[Greenbuilding] Power Vent for Solar Air Heater

John Straube jfstraube at gmail.com
Thu Sep 19 14:57:11 MDT 2013

While I agree that the "almost anything" has an R-value of 0.83, this is true only if the stated R-value includes the standard air film values quoted (which sum to R-0.78).  These films are not part of the the material R-value, but are part of an enclosure value. If we assume 0.83 is a real number then the material is only R-0.06 by the numbers quoted.  I think we can just assume the R-value of poly is zero.

However, R-value includes heat flow due to conduction, convection, and radiation.  The tests, like ASTM C1363, or C518, measure "Apparent Thermal Conductance", then they calculate the R-value (see ASTM C168).  Standard R-value measurements of fiberglass, cellulose, foam etc. include heat flow by the modes of conductance and radiation (about 65/35%) with no flow by convection.

On 2013-09-19, at 4:42 PM, Topher <topher at greenfret.com> wrote:

> On 9/19/2013 3:33 PM, LarenCorie wrote:
>> Polyethylene is transparent to IR, so only has an Rvalue of
>> around 0.83
> What does being transparent to IR have to do with R-value, a measurement of conductive heat loss?
> 0.83 is about the insulation value of almost anything which just blocks the wind (R-0.17 for outside air film (15 mph) and 0.61 for inside air film).  If the plastic is a film which can distort in the wind, those numbers are probably useless.
> Thank You Kindly,
> Corwyn / Topher Belknap
> -- 
> Topher Belknap
> Green Fret Consulting
> Kermit didn't know the half of it...
> http://www.GreenFret.com/
> topher at greenfret.com
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John Straube

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