2 questions for Martin or anyone else who knows
charles3
| Posted in Energy Efficiency and Durability on
1. I’ve just become aware of a manufacturer’s claim that radiant barriers can lose effectiveness over time, either from oxidation or other deterioration. Is this true? If so, do there exist any sheet products that you believe would remain effective indefinitely in typical deep south attic temperatures?
2. I enjoyed your water heater article in the current issue of FH. Which of the steel tank models you evaluated have an easily-replaceable anode rod?
Thanks very much.
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Replies
Charles,
The shinier a radiant barrier, the more effective it is. Any horizontal radiant barrier is likely to lose effectiveness over time due to dust accumulation. Vertical radiant barriers are less susceptible to dust accumulation, but it stands to reason that any radiant barrier won't be as shiny in 10 years as it was when it was first installed. More information here: Radiant Barriers: A Solution in Search of a Problem.
I'm sorry that I don't have a recommendation for a steel water heater with easy-to-change anode rods. Perhaps a GBA reader has a recommendation.
The chief U.S. expert on anode rods is Larry Weingarten. Here is a link to one of his pages: http://www.waterheaterrescue.com/pages/WHRpages/English/Longevity/water-heater-anodes.html
The reduction in performance of radiant barriers over time due to dust & oxidation is remarkably low, even when it looks visibly dirty and none-too shiny.
But that doesn't mean there's much point to them, except as a band-aid where retrofit insulating with a conventional materials is prohibitively expensive or impossible for some reason. Most of the time money on radiant barriers is wasted, and would be better spent on a more comprehensive approach using other materials. You don't have to read the tea leaves too deeply on this to see that its true. See the matrix on page 5 of this document:
http://www.ornl.gov/sci/ees/etsd/btric/RadiantBarrier/RBFactSheet2010.pdf
If you're alread at code-min for R-value and have sealed, insulated ductwork, even if the ducts are in the attic above the insulation adding radiant barrier buys you next to nothing.
The right-most column on the second chart on page 6 puts in in dollar terms. At those low returns you're better off buying grid tied photovoltaic panels to shade your roof with!
In new construction or in re-roofing it's worth looking at CRRC rated high solar reflective index "cool roof" materials- they come in more colors than white these days, and do more for heat rejection than radiant barrier on the rafters. But even for climate zone 3 it might not always have a net benefit from an energy use point of view, since in some instances it reduces more in useful gains during the heating season than the cooling energy use savings it buys you.
You wrote, "The reduction in performance of radiant barriers over time due to dust & oxidation is remarkably low, even when it looks visibly dirty and none-too shiny." What data is that statement based upon?
I'd have to dig it back up- IIRC the effects of dirt & oxidation on radiant barrier was tested in multiple ways back in the mid 1980s by researchers at Texas A & M University, but it's been at least 3 years since I was looking at that- it may have been somebody else's study. TAMU has a huge archive of this stuff online- you might be able to google up the papers, if not the raw data.
The reason the effects of dirt is small is that only tends to build up on one side, and while that side's emissivity and reflectivity goes to hell, the other side tends to still be shiny. One low-E aluminum surface reduces the radiated heat transfer by over 95%, so even if the other side is grungy, most of the benefit there ever was mostly remains. With 95% of the radiated fraction reduced, the convective transfer dominates the heat transfer equation, and the loss of effectiveness on the dirty side barely moves the needle on the overall heat transfer. But even the dirty side still has considerable radiant barrier effect unless it has adhering soot films or is totally oxidized, without even a hint of shine or white-metal.
Is it safe to conclude that GBA approves of the products it allows to be advertised on its site? That is one of the places I saw the claim about oxidation and heat deterioration.
Charles,
Q. "Is it safe to conclude that GBA approves of the products it allows to be advertised on its site?"
A. No. Ads just pay the bills. Caveat emptor.