[Bldg-sim] How can Low-e glass have dramatically lower U-Value?

Blake, Jeff Jeff.Blake at NRCan-RNCan.gc.ca
Mon Aug 4 14:35:23 PDT 2014

All corrections and expansions most appreciated and Mike properly pointed out that my mentioning the interior and exterior exchange of radiant energy is somewhat extraneous to the discussion at hand.

One hair-splitting correction to Mike's addition: Glass is not completely opaque to infrared radiation. Of course, IR is a band of wavelengths and the shorter wavelength end of this spectrum makes it through glass (Near-IR). What we're really talking about here is thermal radiation and the ability of glass to absorb/reflect that spectrum of energy. Low emissivity coatings increase the reflectivity of the applied surface to thermal radiation. Smooth uncoated glass has an emissivity of about 0.90 and the e-coatings have an emissivity in the 0.04 range. Low emissivity coatings allow visible light through but reflect specific wavelengths of electromagnetic energy that is radiative "heat".


-----Original Message-----
From: Michael J Witte [mailto:mjwitte at gard.com] 
Sent: August 4, 2014 09:49
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] How can Low-e glass have dramatically lower U-Value?

Lots of good information in this thread, but I didn't see any reply that explains why low-e coatings have a direct impact on the overall u-value of the window.  And I saw some incorrect discussion of the radiant heat transfer impact of low-e coatings.  Here's my understanding.

Glass is opaque to infrared radiation (look at the detailed glass layer properties in most simulation software and the infrared transmittance will be zero).

The low-e coating is always on an inner face of the window unit, so it only impacts the radiant exchange between the glass layers (or in the case of some windows, between the glass and an inner plastic film layer).  Low-e coatings do not participate in the interior room radiant exchange or the exterior radiant exchange with the surroundings.  So you don't need to be concerned about any double-counting of radiant effects.

U-value is the overall heat transfer through the entire window unit (with no sun present) divided by the temperature difference.  The rated u-value includes the impact of the inside and outside convection coefficients.  As was said before, the heat transfer through the window includes conduction through the glass and gas layers, but it also includes the radiant heat transfer from pane to pane across the gas layer(s).  For a double-pane window, there's conduction through a pane of glass, then conduction *and radiation* through the gas layer, then conduction through the second pane of glass.  This is why the low-e coating has a direct impact on the u-value, regardless of how interior and exterior radiant exchange is modeled.

This FSEC page has a nice diagram and explanation of window coatings.



On 7/30/2014 9:42 AM, Blake, Jeff wrote:
> Randy,
> Some of the commonly used simulation engines do account for the effects of low-e glass on window surfaces (for example, Energy Plus and ESP-r). That is to say that these engines estimate the long wave radiation from interior surfaces (and devices) to the interior surface of the glass and radiated through the various glass panes and interstitial gases to the sky/surroundings.
> Low-e coatings effect the overall U-value of windows because of their effect on reflected long wave radiation. Which low-e coating is most appropriate for the North is important since the coatings will affect both the SHGC and the U-value, depending on their type, thickness and the surface they're applied to.
> The results of a recent study of a Northern Canadian city (in NWT) concluded that hard coat windows, despite their higher cost and relatively higher U-value make sense in terms of both energy savings and cost-effectiveness.
> Jeff Blake, P.Eng.
> Sustainable Buildings & Communities | Bâtiments et des collectivités 
> durables CanmetENERGY | CanmetÉNERGIE www.canmetENERGY.nrcan.gc.ca
> -----Original Message-----
> From: Randy Wilkinson [mailto:randallcwilkinson at gmail.com]
> Sent: July 28, 2014 16:35
> To:bldg-sim at lists.onebuilding.org
> Subject: Re: [Bldg-sim] How can Low-e glass have dramatically lower U-Value?
> I went into this thinking that the Low-e coating would simply change the SHGC and not change the U-Values much.  I suspected that Low-e coatings would NOT save energy in the far North since they would block the wanted solar heat gain.  Since the manufacturers are claiming a U-value improvement from the Low-e coating, It looks like I can't calculate the savings using our normal energy modeling software. If the U-value improvement they give is in lieu of a more direct calculation method, and is accepted by the community (you all), then I will accept and move on to the next problem.
> Thanks to all for your comments.
> Randy

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