Replies

1. Expert Member
   Dana Dorsett | Oct 05, 2015 05:54pm | #1

   Horizontally placed RB with air gaps on one or both sides has assymetric heat flow characteristics due to the buoyancy of the trapped air and the convection inside the air gaps. When the hot side is on the top, the hotter layers of air stay at the top of the air gap(s), and there is no convected heat transfer. When the hot side is on the bottom, the warmer air rises, the cooler air sinks, and it's constantly convecting heat from the bottom to the top of the air gaps.

   But if the material in contact with both the shingles and the roof deck there is no benefit to the IR emmissivity/reflectivity of the material, since there is no path for heat to radiate between layers- it's all conducted (or convected at a high rate in a super-thin air gap, where not incomplete contact.) Without any substantial air gaps, it's thermal benefit is zero.

   I'm not sure what means "....the product has an acceptable R-value for "downward heat flow" to be eligible." in the 'merican vernacular.

   Eligible for what- marriage? Valuable prizes? A free trip to the dumpster, perhaps? :-)

2. 
   wlarsen | Oct 05, 2015 06:00pm | #2

   I assume RB = radiant barrier in your answer, right?

   This contractor is installing the material with no air gaps, so as you stated, there is no benefit to the low-e or reflective properties of the material.

   The material is actually a sandwich of aluminum-foam-aluminum, so the manufacturer is claiming that the insulation part provides different r-values depending on the heat flow direction, when the product is sandwiched between the sheathing and shingles.

   Sorry for the confusion about the word eligible! I work for a program administrator in California, and I'm responsible for verifying whether products meet our standards for financing.

   Thanks for the help!
   Whitney

3. 
   charlie_sullivan | Oct 05, 2015 06:03pm | #3

   Short answer: heed the alarm bells.

   Medium answer: The only time R-value is different for different heat flow directions is when you are considering heat transfer across an air gap. If this product is in contact with the shingles above and the roof deck (or water control layer) below, there's no air gap. So both the contractor and the manufacturer are at best confused, at worst fraudulent.

   Long answer: When you have an air gap and no radiant barrier, the air gap only adds about R1 and and is not really worth analyzing in detail. When you have an air gap and a radiant barrier on at least on of the surfaces facing the air gap, you pretty much eliminate the heat transfer by radiation. So all that is left is convection. Convection does in fact work differently up and down. The materials on either side of the gap work the same for heat flow in either direction, but the convection process for heat to flow across the gap works differently according to heat flow direction. For that reason, the combined structure of a radiant barrier and an air gap works better and has a higher effective R value of the assembly for heat flow down. But it's the convection across the air gap, not the radiant barrier that works differently for heat flow in any direction. But as you know and the manufacturer admits, the radiant barrier feature only works with an air gap and it sounds like you don't have one so there's no such effect.

4. Expert Member
   Dana Dorsett | Oct 05, 2015 06:20pm | #4

   I dunno Charlie, if the sandwich is an inch or three thick the foam's R value is worth something, but foam insulation (at any thickness) is insensitive to the direction of heat flow.

   But it's 1/4" thick it's worth about R1.

   Whitney: Yes, RB= radiant barrier. I still have no clue what it's supposed to be "eligible" for, do you?

5. GBA Editor
   Martin Holladay | Oct 07, 2015 04:25am | #5

   Whitney,
   Q. "R-values - are they different depending on direction of heat flow?"

   A. Material R-values that manufacturers are required to report (a requirement established in the U.S. by a federal law, the R-Value Rule) and R-values that builders are required to report (per the same law) are strictly defined. These R-values do not vary by direction of heat flow.

   A radiant barrier has a negligible R-value -- so close to zero that you might as well call it zero.

   If you use a radiant barrier to create a building assembly -- the classic example is a 3/4-inch or 1-inch air space bounded on one side by a radiant barrier -- then it's possible to measure the R-value of the resulting assembly. The R-value of the air space varies depending on many factors, including (a) the emissivity of the surfaces facing the air space -- and note that this emissivity varies over time, because radiant barriers can get dusty -- and (b) whether the air space is part of a wall, a sloped roof assembly, or a horizontal floor assembly. The direction of the heat flow can also affect the measurement of the R-value of this assembly, as other GBA readers have correctly noted, due to the effects of convection.

   The best reference work that explains these factors is ASHRAE Fundamentals.

6. 
   wlarsen | Oct 07, 2015 11:17am | #6

   Thanks so much for your answers Dana, Charlie, and Martin. I'm so impressed by the community on this site.

   Whitney