Low-e vs solar gain vs window treatments

stimygAugust 28, 2007

I'm building a new house in the Catskills, in upstate New York (zipcode 13756 - that puts us well within the northern heating zone.) I'm trying to decide what windows to put in, as far as energy efficiency and aesthetics. It's a partially modular build, so I think I'm limited to either Silverline or Andersen windows. It's a house with a lot of windows, mostly West facing and some South facing, so it's an important decision.

My architects are recommending getting double pane glass, no special e-coating. First, they say that they've designed the home to take advantage of passive solar gain, even in the winter months. I'm not sure I totally agree - we have one short wall of windows facing South, and the rest facing West. So, while I understand that there will be some solar gain, I'm not sure that it balances the heat lost at night, etc. (This is a vacation house, so we'll only be up on the weekends. And heating the house to some very low temperature, 50-degrees say, during the week. Not sure how that affects things one way or another.)

They also say that the low-e windows block some of the visible light, and make everything seem a little darker outside. I've checked out the windows at Home Depot, and I have to say I agree. (I love bright light, and I do notice the dimming.)

However, I'm worried that they're too concerned about aesthetics, and not being completely accurate about the advantages of low-e windows in the whole passive solar gain vs. insulation argument.

Their solution is to use regular double pane glass, and add insulating window treatments (eg, blinds or shades), that we can open during the day and close at night. That's fine when we're there. Though of course, when we're away, we'll have to leave them either open or closed for the whole week.

It's a bit complicated. Any opinions for me?

Thanks, Tim

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Also, does anyone know of an online calculator that could help me figure this out? I've seen downloadable software, but I think it was PC-based (I'm on a Mac.)

Thanks, T

    Bookmark   August 28, 2007 at 4:37PM
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Just a homeowner here who bought new low-E and argon filled windows 4 years ago. The following link is very useful if you have not used it. The rest of this post is just our experience.


Properly selected Low-E coating keeps your cottage cool in the summer and warm in the winter. Without low-E, even dbl-pane risks condesation problems in the winter and the nasty cold draft at night. It definitely does nothing to keep heat out in the summer especially if the sun shines right through those windows. By mistake, we still have two Milgard dbl-pane on our house and they heat up in the winter just from exterior ambient warmth and they "sweat" in the winter with a cold draft at night.

Our low-E units have no condensations and no cold draft in winter. In the summer, they block out heat and we no longer use any internal window treatment because the no-glare view out is so nice.

As for visible light transmission, it is selectable. In higher latitude, most manufacturers will have models that let more light in rather than keep more light out. We are in CA and our windows are at about 72+% and they are not dim at all. If anything, they make views crisp and clear by cutting out glare.

Note another strange logic. Without low-E, you will have to draw the drapes to keep out the summer heat and winter cold. That does absolutely NOTHING for your view to the outside or for bringing any light into the house. What is the point of having 100% transmission when drapes are drawn?

With Low-E, you will most likely never have to draw the drapes either in winter or in summer because properly selected coating keeps summer heat and winter chill out. You can thus open the drapes to enjouy your glare-less summer views and draft-less winter scenes.

Finally, I suggest checking local mom and pop window stores instead of Home Depot. Your local big box may be very good but our experience with our local big box in CA's south SF Bay area has been dismal. We actually knew more about windows and doors than their salespeople.

Good luck.

    Bookmark   August 28, 2007 at 8:46PM
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Stimyg, sounds like your Architects need to do some research. There are options that will allow you to get the best of both worlds. I'll do my best to answer this with hopes of Oberon answering later. He's the resident Glass Lord and will be able to answer in more depth.

The LOE coating reduces the infrared radiation from a warm pane of glass to a cooler pane, thereby lowering the U-factor of the window. Different types of Low-E coatings have been designed to allow for high, medium or low heat gain. A LOE coating can also reduce a window's visible transmittance, unless you use one that's Spectrally Selective. Spectrally selective coatings reflect certain sun rays, but remain transparent to others. The coatings are used to reflect the infrared portion of the solar spectrum while allowing a higher portion of visible light. This helps create a window with a low U-factor and heat gain while allowing a high visible transmittance.

So you can get the best of both worlds, I know Cardinal Glass has their LoE3. The sell it to different window manufacturers like Milgard, Weather-Shield, Kolbe & Kolbe, and Atrium. Milgard has a trade name of "SunCoatMAX" and Weather-Shield and Peachtree call theirs "Zo-e-shield". See if you have access to these products. The Silverline company is owned by Andersen and I'm not quite sure what the offer. Hope this helps you out. Good Luck!!!

    Bookmark   August 29, 2007 at 7:05AM
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Thanks for the responses so far.

Calbay -- what brand of windows do you have that you like the looks of?

I guess the question for me, though, is whether Andersen or Silverline makes windows that both lower the U-factor but keep the VT high, since it looks like I'm stuck with those choices. From what I've seen, in both those brands the U and VT are linked - low U means low UT. Maybe I'm wrong, though.

The other question is whether the architects are accurate as far as how they've designed the house with regards to solar gain - ie, whether the solar gain in the winter is sufficient to offset the heat loss at non-sunny times. Anyone know of any websites or software or anything else that might help me calculate this?

Thanks -- and Oberon, if you're out there, please do chime in!

    Bookmark   August 29, 2007 at 11:13AM
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Stimyg, I must clarify that our two old Milgards had no low-E because we did not buy that option long ago. It was a mistake.

We ended up going w/ Marvin, wood interior that we stained and aluminum clad exterior. So far, they have held up. I believe any well-constructed low-E windows should do. With good installation, they will work and last a long time, hopefully a lifetime.

I am on Mac's as well and just got an IntelMac. Will see if I can find the S/W that you have been seeking.

    Bookmark   August 29, 2007 at 11:32AM
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good morning,

I am curious that your architect said to get insulating glass without LowE coating because of solar gain considerations. Either the architect is being a bit lazy or else he/she is not well versed on window performance.

If he/she wants to maximize solar gain then you want a LowE coating that maximizes solar gain as well as keeps heat inside your home. You do not want clear glass under any circumstances because the losses (as you noted) exceed the gains when the sun isn't shining on the windows.

A high solar gain LowE coating will balance out the losses and gains to a much greater degree than will clear glass. I know of very few solar gain experts who would dispute that idea, so again I would suggest that your architect may need to do a little reading about passive solar energy performance.

To follow up what Guy mentioned in his post, LowE coatings are designed to block infrared energy. The high solar gain coatings block what is called "far infrared" or "long wave" infrared. Far infrared is what you are getting from your heat source in your home - be it radiant, forced air, whatever. Even when you have solar gain thru your windows that warms the walls and furniture and floors, the heat that you feel radiating from those surfaces is far infrared.

Direct solar gain is "near infrared" or "short wave" infrared. This is the heat that you feel when standing in a sunbeam. This is very nice heat that always feels good - on cold winter days - less good on hot sunny summer days!

When considering passive solar thru windows, you want to allow the near infrared energy thru the glass but you want tyo block the return of the far infrared to the outdoors - again back thru the glass. This is what a high solar heat gain LowE coating does - it allows direct sun heat to pass but then keeps the warm inside air inside.

A low solar heat gain coating, on the other hand, is designed to block both near and far infrared energy. It is designed to keep "all heat" from passing thru the window. If you are not concerned about passive solar gain for whatever reason - for example you live in south Florida or west Texas where solar gain into your home may not be considered a necessarily good thing - then this sort of coating is what you want.

Even in the north country this coating will often (but not always) be more cost effective than a high gain product depending on factors such as actual amount of sunlight available, home orientation, number and size of windows, etc. Again, this is an area where the architect can make a huge difference by desiging a home that will take advantage of direct solar gain in winter and that will effectively block direct solar gain in summer. If you have a home that is designed to those specifications, then a high gain coating may be the best choice.

But, if you have a home that is not designed to make best use of those factors then it may be better to go with a low gain coating instead.

In all circumstances having a LowE coating is better than having clear glass.

Do LowE coatings block visible light? Yes, somewhat. But very few people really notice the difference when the entire home has coated glass. A caveat that not all coatings are created equal and that some manufactuers are much better than others at manufacturing "neutral color" coatings that can be far less noticeable. Often, even experts can't tell if a home has LowE coatings just by looking at the windows.

Calbay is an excellent source of first hand homeowner information. He did his homework before buying and he does a great job of passing what he has learned about his windows.

And off subject...

Guy, now that it has quit raining for a few days, we are going to try to get those windows installed later this week...hoping the eather holds until we get them in! Thanks for the advice and I will likely be bugging you a few more times my friend!!!

    Bookmark   August 30, 2007 at 7:38AM
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Thanks for the detailed and informative response, Oberon. I guess my one remaining question is whether Andersen or Silverline carries low e windows that allow for high solar gain. From what I've seen, it doesn't look like it.

Does anyone have any information otherwise? If not, does anyone have any particular suggestions for good high solar gain / low e windows?

Also - any further suggestions for these windows that don't block much visual light? (I admit to not liking the looks of the Andersen low e windows. But again, I'm super sensitive to light levels, and love bright sunlight. I think I'm part plant.)

Thanks a ton, everyone.

    Bookmark   August 30, 2007 at 1:16PM
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Mac Software for computing energy efficiency:


Some are free, some are not. Enjoy! :-) If you look to the left of the web page after it loads, there is also a link to "Internet" based software under the section titled, "Tools by Platform". I don't remember having any of these 4 years ago when we did our research.

Another correction to my previous post. Our windows (Marvn Ultimate) have visual transmission of 0.48, roughly 48% visible light. Our French doors have 0.40, about 40% transmission. These numbers came right off the window and door stickers we saved. These units face south, southwest and west and all are in banks and we are at a lower latitude, that may be why we get a lot of natural light into the house all year even at only 40%-48% transmission. The "72%" was taken from my notes refering to none low-E windows, sorry for the mix-up.

Hope those software choices have something you can use.

Oberon - thanks for the kind words and thanks for sharing all the insider technical knowledge, wish I had read your stuff 4 years ago.

    Bookmark   August 30, 2007 at 3:00PM
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I knew you would show up Oberon. Your a Super Hero coming out of nowhere to save the day! How's the house coming? Hope everything is going better than before.

    Bookmark   August 31, 2007 at 7:34AM
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There are two types of high solar heat gain LowE coatings on the market - one is hard coat and one is softcoat.

Hard coat LowE coatings are typically easier to "see" than are softcoats and they are also prone to slight discoloration as they age. Nothing significant, but it can be noticeable.

Soft coat coatings tend to be more neutral in color versus hard coats and do not discolor as they age.

That said, Andersen uses a LowE2 coating which is a softcoat. I am curious if your dislike of the coating is based on seeing it installed in homes or if it is a showroom reaction?

A clear glass dual pane window is going to have a visible transmittance, or VT, of about 83% (give or take a little).

Cardinal's LoE178, which is a softcoat high solar gain product, has a VT of 78% (I picked this coating because Cardinal includes glass VT in the product name - note "78").

The coating used by Andersen has a VT of 72%.

Now these numbers can be misleading when dealing with windows because while I listed the visible light that shows thru the glass, window VT listings also include sash and frame components in the VT measurment listed for the applicable window. So where two windows might use an identical glass package, the listed VT could be substantially different based on the ratio of glass to frame components in the window.

So Calbay's VT numbers do not reflect glass alone, they reflect glass plus the particular door/window involved. That is why folks will sometimes wonder why they seem to be getting more light thru a lower VT product versus a higher VT product.

HI Guy, the house is coming along - slowlyyyyyyyyyyyy - lol - we are still waiting for final backfill before we install the windows. Primarily just landscaping considerations, but I am not risking glass when heavy equipment operators are throwing rocks around!!! I have already had to replace one IGU that was broken in shipment. Gets expensive quickly in that case!

Have a great weekend my friend!

    Bookmark   September 2, 2007 at 9:13AM
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Thanks for clarifying the VT number. Have a follow-up question please:

Does the final sticker VT computation includes having the screen in place?

We have noticed that when the screens go on during bug season, there is a definite noticeable dimming effect. On cooler days, we take off all screens internal (casement) and external (dbl-hung) and the difference is night and day. So much more light gets into the house. The external screens do help to keep out some heat on hot days so we put them back on when hot.

My wife would not let me install our french door screen units from Marvin because she dislikes the dimming effect.

Thanks in advance!

    Bookmark   September 2, 2007 at 1:05PM
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Screens are not included in VT calculations.

    Bookmark   September 4, 2007 at 7:21AM
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I have learned a lot from reading all of your comments. How do you feel about mixing Low E windows? We are buidling a home in NC about 3700 feet with mountain views. Cold in winter, hot in summer. Our view side is SW and we have a two story bank of windows about 20 feet wide. While I want the clearest view I am concerned about getting cooked in the summer. I am considering LoE 366. Do you think this is to little VT? Maybe 272 instead? There are a lot of windows under cover with about 70 feet of porch with very little or no sunlight. I was considering LoE 178. For the rest I was considering LoE 272. Maybe I could use LoE178 on shaded (forest) side of the house. Please let me know your thoughts

    Bookmark   September 15, 2007 at 12:57PM
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Window fine tuning is a great idea - using different glazings for different elevations depending on the requirements. I think you would have a hard time discerning the difference in daylight transmission between the LoE366 and the LoE272, so if you can get it I'd go for it.

An interesting issue however with using varying glazings is finding a window manufacturer that will offer them, or offer them for a reasonable price. Many window manufacturers might offer only one type of Low E coating, or perhaps two. Other glazings might not be available at all, or only available for a significant price increase because it is not a standard offering through their normal purchasing channels. LoE366, for example, is difficult to get from most manufacturers either because it is so new, or because they don't buy it from Cardinal. So when window shopping don't assume that all possible glazings are possible; you may have to do a bit more searching to find exactly what you're looking for.

    Bookmark   September 15, 2007 at 7:34PM
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Hi Ed,

I agree with tru_blu that mixing or fine tuning window seclection can be a great idea. But, from your post I am getting the idea that you are anticipating mixing windows in an attempt to balance VT with the light that you anticipate getting thru the window in a particular location.

I also agree with tru-blu that it would be difficult, if not impossible, to coordinate numerous coatings thru a single window manufacturer. Even if they would do it, the cost would likely be prohibitive.

I really doubt that you would be able to see the difference between the various coatings unless you were looking at a tinted coating or else mixing hard coats and soft coats. And keep in mind that LoE178 is a high solar gain product so putting it where the sun doesn't shine (and I really don't mean any sort of pun there!!!) could be potentially counter-productive.

Are you looking for primarily for passive heat gain or are you trying to keep excess solar heat outside of your home? Those should be your primary considerations when selecting windows specifically for controlling (one way or the other) solar issues.

    Bookmark   September 15, 2007 at 9:36PM
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I am looking to place my order this week. Marvin has the LoE 272 quoted for the entire order. The upgrade to the 366 for the large two story bank of windows was very reasonable. They also have the 178 but I have not price it yet for the covered porches. The porches also face the view side.....were he sun will not shine! Thanks, I appreciate your thoughts. Sounds like this is the way to go.

    Bookmark   September 16, 2007 at 4:53PM
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I am looking at getting the loe3-366 Cardinal product. But I am not sure if it is a soft coat or a hard coat. My research tells me soft coat is superior.

Does anyone know what it would be ? I imagine Cardinal being a leader would only do soft coat. I aksi thought since the 272 was soft so would the 366.


Here is a link that might be useful: LOE3-366 from Cardinal

    Bookmark   October 2, 2007 at 5:22PM
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All Cardinal coatings are soft coat, 366 included.

    Bookmark   October 2, 2007 at 7:26PM
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Thank You Oberon, much appreciated. I am having a dealer in next week to quote on this product. Sounds Good.

    Bookmark   October 2, 2007 at 7:29PM
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We have Milgard Low E glass on all of the 8 ft. windows on the South back of our home. Apparently they block 87% of the harmful UV rays that fade certain materials. From Sun-up until about 2:00PM the sun penetrates about 4 ft. into the room. We are concerned about our dark cherry laminate wood floors and a light colored carpet. Is it necessary to install Siloutte blinds that, when closed, block 99% of the harmful rays? We have no need to install them for privacy or decoration, only to prevent sun damage. Hopefully someone will have an unbiased opinion as I need to decide within less than a week. Thanks for your help. Zins

    Bookmark   April 16, 2009 at 9:14PM
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When thinking of fading, most folks think of UV light as the primary cause. However, visible light up to about 600nm also results in fading. This is known as the Krochmann Damage Factor.

Different LowE coatings and other options such as laminated and tinted glass will affect the level of fading damage caused by the combination of UV and visible light, but nothing, short of 100% isolation, will completely protect your floors. You can limit exposure depending on your glass package, but if you are really concerned then the blinds might not be a bad idea.

    Bookmark   April 17, 2009 at 7:48PM
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Hi ZIns,

Oberon offered professional technical details so I will chime in with homeowner experience.

Yes, even with Low-E, there will be fading. Depending on the amount and intensity of exposure and the finish, it is a matter of how much and how soon. In a situation where the sun is very mild and light, by the time fading is noticeable, it may also be time to refinish the floor.

My guess is the floor is not receiving year-round exposure, true? The sun is stronger and deeper during some time of the year and weaker and less intense at other times, is that so?

We have seasonal sun intensity problem too and solve it with a combination of solution.

We use rugs to protect several sections of our floor during mid-Spring through late Summer. The rugs are removed to prevent tripping when we have visitors. The rugs are also removed around Autumn when the sun is low and the exposure is much less intense and shortening each day.

We have old furniture in one area by our French doors and my wife made knitted coverings to cover the furniture. As necessary, we remove the covering when appropriate.

Outside the French doors, we installed retractable awning so that during extremely hot summer days, the awning extends to provide cover without blocking natural light.

Our goal is to not use any window treatment because we enjoy natural light and the views.

Hope this helps a bit.

    Bookmark   April 17, 2009 at 8:27PM
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