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Debunk/confirm radiant reflectors in building construction

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fastline12

Aerospace
Jan 27, 2011
306
I am putting together a pole barn with metal sheeting for myself. I am going with glass batts due to price. 6-8" walls and 12" roof. There has been some large discussion recently on radiant energy passing through the metal sheeting and the use a a radiant blocker would help.

I don't know if I can read much into that. The primary focus of radiant energy is on the outside, heating up the metal which in turn might create its own radiant source but I fail to see how a radiant blocker would do any more than good old glass insulation at mitigating the heat.

I am just trying to determine if I need to specify an additional "something" between the metal sheeting and glass insulation.
 
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faq731-376
7ofakss
 
small terminology comment: I'm pretty sure you meant to write: greybody



=====================================
(2B)+(2B)' ?
 
You're considering using that aluminized bubble packing stuff, right?

If your primary concern is heating from the outside in, the radiant barrier would be effective between your cladding and your insulation. The bubble padding stuff creates its own air gap between the reflective surface and the hot emitting surface, which is necessary for it to function. The same material would be less effective if installed on the interior face of your insulation batts.

It would NOT be a useful replacement for your insulation unless you have no winter heating needs, nor would I reduce your insulation thickness based on the R value claims made by the manufacturers of this stuff.

It's also expensive. So it depends on how worried you are about the place getting hot in summer.
 
From what I just gathered, the bubble wrap stuff would do more than the radiant reflector? I am trying to think of an easy test device for this to build and compare results. Really anything that is shiny is a good radiant reflector and I think I can get basic Al foils products for much less than ones specified as "radiant blockers".

basically trying to consider the corrugated metal heating up and the radiant heat it gives off, how to quantify that as well as quantify any improvement of a radiant blocker vs just glass batts.
 
It's a system, and it's not just the foil. As moltenmetal stated, the air gap created by the bubble is critical, since it prevents convection currents, thereby limiting the mass transport heat flow to pure conduction. Then, the foil helps by reflecting the radiated heat back, which is critical because glass absorbs infrared like crazy.

Obviously, you can have whatever opinion you have and do whatever you want, but it seems silly to try and nickel and dime your way to a reinvention of something that works with something which you think fits "Really anything that is shiny is a good radiant reflector"

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faq731-376
7ofakss
 
Yes, an air gap between the radiant reflector and the hot exposure side of the insulating system is essential to the functioninng of a radiant barrier, assuming that the best solution (i.e. a vacuum instead of an air gap) is not a viable option.

If that air gap is cellular to limit free convection, that too is beneficial.
 
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