Low E Glass Conventional Wisdom?
CloudHidden
| Posted in Energy, Heating & Insulation on
Client reports:
“If you are doing a passive solar design with south facing glass and the appropriate protective overhangs, do not order low E glass for your south facing windows. Just get the standard double pane glass, possibly with argon fill. The low E cuts your solar heat gain by about 50% over the non low E glass, and it’s also more expensive. It’s a difference you can feel when you stand in front of the windows with the sun shining through. You can deal with the higher winter heat loss of the non low E glass by using heavy curtains or other means of insulation at night.
Do buy the low E glass for your north, east, and west windows. It will help reduce over heating in summer, and heat loss in the winter.”
Agree or disagree? Caveats?
Replies
windows are rated for solar transmittance; go by the numbers! There are windows to limit solar gain and windows to allow it.
I will say if passive solar is your gain, your point of entry for solar gain is a point of exit for heat at night. I most strongly recommend thermal curtains. Of course, no one ever takes my recomendation there. But still, I perservere!
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
The blanket statement, "the low-e cuts your solar heat gain by about 50% over the non low-e glass," is not true. Solar gain is a different property from emissivity and can be controlled to be high or low, whichever is desired, with low-e glass.
http://www.efficientwindows.org/lowe.cfm
That's old talk from about 12-15 years ago. In cold climates, use Low "E" and argon at least. In some locations it may be economical to use triple glazing on all sides. You only get the sun for 8-12 hours per day in the coldest part of the winter and have to keep the heat in the other 12-16 hours. Don't forget that on sunny cold days when you have a net gain from south facing windows during the sunny period, you also have heat loss from the windows also due to the high "Delta T" or indoor-outdoor temp difference.
There was a study done about 1996-8 by a Canadian engineering firm (Enermodal) that does a lot of window design and testing. They concluded that overall, the extra R values kept more heat in than they rejected through lower light transmittance.
Here's a good introduction to windows:
http://oee.nrcan.gc.ca/Publications/infosource/Pub/renovate/windowsanddoors/windows-doors.pdf
Edited 6/19/2006 11:09 pm ET by experienced
Edited 6/19/2006 11:20 pm ET by experienced
Agree 100% with previous posters. There are high solar heat gain LowE coatings and low solar heat gain LowE coatings....always go with LowE, whatever the circumstances.
Within the glass and window industry there is some controversy about which LowE coating to use in different climactic regions of <!----><!---->North America<!---->, but there is no controversy about whether or not to use a LowE coating.
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In every region of <!---->North America<!---->, a LowE coating will save energy over clear (non-coated) glass…period. There may be some question concerning which LowE to use and which window surface to coat, but absolutely none about the advantages of the coating.
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LowE coatings are designed to block heat flow…thus, the rather obvious idea of avoiding the use a LowE coating if you want to allow solar energy into your home.
Assuming that you are using clear glass for the benefit of maximum solar gain, while you may have a net energy gain when the sun is shining directly on the window (in the winter…definitely not an advantage in the summer), obviously that is not the case at night when the sun isn’t shining. Also, cloudy conditions or the hours of the day that the sun isn’t shining directly on the window have to be taken into consideration as well.
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At those times, you will have a net energy loss thru the clear glass. Heat always travels to cold unless acted upon by an outside force. Studies have shown that the net energy loss when using clear glass for solar gain exceeds the net energy solar gain in those instances. Basically, your windows, even double pane (without coating), allow a significant amount of the heat in your home to pass thru the window to the outdoors – or in the summer, to allow a significant amount of heat thru the window and into the home.
LowE coatings block a significant portion of the heat transfer thru the glass.
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If a person is interested in feeling the heat of the sun thru the winter, and taking advantage of that heat gain, but that person also wants to keep that heat indoors in the winter (and outdoors in summer) then there are options available.
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A high solar heat gain (HSHG) LowE coating will allow direct solar energy to pass thru the coating and will prevent heat from passing back thru the coating to the outdoors when the sun isn’t shining. This coating offers the advantages of passive solar energy gain with the advantages of the heat-blocking ability of the LowE. Placing this coating on the #3 surface of the IG (the inner side of the interior lite) affords a bit more solar gain advantage as well. 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. LowE coatings are generally placed on surface #2 – interior of the exterior lite – except in extreme northern climates.
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I certainly understand the attraction for the concept of solar heat gain and passive solar heating. It has a certain practical romanticism, but in actual fact, using a Low Solar Heat Gain coating (LSHG) has actually proven to be more effective as a year round energy saver in virtually all parts of the country even though it does block most of the solar gain in winter. The advantage of increased U values of this product plus the ability to block solar energy makes the product very energy efficient. But, as I said, I also like the feel of the warm sun on a cold winter day, so I can understand the attraction of using HSHG products as well.
For someone who is really good at saving energy and and really gets involved in a home’s energy consumption, that peson will save about 12% to 15% on their annual energy usage, when using either type of LowE coating, over using clear glass.
For someone who would prefer to install it and forget it, the savings of using a LSHG LowE over using clear glass will save you close to 35% and using on your annual energy usage over using clear glass. Using HSHG LowE will net an energy savings of about 20% over clear glass.
And, as noted in the original post, if a person wants to spend much of their life opening an closing curtains based on the position of the sun, then they are certainly free to do so. If that person wants to basically live in a window-less cave when the sun isn't shining directly on that window, then they are free to do so as well. And as a side benefit, the insulated heat-blocking shades will also contribute to an increase in window condensation when they are closed.
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Edited 6/20/2006 8:16 am ET by Oberon
Thanks for all that detail!ps. You make windows for a living, or did you just stay at Holiday Inn Express last night? :)
Edited 6/20/2006 9:58 am ET by CloudHidden
Dangit Cloud!!!! I almost spit out my coffee in alaughing fit!
Sure sounds like he knows his stuff. And now I can safely say I learned something today." If I were a carpenter"
*laughing*!!!!!!!
Thanks.....
Actually, I don't make windows and I am not sure that I have ever stayed in a Holiday Inn Express (did stay at a Fairfield Inn a couple weeks ago, but not sure if that counts?)...
People actually pay me to say this same sort of stuff in my day job, and I do have the opportunity to work with - but not for - a few different window companies.
I disagree with the 50% part, 20 to 25%, maybe.
I disagree with heavy curtains having any sort or real effect on heat loss (not counting window quilts, designed and installed specifically to reduce heat transfer). Hanging heavy drapes/curtains near a window will not, and window quilts and other methods are a PITA.
All told, I would not take advice in these matters from "Client reports", whatever that means. IF a client of yours want to base their window selection on such, I would dissuade them if at all possible. Besides, how are your going to tell them aprt and get the right windows in the right holes? A major FUBAR in the works.
I went through this decision process a few years back, concerning the north side of my house, which was to receive a good 30% of the $60k of the windows in the renovation project that was under way at the time. The sun doesn't shine on north facing walls. The cost differential was minor. With a lot of glass, reducing heat loss counts big time. I have no, nor do I ever intend to have any window treatments of any sort. My reasons for my house.
window quilts are a PITA, perhaps. However, it is pretty easy to cut your glass loss at night by 50% or more using them. At least using them on that huge picture window in the family room (hypothetical, of course) would be great.. but in accordance with your statement, most people don't.I don't find them to be that big of a deal in my home, personally. It's more comfortable too.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
We've got window quilts in our house, too, and find them effective and easy to use. Especially the one on the big slider, which makes a considerable difference summer and winter.
If they hadn't been there when we bought it, however, I don't know if I could have handled the up-front cost. From the little bit of price checking I've done, it looks like they cost almost as much as the windows themselves.
Don
I have a 480 sf window wall (30 x 16). My quilt estimate was $10k. Can't save enough on heating costs to justify that.
Holy crap! Sounds like an opportunity for a local seamstriss if you ask me.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
I looked into all sorts of things, including a theater curtains. They started at 3k for unlined, guaranteed to fade material. The big hurdle was the height. Most curtain hardware maxes at 8'. The 16' height moved us to industrial hardware. In the end, there would have been insufficient savings and dw liked the bare look. To pay that amt to hide a fantastic view just never made sense.