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Thread: Patio Door, Low E glass vs Clear glass

  1. #16

    Default first post

    this is my first post to the forum, and it is long - sorry...

    There are two primary types of window coatings available - pyrolitic or hard coat - and soft or sputter coat.

    Softcoats are multi-layered coatings consisting of several different metallic oxides with silver as the primary heat-blocking ingredient. Softcoats are applied in vacuum chambers to finished glass.

    Hardcoats or pyrolitic coatings are applied to the glass while the glass is still semi-molten in the tin bath portion of the float process. Basically, the bottom side of the glass will have a coating of tin from floating in the bath (as does all float glass), but the topside will also have the layer of (primarily) tin oxide - which is the LowE coating.

    So in one sense, glass with a pyrolitic coating has two "tin-sides", but one - the LowE side - of them is much thicker than the other, although only one coat affects energy performance.

    Often, the pyrolitic coating is applied to the #3 surface of an IG unit if the unit is intended for use in a heating-dominated climate - but not always since there are also solar-reflective pyrolitic LowE coatings that are applied to the #1 or #2 surface of an IG unit to reflect solar heat gain in cooling dominated climates.

    Sputter LowE coatings are also applied to the air-side of the glass, primarily because sputter coats adhere better to the air-side, but also it is possible for the slight metallic (tin) layer on the tin-side to affect the performance of the coating.

    A sputter coat is applied in multiple layers, it may be about 7 to 11 layers, depending on the coating. People really don't realize, or can appreciate, how thin a typical sputter LowE coating is. Sputter coat folks measure the thickness of each metal oxide layer, as well as the finished coating, in angstroms, or by how many atoms thick the coating is. A typical softcoat LowE coating is generally less than 1000 atoms thick.

    Sputter coats are generally applied to surface #2 of an IG unit, although they can also be applied to surface #3. Consider a typical IG unit - two lites of glass separated by a spacer.

    The exterior of the exterior (as you said) is surface #1. The interior of that lite is surface #2. The other lite then has surface #3 and #4 - with #4 being the surface that is actually exposed inside the home.

    In a heating dominated climate, there are two reasons for placing the LowE coating on the #3 surface of the IGU. First is to allow for solar heat gain in the winter and second to block the transference of the heat inside your home to the outside.

    Which brings us to High Solar Heat Gain (or HSHG) coatings and Low Solar Heat Gain (or LSHG) coatings…

    What does that mean? Well, all LowE coatings are designed to block far - or longwave - infrared energy. This is the range that includes typical household-produced heat. This is also the frequency range of heat that is produced when the sun warms an object – the heat you feel when you touch the sidewalk on a hot, sunny summer day. While direct solar energy is shortwave IR, the heat released by a sun-warmed object is longwave IR…and hopefully that made sense.

    A typical hardcoat or single-silver layer softcoat works in this application since both types of coatings block the far infrared energy - thus keeping winter heat indoors - but neither is designed to be effective at blocking shortwave infrared - thus "allowing" solar heat access to the home - winter or summer.

    Placing a high solar gain coating on surface #3 maximizes the level of solar heat gain thru the IG unit which can be an advantage in winter and can also be a disadvantage in summer.

    A Low Solar Heat Gain product, on the other hand, is designed to block both near and far infrared energy. It will keep heat - including direct solar gain – from passing thru the window in both summer and winter.

    These coatings are placed on surface #2 to maximize effectiveness against direct solar gain by blocking solar heat before it can pass into the airspace in the IG unit – and into the home.
    Last edited by Oberon; 11-23-2007 at 07:26 AM.

  2. #17
    DIY Member thegallery's Avatar
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    Oct 2006
    Philly, Europe


    Wow Oberon, that was some post! I haven't checked the Wikipedia entry for Low E in a while but I reckon a lot of your information would go well there.

    I have the typical home window glass as you mentioned but not sure which of the two versions I have. I went with American Craftsman 9500 windows at Home Depot and they say, "U Factor: LoEČ Glass - 0.35". I did not get Argon filled ones.

    I'm not sure if they are the greatest windows in the world but they seem to work great. I did the Patio door a couple weeks ago and all eight windows last weekend (yes, quite proud of myself!). It's impossible to tell how effective they are but compared to the previous dilapidated aluminum single paned windows I sense already that the heater is firing up less than normal. The patio door, which work next too, is a huge difference. The interior glass is still cold to touch, but I'm sure not half as cold as before.

    Thanks again for your post. I'll have to get it a couple more reads to let all the info sink in!

  3. #18
    DIY Member thegallery's Avatar
    Join Date
    Oct 2006
    Philly, Europe


    Quote Originally Posted by BigLou View Post

    Low E windows are going to greatly reduce the amount of sunlight that passes through the window usually by about 45-50% if you tell me what direction the door faces we can do a calculation on how many BTU's you will get in potential sunlight compared to how many BTU's you will lose with the reduced R value window to see if there is any solar gain. My best guess would be that the Low E is the way to go

    Lou, sorry I missed this post. So I ended up doing the Patio door with the LoE. It was $360 including screen from Home Depot.

    Because of the huge hill right behind me I only get maximum 3 hours of direct high angle sunlight in the summer and no direct light in the winter. It is pretty light though because the sun is just behind the trees. I just doesn't get hit directly. The door faces South West. It's a standard 6' door and rated with a U factor of .37.

    Wouldn't you know that the stock window was 1/8" larger than my opening! So I had to re-frame one side, (which was just as well as I found some rot in the bottom). But added to the when I pulled out the old door the concrete sill had deteriorated so I had re-concrete the sill too. (I'm hoping the repair will last at least 10 years). Anyway, the job is done and my dogs are happy. It's so much easier to open, and I can't feel any air coming through.
    Last edited by thegallery; 11-23-2007 at 07:59 AM.

  4. #19


    good morning thegallery,

    LowE really does work. Some folks would suggest that LowE is simply a marketing ploy to get more money from the consummer, but as you have seen the addition of LowE in a dual pane window really does make a difference.

    As a general comparison, the interior glass temp of a single lite window when it is 0 degrees outside and 70 inside is going to be about 16 degrees. Add a storm window or go to dual pane window and that temp jumps up to about 43 degrees.

    Add LowE and that temp jumps up to about 53 degrees...so while the glass still feels cool to the touch, it is a good bit warmer than before and as you have already noticed it makes a difference in the room.


  5. #20
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
    Join Date
    Nov 2005
    Central Florida


    Wouldn't any thermally conductive surface below body temperature feel "cool" to the touch? If I take a glass tumbler out of the cabinet it's at room temperature, but definitely feels cool.

  6. #21


    I would agree completely. One measure of a windows performance is "how comfortable" the room feels.

    Heat goes to cold. Nature likes balance. The colder the windows, the more uncomfortable it is to stand (or sit) close to them because they are drawinf heat from your body in an attempt to equalize temperature between you and the window. So increasing the temperature of the glass surface really does make you feel physically more comfortable.

  7. #22


    great posts about the Low E coatings I learned a lot about how the different coatings are applied. Do you work in window manufacturing ? I would agree that more often then not Low E is a worth while thing to do


  8. #23
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    A second big advantage of low-e is that it stops a significant amount of UV, which helps slow down fabric fading and rotting. Most of the coatings are on a thin film of plastic suspended between the glass in the better systems...in fact, some have more than one film between the glass.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #24


    Hi Lou,

    I do work in the window/glass industry primarily in the areas of quality, product development, and certification.

    I don't work for any particular window company, but I have worked directly with many different window companies.



    Although there are companies that still use the system of having their LowE coating applied to a plastic film that is suspended between the lites of the IG unit, that option is somewhat uncommon when compared to "standard" IG construction (i.e. LowE on glass). I would suggest that less than 1% of IG units made today use that particular system.

    Unfortunately, the LowE coated film system had several significant problems, that are still something of a headache to many of the window companies that used it, in the 1990's and many folks who had been using it dropped it at the time because of those problems.
    Last edited by Oberon; 11-28-2007 at 04:44 AM.


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