Low-e Glass … A Nation Divided

Low-e glass, first introduced in 1979, has transformed window energy performance. Low-e glass is manufactured with a microscopically thin and transparent layer of metal or metal oxide that reflects infrared “heat” energy back into the home, greatly enhancing the thermal performance of the window.

In the simplest of terms, there are basically two kinds of Low-e glass:

  1. Low-e glass with a low Solar Heat Gain Coefficient (SHGC) that also reflects and keeps much of the Sun’s heat energy out of the home. This is the best choice in climates dominated by cooling.
  2. Low-e glass with a high Solar Heat Gain Coefficient (SHGC) that allows the Sun’s heat energy into the home. This is the best choice in climates dominated by heating or for south facing windows in climates with a mix of cooling and heating requirements.

All this seems pretty straight forward until you actually attempt to purchase a window with Low-e glass. But before I get into that let’s take a look at the climate zones in the U.S. used by Energy Star to determine the qualification criteria for windows and skylights in an Energy Star rated home.

Energy Star Climate Map

The Northern zone is the perfect candidate for a Low-e glass with a high SHGC and the North/Central and even the South/Central can greatly benefit from high SHGC Low-e glass on southern exposures. Unfortunately, even though the average American family spends far more on heating than air conditioning, both Energy Star, LEED, and the International Energy Conservation Code (IECC) seem to be color blind when it comes to space heating and the benefits of Low-e glass with a high SHGC.

Energy Star requirements for SHGC seem to be all about cooling as the following requirements for SHGC for each climate zone indicates. The IECC is no better, only requiring a SHGC of ≤ 0.40 for any residence with less than 3,500 Heating Degree Days (HDD). LEED only takes the cooling bias further by requiring even lower SHGC’s in the South/Central and Southern climate zones.

Energy Star Window Criteria

As an unintended consequence of the regulatory bias in favor of cooling, window manufacturers have all but abandoned Low-e glass with high SHGC’s. The result is a nation divided, with more than half of the country left out in the cold without ready access to high performance windows that take advantage of the free solar energy that strikes our windows everyday. With very few exceptions you just cannot find a window manufacturer willing to give you the glass options we need and require in heating dominated climates.

So what’s the consequence of the current regulatory bias? First of all it makes no sense, according to the U.S. Energy Information Administration’s (EIA) 2001 Residential Energy Consumption Survey, Americans consume more than 7 times more energy for space heating than for air conditioning. To get an estimate of what’s on the table for high SHGC, Low-e glass in terms of the potential energy savings I assumed a 10% improvement in heating costs for homes in the U.S. with more than 4,000 Heating Degrees Days. Based on the same 2001 EIA Survey, that would amount to an annual savings of 0.362 quadrillion(1 followed by 15 zeros) Btu’s per year. In monetary terms, that’s about $475 billion dollars worth of natural gas!

When the government gets serious about Global Warming maybe they’ll fix the cooling bias in the regulations, but until then here’s where to go to get a Low-e windows with a high SHGC:

Just want a source for the glass? Try Pilkington North America.

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19 responses to “Low-e Glass … A Nation Divided

  1. How True!

    We are planning a replacement of 36 single pane windows and I absolutely cannot find the high SHGC/low e glass windows for the install. PPG makes a fine glass in Sungate 500 but window manufacturers are not using it?

    Our home was designed by Us 34 years ago to be heated passively in house orientation on the lot, eave overhang dimensions etc. Now we would like to take advantage of the low e technology but do not want to give up the solar heat gain nor the visible light transmission. What a conundrum

    Jon Doornink

  2. Jon, besides the companies listed in my “Nation Divided” post, if you can afford wood windows, Marvin has told me that they will spec whatever glass you want into their windows. You will have to the call their tech support for architects and design professionals at 800-346-3363. Another option, is to have a local fabricator make fixed insulated glass units (IGU’s) that you can frame directly into your existing south facing openings.

  3. In Canada We Care

    This piece you have written in reference to Low E availability in relation to SHGC is right on the mark as it is in the U.S.. Contrarily in Canada the :”Energy Star Program” has (2) two routes to qualify windows for the program. One being as it is in the U.S., the U value route, were only heat loss is considered. The other being what is referred to as the “Energy Rating” (ER) route. In the latter method passive solar gain is weighted in. As a result the high SHGC products (hard coat low E’s) become the products of choice. I work as a sales manager for one of the leading fiberglass window manufactures and train our sales staff to look at this whole matter from a case by case viewpoint. As for example were dwelling designs that have elements like large overhangs on southern elevations and perhaps construction materials that can absorb and hold this energy, hard coats are recommended for these areas. Then again on southern (unshaded) and western elevation with large glass to wall ratios soft coat Low E’s (low SGHC’s ) would be recommended in such instances. In general both types are often used in the same building. In the end I find the greatest thing to consider in all this is occupant comfort when trying to work through all the variables.

    For those interested in the way windows are viewed in terms of the Canadian “Energy Star” program one can go to: http://www.oee.nrcan.gc.ca/energystar/english/consumers/window.cfm?text=N&printview=N

    Regards,

    Phil Warnell
    INLINE Fiberglass Ltd.

    P.S. If you wish you could mark us up as a window company that can supply this high SCGH (hard coat) Low E products

  4. Thanks for sharing this information. Really is pack with new knowledge. Keep them coming.

  5. Writing an article about this topic for a building magazine. Would like to chat.
    Thanks!

  6. I live in Tucson, Arizona and have begun to makeover my home with the goal of increasing energy conservation and taking advantage of passive solar design.

    I plan on replacing the old steel frame single pane windows with double pane windows. I heard that the south facing windows should not be low -E.

    Can you direct me to more information and should the south facing windows not be low-E in Tucson, Arizona?

    Thanks.

  7. Great question Victor,
    Your typical low-e glass package in this country will have a very low Solar Heat Gain Coefficient [SHGC] and block about 75% of the incoming solar radiation from entering your home. That’s why “you’ve heard that south facing windows should not be low-e”. However, you CAN specify low-e glass with a high SHGC and get the benefit of both low-e AND the solar gain you need for a passive design. Since Tucson maxes out at about 400 Heating Degree Days per month in the winter, you don’t have much of a heating load to consider. I would either use a standard low-e glass for every exposure, or if you have a south facing overhang that will block out any direct sun for your 8 month cooling season, then go with Marvin Windows for at least the southern exposures. They are the only mass producer I am aware of that will allow you to spec in a high SHGC glass package. Tell them you want a SHGC of at least 0.7 (that means that 70% of the available solar energy striking the glass will be able to enter your home).

    Thanks for your comment and all the best with your project,

    John

  8. Good article! Some one needs to write a book on the new views and realities of solar design.

  9. The situation has been worsened with the American Recovery and Reinvestment Act of 2009 (ARRA), which offers a tax credit for home improvements that include window and door upgrades. The tax credit requires the window to have a maximum U-Factor (inverse of insulation R-Factor) of 0.30, and a maximum Solar Heat Gain Coefficient (SHGC) of 0.30.

    This may have two effects: (1) homeowners may forego their desired passive solar heating in favor of a short-term savings, (2) manufacturers may gear up to make only those windows that will qualify for the tax credit.

    I wrote to Energy Secretary Steven Chu and to both Colorado senators to ask for a review of the ARRA. If we as a country are going to encourage energy efficiency, we should do it more intelligently. I encourage you to write to your representatives in support of a review of ARRA terms, especially the SHGC maximum.

  10. This discussion still leaves those of us in mixed climates in a quandry.

    The answers seem clear for Manitoba and Arizona, however my Seattle home is a puzzle.

    I live in a home not designed specifically for passive heat gain, however our WEST facing windows crank our home temperature into the 90’s every summer. That’s hot for Seattle!

    With west facing windows, I’m not sure how much heat gain I’m receiving in the winter.

    How can I go about researching the best windows for my home?

    many thanks

    Jason

    • Thanks Jason — I lived in Seattle for many years. I would recommend low-e glass with a high Solar Heat Gain Coefficient (SHGC) on your south facing windows and low SHGC on all your other windows. I have found that Marvin with work with customers that want a high SHGC solution.

  11. ch clear day. With the low E glass, what is the anticipated effect on the fading from UV rays?

    [I assume your question concerns the UV resistance of a high SHGC low-e glass? If so it is still very good. If you’re interested in specs checkout PPG’s Sungate line of low-e glass]

  12. ı live in turkey, istanbul. ı should take SHGC no more than 0.26! that means ı should live behind a black glass! this is not comfortable!

    does any one has an idea for the issue? ı should confirm that ı comply with the ASHRAE 90.1 -2004 reqs ???

    • Try external shading, either active or passive. (trees, manual external shutters, solar controlled external shutters, overhanging elements on south facing exposures, primarily north facing glass, etc.)

      • thank you!

        one more question 🙂

        how can i calculate glazing/ wall area? 🙂

        glazıng area/all facade (glazing+wall area)
        OR
        glazing area/just wall area (facade – glazing)

      • Calculate your south facing glazing based on glazing per floor area. The rule of thumb for passive solar is 7 to 12% south facing glass based on the floor area of the entire home. Your west and east facing glass will be additive to that so you can either make a judgement based on your experience living in the home without any south facing glass, or have a local professional size the south facing glass using computer modeling. I’m assuming you’ve done everything possible relative to adding insulation and weatherizing.

  13. Hi John (& any others w/knowledge to offer),

    I am renovating a small 2-story brick row-house in Philadelphia. My side wall (45’long) faces south, and I am having a mason make window openings. I’ve been trying to maximize glazing area until I read about the more imp’t considerations of SHGC.

    My main problem is that floors are old hardwood — not much thermal mass. I plan to heat only with passive solar and a wood stove. I do NOT plan to cool with electricity, so I’m not concerned about saving energy in summer.

    That said, could you recommend a ballpark SHGC/u-value tradeoff, and an appropriate glazing area? I’m looking at about 30 sf south glazing for a 144 sf bedroom — but again, no thermal storage. At a much greater cost, I can increase south glazing to about 42sf, but I dunno if it’s worth it…

    many thanks!

    jacob
    low-income artist.

    • If I understand correctly, you have the end unit of a row house with a window-less south facing wall. That would mean that all of the rooms that you might be adding windows to already have windows that supply ventilation and in the case of the bedroom(s), code compliant egress. If that is the case, then your cheapest window option with be to have a local supplier fabricate insulated glass units (IGU’s) that would be framed into the new openings. These IGU’s (at a minimum) should be dual glazed, argon filled, use super spacers, and incorporate a low-e glass with a high SHGC similar to PPG’s Sungate line of products. You can add mass with water in barrels or other containers. You should also consider insulated shutters or blinds on all (or at least your most inefficient windows) to minimize night time losses or for use in rooms you are not continually using.

  14. John:

    As a fellow Coloradoan, I’ve been trying to find a reasonably priced replacement window for the south facing exposure on our house that has a low u and and a high SHGC.

    Sunrise Windows has an option with a .31U/.50 SHGC window. As it turns out, they sell windows that can have an option of an inside the window blind. In order to prevent the blind from rubbing off the low e, they offer a hard coat option. It is available even when you don’t buy the blind option.

    BTW, they have several lines under different names. In addition to Sunrise, there are Vanguard, Restorations and Verde.

    Their frames are foam filled vinyl, not the greenest, but still very efficient thermally. 0.31U/.50 SHGC is good but certainly not the best. I can’t tell from their website, but they don’t appear to use super spacers in their insulated glass units and for our altitude you will not get argon fill, you’ll get air fill with a breather tube. However, Sunrise is probably a good low cost solution for a high SHGC operable replacement window.

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