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Windows and low-e coating

I have always understood that low-e windows worked in cold climates when you had the low-e on the exterior side of the pane closest to the interior (surface 3) and vv for hot climates (surface 2). This is stated pretty clearly in the book Residential Energy, for example, which most energy auditors use to train from, and I just checked with the DOE website and they say the same thing.

Today, however, I had 3 different conversations at a green building conference. One was with a rep for a major window manufacturer who agreed with the surface 3 idea. One was with a rep for another major window manufacturer who said they used to do that but now always put it on surface 2 because when they had it on surface 3 the pane would heat up too much thus heating the airspace too much and the seals would break and they had too many warranty problems. The third conversation was with a very well-known green building expert who said they are put on surface 2 as they are really heat mirrors and reflect heat back onto the room.
The name "low-e" would make you think that originally it was intended for surface 3 in a heating climate.
Can anyone enlighten / confuse me further?

Asked by Anonymous
Posted Oct 25, 2009 9:09 PM ET
Edited Oct 26, 2009 7:21 PM ET


13 Answers

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Here's information I've gathered over the years:

"In very general terms, you would look for [the low-e coating to be on] surface 2 in an air conditioning climate and surface 3 in a heating climate. The U-factor of the window remains constant in both placements but the SHGC varies." [Tex McLeod, McLead Associates, Toronto]

"Placing this [low-e] coating on the #3 surface of the IG (the inner side of the inner light) affords a bit more solar gain advantage... But placing the coating on that surface will make the coating less effective in the summer months in keeping outside heat outside, so there is a trade-off. Low-e coatings are generally placed on surface #2 — the interior of the exterior light — except in extreme northern climates." [A window expert who posts on the Web using the nickname Oberon.]

Answered by Martin Holladay
Posted Oct 26, 2009 4:56 AM ET



Your thinking is correct and the "experts" who confused you are wrong. LowE means low emissivity of infra-red spectrum light (sensible heat). It is not a "heat mirror" (though that is the brand name of windows with intermediate films) because it does not reflect light the way a mirrored glass does.

The reason that a lowE #3 window feels warmer when standing next to it is not because it reflects your heat back but because the inner pane is not losing heat and has a higher temperature so that there is less radiant heat transfer between your body and the inner glass (it raises the mean radiant temperature of the interior space).

Most heat conduction in a multi-glazed window is by infra-red radiation between the glass panes (since the intermediate air layer is a poor conductor), and of course in the glass frame and window sash. So, putting a lowE coating (a few molecule thickness of metal oxide) on the outside of the inner pane (surface #3) minimizes radiant heat transfer to the outside in a cold environment. Placing the lowE coating on the inside of the outer pane (surface #2) minimizes heat transfer inward in summer (or in winter when the sun is shining, which is an advantage for solar gain).

Thus, lowE surface #2 is appropriate for an air-conditioning dominated climate, and lowE surface #3 is appropriate for a heating dominated climate and superior for a passive solar strategy. In a properly-designed passive solar house, heat gain in summer is controlled by overhangs or other shading of the high summer sun which, because of its steeper angle of incidence, does not transmit through the glass as well as low winter sun.

The air between panes would overheat and undermine the seals when a lowE coating is applied to both surface #2 and surface #3, which prevents radiant transfer in either direction and overheats the intervening air by direct conduction.

Answered by Robert Riversong
Posted Oct 26, 2009 11:41 AM ET


For those who are interested in delving a little deeper, clear glass has an emissivity of 0.84 to 0.92 while lowE glass has an emissivity of 0.35 to 0.05 (aluminum foil is 0.04).

The emissivity value of a material is a fraction of the emissivity of a perfect black box radiator (E = 1), and can vary between 0 and 1. Some materials may surprise you, as the E value is not simply related to color or texture. Wood, for instance, has an E of 0.85 to 0.95 and human skin has an E of 0.98 - an almost perfect radiator - which is why we love being near a radiant emitter and lose heat so quickly through our heads to the winter night sky.

But if passive solar gain is the design goal, which surface has the lowE coating is only part of the picture. The NFRC-rated solar heat gain coefficient (SHGC) is the fraction of incident solar energy that a total window unit will transmit, which is dependent on several factors. In the north country, a SHGC of 0.5 or better, with as low a U-value as possible, will offer good passive solar performance. On the south facade, the SHCG can be more important in reducing total annual heat load than the U-value.

Answered by Robert Riversong
Posted Oct 26, 2009 7:04 PM ET


Thanks so much for these answers!

Answered by Amanda Evans
Posted Oct 26, 2009 11:27 PM ET


Trade off
We are building a passive solar cottage with a direct southern exposure. I decided to use NFRC on all the non -southern windows because I wanted better visual transmittance (VT) for the south view.
Also, of course, I got a higher SHGC by using clear on the south. We will be learning so much as the seasons come and go. We have a very small mechanical footprint, just ERV and two ductless split systems. However at the insistence of our HVAC person, we will have portal electric hydronic baseboards as the back up since we live in New England.

Answered by rita addison
Posted May 19, 2010 4:56 PM ET


"NFRC" is the acronym of the National Fenestration Rating Council, the agency that sets the rules for testing window glass performance.

NFRC is not a glazing type.

Answered by Martin Holladay
Posted May 19, 2010 6:46 PM ET



Clear glass is rarely beneficial in a northern climate for passive solar design. LowE glass so outperforms clear that it has become the industry standard. What is important is to balance SHGC (solar heat gain coefficient) with thermal efficiency (low U-value) to optimize the winter season net solar gain (gain minus loss).

In most northern zones, a high SHGC double or triple glazed lowE window is the optimum for passive solar efficiency.

Answered by Riversong
Posted May 19, 2010 6:51 PM ET


The difference between putting a high solar gain low-e coating on surface number 2 or number 3 is minimal compared to the difference between having the coating on one of those surfaces or NOT having it at all. And since, if you purchase a pre-manufactured window that has an NFRC rating, the window company makes the decision for you regarding the surface on which the coating is applied, this thread seems to be missing the point.
To me, the point is that for heating dominated climates you want a high gain low-e window, meaning as low a u-factor as possbie and a high solar heat gain coefficient (SHGC). If you purchase a pre-manufactured window, the coating will likely be on surface #3, but again, don't obsess about that. If you are designing a passive solar home in ANY climate, where the windows are shaded during hot times of year, you want the same type of glass. As mentioned by Robert Riversong, a high solar gain low-e coating will outperform clear glass because while it WILL slightly reduce gain in the winter it will do a better job of keeping that heat gain from getting back out the window than will a window with clear glass.
If windows aren't shaded, and air conditioning is included, use a low solar gain (low SHGC) window with as low a u-factor as possible (although SHGC is the most important consideration). This type of window is actually a spectrally-selective low-e window. It's important to understand that the low-e coating has little effect on wavelengths of solar radiation. It's the spectrally selective qualities of the coating that give it the 'heat mirror' effect. These coatings reflect the wavelengths of solar radiation that we humans DON'T see. Thus they act as mirrored glass to non-visible solar wavelengths, but are highly transmissive to visible solar wavelengths. The emissivity of the glass is NOT what determines whether solar gain is transmitted or reflected. Emissivity comes into play when we're considering TERRESTRIAL radiation - the kind that you and I radiate or that warm objects such as heated thermal mass or hot asphalt paving radiate. Low-e coatings (on whichever surface!) will slow down the radiant transfer of long-wave infrared radiation (which is NOT a component of direct solar radiation) from one side of a window assembly to another. Low-e coatings will keep long-wave radiant energy on whichever side of the window assembly is hotter. So in the summer a low-e coating keeps radiation from hot driveways or hot sidewalks from reaching the interior. In the winter, or when outside temperatures drop considerably below inside temperatures as often happens even on hot days in a hot/dry climate, the low-e coating keeps TERRESTRIAL long wave infrared radiation inside. Low-e coatings in and of themselves don't have much effect on solar wavelengths. You need to add other elements to the coating which make it spectrally-selective in addition to being low-e if you want to limit solar radiation.
The best explanation of this is on the web at http://www.efficientwindows.org.

Answered by Bill Burke
Posted May 20, 2010 1:14 PM ET



If lowE coatings reflect only terrestrial long-wave radiation (such as give off by our bodies), what effect do they have when the house is occupied by extraterrestrials? ;-)

Answered by Riversong
Posted May 20, 2010 6:12 PM ET


There is a interesting commentary in the May issue of Solar Today by David Bainbridge and others. They lament the lack of choices in coated glass windows. With metal-coating technology, glass performance can be "tuned" to different solar gain/heat blocking requirements for different orientations - if a window manufacturer would offer it.

Answered by gred
Posted May 23, 2010 10:21 PM ET



Several custom window manufacturers do just that.

Answered by Riversong
Posted May 23, 2010 11:08 PM ET


They surveyed several major manufacturers and found no interest in producing a variety. Their bigger point was that the prescriptive legal, regulatory environment we are in doesn't give due credit for optimized passive design - window are speced to reduce heat loss only, not to provide for net energy production for south-facing windows.

Answered by gred
Posted May 24, 2010 8:07 AM ET


Check out the Canadian window manufacturers -- expecially the manufacturers of windows with pultruded fiberglass frames. To learn more about where to get high-solar gain glazing, check out these articles:

High-Solar-Gain Glazing

Choosing Trople-Glazed Windows

Passivhaus Windows

Windows That Perform Better Than Walls

Answered by Martin Holladay
Posted May 24, 2010 9:13 AM ET

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