Clear Glass Windows vs Low-E Glass?


Old 06-11-05, 08:28 PM
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Clear Glass Windows vs Low-E Glass?

We live in Colorado at 9,000 ft...we are looking at replacing our old aluminum windows with fiberglass windows. We do not want to loose the heat gains from the sun, so are wondering if clear glass would be better than low-e glass. Does anyone know anything about which would be better? Also wondering about triple pane vs. double pane windows. Has anyone heard of Thermotech Windows out of Canada?
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Old 06-12-05, 05:39 PM
Join Date: Mar 2000
Location: Arlington, WA
Posts: 9,238
Go with Low-E glass. What little heat gain you lose in the summer will be more than offset by the savings on your heating bill in the winter!!

Never heard of them.
Old 06-13-05, 04:31 PM
Join Date: Oct 2004
Location: East of the Mississippi
Posts: 177


This is going to be more than a bit long, and also a bit technical, so please bear with me.

You have really four available options: clear glass, hardcoat LowE, "standard" softcoat LowE, and "high performance" softcoat LowE.
For the sake of this discussion, I am going to drop clear glass and high perfomance LowE and concentrate on the standard softcoat and the hardcoat products.

But, first a few comments on energy efficiency and window performance.

Glass, by its nature, has very little insulating value. A sheet of double strength glass (3.0mm or 1/8"), alone, has an R-value of about .8. An R value measurement for a specific window is probably based on "center of glass" reading....A good dual pane IGU (Insulating Glass Unit) with a softcoat LowE coating and an argon gas infill can potentially get about an R-6. A triple pane IGU with a krypton gas infill can achieve an R-10.

But, all that said, R value is not the best measure of a window’s energy performance...U value is used in the industry and it is a much better measure for determining a window's energy performance (actually it is better for whole house performance as well, but the fiberglass insulation folks have conditioned people to think in R value…).

Anyway, R-value is based on Resistance to heat flow. Heat always goes to cold. When you touch that really cold floor with your bare feet and you feel the cold spreading over your feet and up to your ankles and higher, it is not really that the cold is penetrating your body, rather what you are feeling is the heat being drawn from your body thru your feet to the cold floor. This is known as conduction.

Heat travels in three ways: conduction, radiation, and convection. All three of these factors affect your comfort level and all three of them affect, and are affected by, the performance of your walls and windows.

One really good measure of conduction is to take an aluminum bar and hold one end in your bare hand and one end in an open flame. Everyone reading this knows how long you have before you will drop the bar because the very sudden rise in temperature will certainly affect your ability to hold on.

Radiation or, radiant heat, is what you feel when the sun warms you as you are lying on the beach (except when you gain warmth by touching the warm sand around you, that would be conduction). Radiant heat is what you feel when you stand close to (but not touching) a steam radiator or a radiant heater. Radiant heaters use infrared radiation (IR) to warm you (or other objects), and IR is also the heat you feel directly from the sun....Your primary question deals with this sort of heat from the sun. You don't want to lose it and you fear that a LowE coating will deprive you of that heat gain in the winter.

Convection heat is warm air movement such as a forced air heating system. A convection current is set up as the warm air moves and is replaced by cooler air that is then warmed and the cycle continues.

As I mentioned earlier, windows are normally measured in U-values and not R-values.

U value measures how much heat is passed thru an object, or said another way, how much heat an object will pass; versus R-value which is resistance to heat flow. Although it doesn't sound like much of a difference, it does come into account much more when doing actual energy performance calculations.

A multiple pane IGU is designed to do two things.

First, it maintains a sealed dead-air space between two lites of glass. This dead air space helps to moderate the differences between the outside air temperature and the inside air temperature. The outside lite will approach the temperature of the ambient air and the inner lite will attempt to reach the inside ambient temperature. The inner lite will not reach the interior temperature because that heat will be radiated toward the exterior lite. But, while the inner lite will not reach equilibrium with the inside air temperature, it will still be warmer than the outside lite for a net gain in room temperature and especially comfort. Additionally, the air within the IGU space will be warmer than the air outside the house because of the warmer interior lite. Again adding to the net gain of the entire unit.

One might comment that using single glazing with a storm window will accomplish the same. It MAY do so in the best of circumstances, but unfortunately the window MUST be a sealed airspace for optimum performance (otherwise heat will escape thru the leaks) and any airspace wider than about 7/8" begins to lose performance because of convection currents that form between the lites within the airspace.
The outer lite and the inner lite in an IGU have to be close enough to allow the inner lite to radiate heat to the outer lite. If they are far enough apart, not enough heat will transfer and the differences in the air temperature near the two lites will cause a convection current to form. This current will very quickly suck the heat out of the inner lite and negate much of the advantage of the multipane IGU.

You can also have convection currents in your house. A single pane window or a dual pane without a LowE coating will be colder than a dual pane with a LowE colder...this is why it was so common (and still is the normal way to do things) to put heat sources under windows on the exterior walls. Essentially, heat from the heat source will rise and will warm up the glass enough to prevent much cold air from entering the room. What is really happening is that that heat is being sucked from the room, but if you can pump enough heat at the heat sink (window), then you can keep the room are simply overwhelming the amount of heat that nature can take out of the room.

Now, what if you could include something that would reflect the radiant energy back into the room? And that is the idea of LowE coatings. LowE coatings reflect IR energy, or heat, (they also reflect UV energy as well, but that is for another post). If the heat is outside, the coating keeps the heat outside. If the heat is inside, the LowE coating helps to keep the heat inside.

In the winter, the LowE coating will reflect the IR (or heat) back into the house, where it started.
In the winter, the interior heat warms the interior lite. The interior lite then radiates that heat to the exterior lite (a process I mentioned earlier). BUT, there is now a difference...instead of the outer lite absorbing that heat and radiating it into the night, because it has the coating it reflects that heat back into the house. This does two things...first, it warms the interior lite and allows it to come much closer to the inner temperature of the room; second, the warmer interior lite also tends to warm the airspace which in turn helps to keep the interior lite warmer. See the pattern?

In summer, the IR energy is reflected by the exterior lite which allows the interior lite, and subsequently the airspace, to remain cooler and closer to the inside temperature of the house.

Does all this work? It does, and it works very well. This is not just conjecture, it has been tested thousands of times in lab environments and in the field. There are some very specific test and evaluation projects that prove without doubt that it works.

Two things to look for when looking for a window - U value or factor, and Solar Heat Gain Coefficient.

The U-factor measures how well an IGU prevents heat from escaping. U-factor is going to generally be between 0.10 and 1.20. As I have mentioned, the lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.

Solar Heat Gain Coefficient (SHGC) is the measurement that actually measures how well a product blocks heat caused by sunlight... SHGC is the actual measurement of solar radiation or IR energy, or solar heat (pretty much all the same thing), that passes thru a window either directly transmitted or absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits. Now is where we get the argument between the hardcoat and softcoat (or northern versus southern) LowE folks.

Hardcoat, or pyrolytic coating folks say that you should use a hardcoat LowE on the #3 surface. The hardcoat does not block IR energy as well as softcoat...and they admit that as well. But, they turn it around to say that hardcoat is better in cold environments than softcoat because since the hardcoat does not block IR as well as softcoat, the hardcoat will allow more of the suns heat into a room - passive solar.

And, they are actually correct as long as the sun is shining directly on the window. Of course during the other 20 hours of the day the window is losing a fairly substantial amount of energy because the hardcoat has a lower U value as well.

Hardcoat folks favor a higher number SHGC in the north because they believe that the additional heat that is transmitted thru the coating (which does not block heat as well as the softcoat does) is an advantage in absorbing the sun’s energy in the winter…and softcoat folks feel that because the higher SHGC number is related to increased heat flow thru the glass, the gain in sun-heat energy is not justified when compared to the losses due to the poorer U-value…and the fact that in the summer that same higher SHGC will allow more of the sun’s heat into the home and raise cooling costs as well.

The hardcoat guys counter by saying that in the winter the sun is lower and shines directly on the window so the heat gain is significant and does offset the greater loss because of lower U-value, and in the summer the sun is higher and the house can be equipped with awnings or a soffit capable of shading the window in question.

Essentially what you want is a LowE coating that has a great U-value and a "poorer" SHGC value.

Back to high performance softcoat and clear glass.

As I have mentioned, glass is really a lousy insulator. Dual pane is betteris better than single pane for example, but it is not really the answer to your need. Clear glass has a terrific SHGC, if you want the maximum amount of the sun's heat to enter your home, but the very poor U value will allow that heat to quickly exit your home when the sun isn't shining on the window. That is especially true for windows that are not directly affected by the sun's energy. Clear glass is definitely NOT the best option for your situation.

High perfomance softcoat LowE is the opposite end of the spectrum. It is going to block virtually all that sun energy from entering your home, but it is also going to keep the home's heat inside when the outside temperature is cold. It has a great SHGC value for Arizona, but it may not be exactly what you want at 9000 ft in the Rockies. Also, probably not the best solution in your situation.

So, we are left with the two other options I mentioned - "standard" softcoat LowE and hardcoat LowE....details discussed previously.

About 85% to 90% of LowE used in residential applications in North America is softcoat. There is a reason for this is simply a better product. Softcoat is much more color-neutral and much less likely to get "hazy" over time - a problem that does plague hardcoat. But, hardcoat is much more prevelant in Canada because of the solar gain issue. The hard coat manufacturers have done enough marketing to convince many people that they have the better product for colder climates.

"Standard" softcoat LowE has SHG numbers quite similar, but not quite as "good" (good being defined as the ability to allow the suns energy into your home) as hardcoat and U values that are slightly better than hardcoat.

Ultimately, you might simply be at the mercy of the window company that you are is the window company that determines what coating you will have.

I presented a LOT of information here, and would be more than happy to clarify specific points or questions...hopefully, it made some sense!

I have more on dual pane versus triple pane and gas infills as well, but this is way long enough already....

Anyone read this far?
Old 02-17-09, 07:52 AM
Join Date: Feb 2009
Location: Atlanta, Georgia
Posts: 2
rear double doors

Dear Oberon -
Thank you for posting all of that information. I am somewhat confused but nevertheless determined.

I am seeking to replace my rear glass double doors with a set of doors and a new jamb. I want the doors to feel very stable and have excellent energy efficiency. To stay in keeping with our older home, I am planning on installing antique hardware.

I visited Randall Brothers here in Atlanta and they have offered the following:
- 64"x80" one-light each
- 1 3/4" thick
- fir
- double pane
- Hardcoat Low-E argon gas
- R value of 2.71 and U value of .52

From reading your post, these numbers don't seem that great for energy efficiency. Do you have any advice or a suggestion for another retailer? THANKS!
Old 02-17-09, 05:54 PM
Join Date: Oct 2004
Location: East of the Mississippi
Posts: 177

Did you actually read that whole thing? I couldn't get thru it and I wrote it!

I hope that I have improved my question answering skills in the past four years...

Anyway, thanks for the kind words, but somehow I think that I should be apologizing for putting anyone thru that!

To answer your question,ithout going all verbose (I still can' keep my answers short and sweet), you are in Atlanta. You want to keep heat out of your home for the most part, especially direct solar gain.

Those numbers (trying to think of a polite way of putting this)...they are less effective than what you want. The claim of .51 U is probably accurate for the entire door. The R-value of 2.71 is probably center-of-glass. Since R=1/U, the "true" R-value for the door would be just about 1.96.

Seriously, you want to look for a door with a LoEł-366 or Solarban 70XL coating. These are both triple silver softcoat LowE coatings that will keep summer heat outside where you want it to be and keep winter heat inside, also where you want it to be.

They will allow you to control your environment much better than the numbers that you presented in your post.

You want to look for a SHGC (solar heat gain coefficient) and a U-factor, both below .40. There are lots of options available, possibly even from the folks that you mentioned.
Old 02-17-09, 06:56 PM
Join Date: Mar 2000
Location: Arlington, WA
Posts: 9,238

I DID read through the whole thing. Just keep on coming on!! (I've had a couple of posts like that, and I can't believe that I wrote the thing, let alone was able to reread through it a couple of years later!!)


The ONLY problems that I see with the door that you are talking about is the gas fill in the glass and the hard coat Low-E. You don't need them. Gas fill is GREAT in a really cold climate -- Canada, within a few hundred miles of the Great Lakes -- the COLD places. But Hotlanta and CA, -- we just don't get cold enough to justify the added expense -- especially in something as small as the glass in an entry door. And hardcoat Low-E?? Reread what Oberon said. 90% of everything in the US is softcoat because it works better. You'll pay extra for the hardcoat, and the money will be wsted.
Old 02-20-09, 06:32 PM
Join Date: Oct 2004
Location: East of the Mississippi
Posts: 177
Thanks Lefty! But I still wonder what I was thinking!
Old 02-20-09, 07:39 PM
Join Date: Mar 2000
Location: Arlington, WA
Posts: 9,238

YOU wrote it -- I didn't. I have no clue what you were thinking!! (That, and the fact that you wrote it about 4 years ago!!)


The doors that you are talking about are about as efficient as they are going to get. IMHO you are wasting $$ going with the hardcoat low-E and the gas fill. You are just not in a climate that needs those things. Save the $$ of the hardcoat and go with soft coat -- it's all you need, and it's more readily available. Gas filled is fine if you were looking at tmps dropping below 0 degrees and staying there days on end. Otherwise, it's a waste. And the R value and U value that you are talking about is due the the WOOD, not the glass.
Old 02-22-09, 06:27 AM
Join Date: Feb 2009
Location: Atlanta, Georgia
Posts: 2
Oberon & lefty -
Thank you both for your insight. I have emailed some door manufacturers with the specs that I now know are necessary. I also plan on becoming a member at Southface Energy Institute ( to learn more about energy efficiency. THANKS!
Old 07-06-14, 06:55 AM
Join Date: Jul 2014
Location: USA
Posts: 2
A real DIY question on Low E

Years have passed since Oberon's post, but I have read it a couple times and hope he is still active on the site!
My situation is a little odd: I have dual pane windows where the in re pane is essentially a storm window... It looks integral, but is actually removable after moving 8-10 small latches. It has a rubber seal between it and the window frame.
I haven't tested yet to see if there is a low E coating or not... House was built in 1988, and I think these are Marvin windows.
Heat gain in the summer is my problem. Standing in front of window you can feel the suns heat...and two rooms especially got very warm.
My thought is to apply a low E silver film to the windows.

I was thinking interior side of exterior pane, thinking that this is the best point to reject the sun without being outside where it can be scratched. I was also thinking that this is where any LowE coating might already exist?
I'm concerned about having the interior air gap becoming an oven and causing issues with the wooden sash.

Appreciate any thoughts on this all... Thanks in advance!
Old 07-06-14, 07:26 AM
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Join Date: Dec 2004
Location: USA
Posts: 20,859

They sound like Pella windows (should have their name on the crank) and you may have single pane sashes with the removable interior single pane panel. They also offered that window with a double pane IGU on the outside and the removable interior single pane panel.

IMO, rather than applying any kind of low-e film to the glass (basically like tinting the glass) you might be farther ahead if you just got new sashes, which could be ordered with the latest glass options, as replacement sashes are still available from Pella. You could also upgrade them to double pane + the 3rd storm panel at the same time.
Old 07-06-14, 08:27 PM
Join Date: Jul 2014
Location: USA
Posts: 2
Good Input

Yes, of course you are correct... I don't know why I was thinking Pella, and typing Marvin!
Your thought is an interesting one, and I will contact Pella to see how much I'd be in for to replace windows. I have a feeling it will be very expensive vs a film, even if the film is a temporary solution while I accumulate window replacements.
I had actually contacted them a couple years ago about having them come out to see what could be done with my windows (repairs for broken mechanisms, screens and doors.... like a mid-life upgrade) and they didn't seem to know how to do fixes, only sell new windows. It could be that I should have had them come out anyway to buy new where it made sense.
With that said, do you have a view on the application of the film If I were inclined to try it? (which of the surfaces to apply the film to, the inside of the outer glass or the outside of the interior pane)?
Thanks again!
Old 07-06-14, 08:38 PM
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Join Date: Dec 2004
Location: USA
Posts: 20,859
IMO it would best go on the inside of the outer pane. But I'm not the glass expert, Oberon is!

I think Pella likes to play dumb when it comes to repairs for the exact reason you mentioned. They would rather replace windows than tune-up or repair them.

Oh, and now that I think about it, I'm pretty sure that you could only replace the sashes on the windows that actually operate/open. If you have any fixed sashes, Pella did not make them easily replaceable until maybe 10-15 years ago when they changed the way their fixed sashes are held in place. On windows of your age, I'm pretty sure that the fixed sash is nailed in place from the outside perimeter of the jamb, making it impossible to remove without destroying the entire window. Handy.

Lowes might be another source you could look into for special order replacement parts. In Omaha, they can usually beat the prices I get from this area's Pella Window and Door Store.

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