reflective barriers 1

[superheat]

I would like some quality info on the effectiveness of reflective barriers. I could calculate the effective R-value of ducts in the attic, but the geometry gets quite involved.

I have looked at the ASHREA handbooks, but find it lacking. There is nothing about temp difference. It would seem to me that 60F air moving trough a duct in a 150F atic would have a large effect compared to that same duct in a 100F space.
I have looked at various manufacturers web sites but am not happy with the data. Any thoughts or idea would be appreciated.

 

[davefitz]

not my field of expertise, but it seems that they sell the R-19.5 bubble wrap, which might be used to insulate the outside of the duct. Not sure of NFPA flame rating , though. It uses 2 layers of bubble wrap and 2 reflective layers of mylar or aluminum .

 

[superheat]

That is the kind of info I have been getting. I am sure it is R19, in an extreme case. I asked one of those manufacturers about the conditions of the test. They did not respond to my e-mail. Radiant barriers work, but without the details it is hard to design with them. If manufactures do not give details, I can't help but think they are trying to sell snake oil potions.

Liquid N2 is transported using radiant barriers and vacuum break between them. That case has a DT about 400F.

 

[jb48]

superheat,

The temperature of the air in the attic does not affect the performance of the radiant barrier. At low temperatures, below 1000F, the absorptivity and emissivity of the air can be neglected. What you need be concerned with is the temperature of the surfaces involved, in this case the temperature of the radiant barrier, the duct, and the underside of the roof. Since the duct is totally enclosed in the attic, you can assume that the shape factor for radiation from the duct to the enclosure is 1.0, since any radiation leaving the duct eventually reaches the enclosure boundaries. The shape factor for radiation from the enclosure to the duct can then be found by area ratio. Then solve for the radiant barrier surface temperature, by setting radiant transfer from enclosure to duct, plus convective gain from air inside the enclosure to the duct equal to heat flow from the radiant barrier to the duct fluid (conduction through insulation and convection at inside surface of duct). The solution is iterative, so you may want to set up a spreadsheet, to do the repetitive calcs. Once you have the surface temp of the radiant barrier, it can be substituted back into the transfer equation to calculate the radiant heat gain.

 

[cbiber]

Or you can use an effective radiant heat transfer coefficient. It avoids the iterative solution that's accurate enough for most purposes if the temperature differences aren't too large (50 C?).

http://www.coolingzone.com/Guest/News/NL_JULY_2001/Cathy/July_cb_2001.html

 

[superheat]

JB There are at least 2 surfaces in the attic, the roof and the ceiling. they will be at different temps and have different radiation. I solved to problem you discuss in a college class. We had to come up with a relationship between shape factor between an infinite plate and a rod based on the distance between the plate and rod. That problem was hard enough. 2 plates at an agle to each other would be a harder problem.

Thank you cbiber, that is what I am looking for. Actually I am lazy, I would rather have a journal article that lays everything out in a nice neat graph. But if this were a perfect world I would be rich as well as good looking.

 

[samv]

superheat - I am curious, have you concluded that reflective barriers are an effective way to insulate ducts?

samv

 

[superheat]

Reflective barriers are very effective in attics. There can be 100F to 150F difference between the air in the duct and the surfaces in the attic. Radiation is probably the largest form of heat transfer in some systems.

I was wondering about R30 insulation in the ceiling joists compared to R4 on the ducts. I have seen several people claim to be benifiting the insulation value by piling loose-fill on top of the radient barrier. I was trying to see how much loose fill needs to be put on top of the radient barrier to get back to the same heat transfer. My calculation shows about the first 4 inches on top of the radient barrier would be useless in slowing heat tranfer in the heat of the day. I have yet to test my calculations.

I was also wondering about installing reflective barriers on the roof decking.

 

[davefitz]

The BOCA code has some recommendations for radiant barriers , and also references some test analyses of its effectiveness in different condigurations.

To be most effective in the attic for ducts, there should be a air gap on both sides of the foil to allow low emmissivity cedit for both surfaces, and it also helps to lay reflective cloth under the roof sheathing .