Thread: R values for foil-backed board

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1. Ed2
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Nov 2012
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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. 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. Senior Member Rank 2
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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. Ed2
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Nov 2012
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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?

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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. Ed2
Junior Member
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Nov 2012
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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?

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