected from UV radiation and the elements, it will last a long time, but I don't think that is what is meant by the degrading. Some manufacturers of foam will claim some pretty high R-values. They often use a process that will capture an inert gas in the foam during manufacture that increases the R-value. Over time this inert gas will leach out or "outgas" from the foam and be replaced by air. That will reduce its R-value over time and I think that is what the degrading is referring to.

    I'm not sure that foam manufacturers use that initial high R value anymore, I used to see claims of R-8 per inch. An R-6 per inch is a lot more reasonable but I would not be surprised if the foam eventually degraded to R-5 per inch. I'm not saying that they do, I'm just saying that I would not be surprised if they did.

  4. #4
    Join Date
    Nov 2012
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    7

    Default Re: R values for foil-backed board

    Thanks for the replies. If I understand correctly, a foil side open to a room uncovered is of little value. Is this correct? If it is 3/4 inch from a sheath, how much of the R-value is from the space and how much from the foil? What if the space is tiny like a millimeter? If the space is not air-tight, is there still any value from the foil? Is there any value from the double foil by having the foam board between the foil layers?

  5. #5
    Join Date
    Dec 2007
    Location
    Tennessee
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    1,381

    Default Re: R values for foil-backed board

    An uncovered foil surface facing an open room does have value. It blocks on of the three ways heat is transferred from the room to the wall, but I don't know about you, but I don't think I'd want to be in a room with foil covered walls. As for two foil surfaces facing each other, optimum distance is 3/4", it has to do with suppression of convection. at 3/4" and less, there little or no convection.

    Air also conducts heat so as the surfaces get closer together, there is some loss, but its only a little. One mm might be a bit close though. Your last question, foam insulates better than dead air.
    Last edited by keith3267; 12-17-2012 at 08:38 PM.

  6. #6
    Join Date
    Nov 2012
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    7

    Default Re: R values for foil-backed board

    Thanks, Kieth. I'm sorry to be so dense, but I'm still confused. You said in the first reply, "A foil surface would be around 0.03 to 0.1.". So I took it that foil to an open space is of negligible value. Am I missing something?

    In this case, I'm trying to do a temporary fix for the Winter, as I want to modify the roof and structure next year. So I decided to use the double foil-backed board on the upstairs ceiling, thinking the foil would be a benefit. Now I wonder if I wasted my money on the expensive foil board, except I figured I could reuse the board in the final structure. The other question is about whether the two foil layers is any value here. Is there any extra block from the upper foil?

  7. #7
    Join Date
    Dec 2007
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    Tennessee
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    Default Re: R values for foil-backed board

    I was talking about the emissivity or the foil, not its R-value. The emissivity is how much radiated energy it will either absorb or how much it will radiate. The lower the emissivity, the less energy it will absorb or radiate. This does not affect the energy it will accept from direct contact through conduction.

    Think of a wood stove. These are often a flat black in color. As you get close to the stove, you feel the heat in two ways, the primary heat you feel is the radiation. This is long wave energy (infrared or IR) that is contacting your skin. The other heat you feel is the hot air. If you take a piece of aluminum foil and put it between your exposed skin and the stove, you will feel much less heat, especially as you get closer to the stove.

    When these long waves strike a surface, they must be either absorbed or reflected. Since the aluminum foil has a very low emissivity, the side of the aluminum foil that is next to the stove is reflecting most of the radiating heat that is striking it, that is it is not absorbing it. What little radiation is absorbed combined with the heat transfer from direct contact with hot air is heating up the aluminum foil, but on the skin side, since the aluminum foil radiates very little energy, your skin does not feel the heat from it.

    The radiation from the stove heats the air around the stove. There is also a little heat transfer from the air due to direct contact of air molecules with the surface of the stove. So the air is heated by radiation and conduction. The warm air rises and moves about the room. As it moves across surfaces of the room, it transfers heat to them via radiation and conduction. The movement of the air is convection. The surfaces of the room are also being heated directly by the stove through radiation alone.

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