radiant coating on duct insulation

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  #1  
Old 10-31-01, 09:23 AM
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I have a question for rbisys. I have read all of your posts. Most duct insulation has a shiny surface, but only R-2 FG. What is the true insulating value with 45* air inside and 180* attic? Is there a better way to insulate ducts?
 
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  #2  
Old 11-03-01, 04:41 PM
rbisys
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Greetings, Kkim

Heating ducts in the attic are disasters, PERIOD.
What value is "R" 2? The fiberglass industry has the American consumer paranoid over "R" values. The "R" values that are used are obtained from a test procedure that was and is controlled by the fiberglass industry. The one that is used is C-236. This test is a 30 deg temp differece test that minimizes the radiant energy factor and is bone dry. Now, knowing that fiber glass is at least 98% air spaces and the material itself is only about 10% efficient against heat energy you tell me what value "R" factors are. This is why the fiberglass industry, the US government and utility companies have strived so hard to prevent the spread of RB usage.
Now in the case of your duct it is obvious the aluminim foil is the primary insulator,BUT, you have a large radiating surface and air moving over it so its effectiveness is diminished.

If you look in an engineering manual you'll see the emissivity values and a formula to figure the BTU's that are radiated from one surface to another per a given temp difference. The chart I have shows a 2 BTU gain into a room from a RB that is 110% to a 75 deg floor. By the way at those temps a fiberglass insulated ceiling will radiate abiut 37 BTU's /hr/sf. This RB BTU value does not change much over a fairly large winter/summer temp difference. But again walls and ceil'gs are a low air movement area and the dynamics for ducts are different. I recommend to my customers that they use foil covered ducts if there is no other placement option. This is all I can recommend because that's the only option I know of. The other factor(an aside) is that a 2 story house insulated with RBs will not usually require an upstairs heat/a/c unit so the ducts stay in the house. Consider, the upstairs temp of a RB house
will be about 81-82 degs on a 95 deg day. The 1 st floor about 78 deg.

The only other method that I find superior is foil wrapped foam ducts, but then, most home buyers don't want to pay the extra up front. They would rather pay it over and over again in their utility bills.

Thanks for considering my opinion.
 
  #3  
Old 11-04-01, 08:59 AM
Insulman
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The actual material covering duct wrap insualtion is a reinforced foil known as FSK (flame spread kraft) The reason it is designed this way is for its ability to retard fire.. Generally all walls in a residence require a min 1 hour fire rating, 2 hours at attached garages.. This is usually accomplished when the drywall is installed.

However in areas that are exposed ie...attics etc that have mechanical systems that penetrate to the home it is required to have a one hour or more fire block. That is the purpose of the FSK facing on the duct work..

Good Luck

Jim

 
  #4  
Old 11-05-01, 09:50 AM
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Thank you for the reply.
Most of the systems around here are in attics. I try to run the ducts in the attic floor joist space. Then they cover the ducts with insulation. This removes the RB effect. From what you have said, that decreases the insulating effect. could I install a second radiant barrier on top of the ducts? Is the material you use stiff enough to hold itself up in the center if I tack the RB to the floor joists? Could you give me a good source for RB material? I would like to experiment after I pull out my engineering manuals.
 
  #5  
Old 11-05-01, 10:17 AM
Insulman
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I am not a huge advocate of RB in attics.. especially when laid over insulation... The Dept of Energy web site says rb is only effective until it gets dusty then it loses its reflective capabilities. They predict this will happen within 2-10 years...

Do you blow your attics with insulation after the duct work is installed. I would think that would accomplish more than a reflective barrier that will lose its value in such a short time...

The web site I got my rb info is

http://www.ornl.gov/roofs+walls/radiant/rb_05.html

Good luck

Jim
 
  #6  
Old 11-08-01, 12:21 PM
rbisys
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Greetings, Kkim

Ah, we have an engineer. Thank GOD for small favors. Look up the emissivity factors and you will see the % of effectiveness for all common building materials. Glass is less than 10% efficient, as is, wood & gypsum. Next check petroleum products for foam efficiency, avg. 20%. Next the metallics, alum. 97%. There should also be a formula for calculating heat emissivity, any direction, on the preceeding page. If you do the calcs you'll find that a FG insulated ceiling, which can get up to 110 degs on a 95 deg day will radiate about 37 BTUs / hr/sf to a 75 deg floor. A single RB in the joist airspace will radiate about 2 BTUs TO the ceil'g. This does not include the heat transfer thru the joists. On a 95 deg day the FG attic temp can get up to 160 degs. The RB about 106 degs. This is why FG salespeople are so afraid of RBs. They know that it's only a matter of time before RBs take over.

You can put a 2 layer RB over the FG in your attic. Not only will it drop the duct temp, it will also drop the FG temp thereby reducing the a/c costs. You will also get some winter savings, but not as much, as the delta T is lower for winter, plus there are some other factors involved.

The reason Insulaman is not a fan of RBs is because he is a FG salesman. Insulaman does not not have any experiences with RBs and knows only the propaganda that the FG industry puts out. He is an order taker, which most FG salesman are.
My experience with FG salesmaen is that when they encounter a RB home that is obviously performing better than a FG home, they brush it off and try to make up some excuse for the FG poor performance.

I have standing offer that any insulation dealer, buider, home owner, engineer, etc. that can produce an "installed, summer/winter, condition test" from an independent, government approved lab showing that FG gets its advertized "R" values, I will install their job free of charge, labor and material. Regardless of size. Details to you upon request. O-C did this in the late 60's and the best they could get out of their 3 1/2" batt was about "R" 6 in a 2x4, brick veneer wall. This was their test in their lab.

To elaborate a little more,(can you take it?), let's look at the increased efficiency of adding a "R" 19 batt to an existing "R" 19 attic batt ( or blown). If you look at a chart showing the increase efficiency for each added "R" value, you'll see that going from a "R" 19 to "R" 38 only has an increased efficiency of about 7.5%. Multiply this times the 20% heat loss thru the ceil'g and you see that you'l probably never pay for the add "R" 19. In fact some lab test have suggested that the add'l mass absorbing summer heat could actually increase a/c costs. Many contractors warn home buyers/building that they will never pay for the add'l costs of going from a 2x4 wall to 2x6 wall. O-C tried to push the 2x6 balony in the mid 70's with the Akansas Story. Anmother FG wipe out.

I think Insulman should get a copy of this from O-C and put it on the internet.

I have been laying RBs over FG for over 25 years and have had no problems. The customers love it because it reduces the a/c costs so much and the home is definitely more comfortable, PLUS, you don't have to listen to the a/c run constantly all day and part of the night.

Regarding dust. Laboratory tests over several decades have shown a maximum reduction in effectiveness of about 20%. This leaves the RB at about 77% efficent,or, about 7.5 times more efficient than FG. In order to minimiize this effect a 2 layer material is used and that eliminates that problem. So even if you have a single layer in the attic it will still provide plenty of performance. You would probably have to paint the RB flat black to not outperform FG.

Foil wrapped ducts are not that effective against fire if the temp goes over 1100 degs. Don't put the ducts in the attic. I know, its too late for your attic.

By the why there is a paint for the insde of exterior walls that is 30 -40% efficient, or, at least 3 times more efficient than fiber glass. How many mils are we talking here?

Let us know what your research shows.

Thank you for considering my opinion.
 
  #7  
Old 11-12-01, 09:07 AM
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I have been looking at different internet sites. They are mostly ads. The hard part about the calculations (other than somebody stoled my best book)was getting convection in line. Radiant covers radiation heat loss. Conduction- well that metal foil has no resistance to conduction. Convection is the kicker. If the air does not move, there is no convection. It reduces to conduction of air. I'm still playing with it.
I insulated part of my recovered attic last year. I plan on installin RB in the other part this year and see which is better. The attic is livable- 12 ft wide but only 4 ft of that is 7 ft tall. ceiling slopes to a 4 ft. knee wall. I have a 15,000 BTU window shaker in the finished part now. It was left from the time when there was next to no insulation. It worked OK never great. I plan on adding duct work for 18,000 Btu of central air to both parts of the attic. 12' X 35' plus 12' X 12'.
Thanks for all the info. Never really thought about more than 1 RB layer.
 
  #8  
Old 11-12-01, 03:49 PM
rbisys
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Greetings,

Sounds like a i 1/2 story type construction.

The rafters that are exposed to the room area are the conductors of a large amount of heat energy. To isolate them install one layer between the rafters, about 1" from the bottom surface and then a second one on the surface of the rafter like a vapor barrier. Then install 7/8" steel furring strips ACROSS the bottom of the rafters. This will isolate the rafter heat. On the vertical wall sections install two layers but do not install on the out side facing surface as you might have moisture problems.

Conductuon in a house 12-15%. Radiant about 75 -80%. Convection about 7%. There is no convection downward in an attic. Radiant gain in attic, over 90%in summer.

Don't put ducts in uninsulated attic area. Keep ducts on inside of living space.

Thank you for considering my opinion.
 
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