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mattie

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Reply with quote  #1 
Hello all
This post comes from looking into R values for the Missori U air filled plastic films, used as glazing on the greenhouse.
Heres is the spreadsheet for polycarbonate vs air pumped plastic.The air cavity between the sheets of plastic is 4 inches =100mm or .1m .the 6mil plastic film when converted to mm=0.15mm or 0.00015m.For the other conversion I multiplied the R value by 6 for SI units to US values.

Theres a big difference in comparing both R values.If i have made any mistakes please point them out no point in sharing wrong information.Perhaps theres some application for air filled plastic regarding solar panels unless temperatures get too high.I uploaded two different files in case of opening issues with older versions of excel.
Regards Mattie

Garage_Hermit

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Reply with quote  #2 
Nice, Mattie.

I did an "independent" web-search for "R value of 10-mm twinwall", and it said, 1.9.

So that is very close to your Excel value.


For anybody wondering about the enormous difference between the two glazing options, that is, R 25 for the PE-film sandwich, and R 1.8 for the twinwall, don't forget that the PE solution is TEN TIMES thicker than the twinwall [redface]

=============================
As quoted elsewhere, above a one-inch air gap, the effectiveness of an air gap diminishes.

Whence my suggestion to use multiple films, and split the one big 10-cm air gap, up, into FOUR x 25-mm air gaps...

I will post the justification for this later (otherwise I will crash my post...)

G_H


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mattie

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Reply with quote  #3 
Hello G_H
I went ahead and posted this as above seemed like a summary of your previous reply.There is still some info regarding solar applications from you missing so perhaps underneath this is good place to send it.

From previous post "beyond one inch thick, the utility of an air space declines, whence the advantage (for THICK air spaces...) of dividing, by increasing the number of PE sheets, to create additional barriers to heat"
So your logic here suggests that a better design on the Missouri U would have been four 1 inch air gaps ,or 5 instances of plastic film making up the four layers of air.

The air pump would then need some sort of manifold or connection to feed 4 separate sections of air creating a plastic version of quadruple glazing.I know that with window design the air gap is kept low because of losses
This from wiki "The maximum insulating efficiency of a standard IGU is determined by the thickness of the space. Typically, most sealed units achieve maximum insulating values using a space of 16–19 mm (0.63–0.75 in).

So a lower spacing would seem a to be a better option here (0.63–0.75 in). 
More wiki "Argon, krypton and xenon are often used since (at normal temperatures) they do not carry heat in rotational modes, resulting in a lower heat capacity . Argon has a thermal conductivity 67% that of air, krypton has about half the conductivity of argon.Krypton and Xenon are very expensive. These gases are used because they are non-toxic, clear, odorless, chemically inert, and commercially available because of their widespread application in industry."

I'm not sure  how practical it is to use any of this in a DIY manner and i have mentioned argon before on another thread.Or if the gasses can be sourced easily(it says commercially available above) have seen some argon canisters on line so for smaller applications so maybe feasible, larger scale not so cost effective over using air.

I could very easily be drifting into the land of over complication here and air could be the most easily applied solution. I wonder how well this concept would work with solar panels to provide insulation?
Regards Mattie
KevinH

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Nick Pine used argon between polycarbonate: http://www.builditsolar.com/Projects/Sunspace/PineSunSpace/PineSunspace.htm

Kevin H
MN
SolarInterested

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Reply with quote  #5 
There's also a greenhouse design that uses soap bubbles pumped between the layers at night for extra insulation.
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mattie

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Thanks KevinH and SolarInterested
Some good info shared. I see Nick Pine used a pump nozzle somehow to fill the polycarbonate with argon from a gas cylinder.The soap bubble greenhouse is a new concept to me ,looks like an interesting one for sure.
Another thought here is if an air and argon are both used to fill a larger space ,like in the case of the greenhouse air pumped cavity between plastic films,would the air and argon remain separate to each other or do they mix and create a more uniform overall improvement, than if the air alone between plastic films?
Regards Mattie

Garage_Hermit

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Reply with quote  #7 
Guess the gases would mix, like nitrogen and air are already mixed (air being 80 percent nitrogen and 20% oxygen, approx.).

=====
Otherwise, here is the justification for splitting up a big air gap, into several smaller air gaps:

Cavities and Air Spaces

An air space is a planar volume of air contained on two sides by some elements of an envelope assembly (drywall, brick, insulation, etc.). As mentioned in insulation, air spaces are commonly built-into wall constructions to help reduce heat transfer when multiple layers are in series.

Air has a high resistance to heat conduction, but it has almost no resistance to heat radiation, and little resistance to heat convection outside of the thin air film touching surfaces. When conduction, convection, and radiation all occur at the same time, the overall thermal resistance of air spaces becomes virtually independent of gap width when it is greater than around 1" (2.5 cm). 

The resistance of a thick air space can be increased by subdividing it into several thin layers. The resistance of the whole space is then the sum of the resistances of the thin air spaces, plus the resistances of the separators. Triple-pane or higher multi-pane windows use this strategy.

- See more at: http://sustainabilityworkshop.autodesk.com/buildings/total-r-values-and-thermal-bridging#sthash.IjTGbO5P.dpuf

Cavities and Air Spaces

An air space is a planar volume of air contained on two sides by some elements of an envelope assembly (drywall, brick, insulation, etc.). As mentioned in insulation, air spaces are commonly built-into wall constructions to help reduce heat transfer when multiple layers are in series.

Air has a high resistance to heat conduction, but it has almost no resistance to heat radiation, and little resistance to heat convection outside of the thin air film touching surfaces. When conduction, convection, and radiation all occur at the same time, the overall thermal resistance of air spaces becomes virtually independent of gap width when it is greater than around 1" (2.5 cm). 

The resistance of a thick air space can be increased by subdividing it into several thin layers. The resistance of the whole space is then the sum of the resistances of the thin air spaces, plus the resistances of the separators. Triple-pane or higher multi-pane windows use this strategy.

- See more at: http://sustainabilityworkshop.autodesk.com/buildings/total-r-values-and-thermal-bridging#sthash.IjTGbO5P.dpuf
Cavities and Air Spaces

An air space is a planar volume of air contained on two sides by some elements of an envelope assembly (drywall, brick, insulation, etc.). As mentioned in insulation, air spaces are commonly built-into wall constructions to help reduce heat transfer when multiple layers are in series.
Air has a high resistance to heat conduction, but it has almost no resistance to heat radiation, and little resistance to heat convection outside of the thin air film touching surfaces. When conduction, convection, and radiation all occur at the same time, the overall thermal resistance of air spaces becomes virtually independent of gap width when it is greater than around 1" (2.5 cm).
The resistance of a thick air space can be increased by subdividing it into several thin layers. The resistance of the whole space is then the sum of the resistances of the thin air spaces, plus the resistances of the separators. Triple-pane or higher multi-pane windows use this strategy.

(courtesy of: http://sustainabilityworkshop.autodesk.com/buildings/total-r-values-and-thermal-bridging )


G_H

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mattie

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Reply with quote  #8 
Hello G_H

"the overall thermal resistance of air spaces becomes virtually independent of gap width when it is greater than around 1" (2.5 cm)" or 25mm

"The resistance of a thick air space can be increased by subdividing it into several thin layers. The resistance of the whole space is then the sum of the resistances of the thin air spaces, plus the resistances of the separators. Triple-pane or higher multi-pane windows use this strategy."
Good info.

So looking again at the excel sheet it seems there are missing variables not factored into the calculation that will drop the R 25 result it seems as the 4 inch gap is over the recommended 1 inch or less threshold. Multi layer plastic films as you have mentioned spread over the 100mm or 4 inches @ 25mm or less seems to be the way forward here.
 
I suppose what i was getting at earlier with the argon air mix, is would it create pockets of argon in the air cavity that have better insulating properties than that of air or does the mix improve the overall R value.
It would be an interesting experiment to try i think.
Regards Mattie
mattie

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Reply with quote  #9 
Hello all
Something else that i later thought of and forgot to add regarding multilayer plastic films is,with each layer of plastic light transmission will drop .Here is a website article with many different glazing types and pros and cons for each.

http://yukongreenhouse.weebly.com/glazing.html

Another concern with even dual layer plastic is dust will gather on the inside of the plastic resulting in a drop in light levels internally.Maybe its a simple case of using a filter that can prevent this occuring but worth mentioning to anyone else considering this route.
Regards Mattie
Garage_Hermit

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Reply with quote  #10 
Good points, all.

Still, what are the respective "weightings of the pro's and con's ?

Guess the only way to find out, is to build a model and test it !

(right on for the dust filter -- or rather TWO - one upstream, one downstream of the compressor...)
(sorry -- just upped the cost, and dropped the efficiency) [frown]

G_H

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