Low-E vs regular windows in a south-facing room?

soylentgreen
soylentgreen Solar Expert Posts: 111 ✭✭
in Energy Use & Conservation #1
The master bedroom in my house has a bunch of dual-pane windows that are fogged and have lost their seals. I've decided to get them replaced. I'm not sure whether to get regular glass vs. Low-E glass.

In the winter: the deck is not that hot, but the low angle sun shoots under the roof and directly through the windows, warming the room via radiation, which is nice.

In the summer: the deck is very hot, but the high angle sun does not hit the windows because of a 3-foot roof overhang, so most of the heat seems to be coming from convection rather than radiation. The heat is not wanted in the summer.

In this situation, would Low-E windows be better as opposed to regular old dual-pane windows? (Note: I am getting the windows replaced).

My thinking: new dual pane windows, since they are properly sealed, might be better at blocking Convection in the summer, but would still let the direct Radiation through in the winter.

Additional details: Climate is Coastal San Diego, so it never gets that hot or that cold. The Low-E windows will add about 50% to the price.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #2
    Re: Low-E vs regular windows in a south-facing room?

    Your thinking is right on with the regular glass if you do not have direct sunlight on the window that causes you a problem. I did put Low-e on a few windows on the back of my house ( South Side ) where they got direct sun in the summer. I don't know what kind of windows you have, I had double pane from the 70's there no comparison with the new ones, best money I ever spent, 10" of extra Insulation overhead runs a close second. When it's 0 on the outside they are the same temperature as the inside room is.
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  • Lee Dodge
    Lee Dodge Solar Expert Posts: 112 ✭✭
    #3
    Re: Low-E vs regular windows in a south-facing room?

    If you choose low-e windows, make sure that you get the glass package that you desire. Some packages are designed for low solar heat gain, and some for high. Both types will reduce radiative heat losses from the house. The low solar gain will be slightly better in insulation value (u-value, which stands for thermal conductivity), but they will reduce solar gain by more than a factor of two compared to the high gain windows. Many window manufacturers use Cardinal Glass in their windows. Cardinal calls their high solar gain windows low-e 180, and their low solar gain windows low-e 272 or 366 depending on the exact specs. You might want to take a look at the Cardinal Glass catalog, http://www.cardinalcorp.com/. In your area, you will likely get low solar gain unless you specify otherwise. Local building codes will specify that you use low-e windows of some type, which is probably wise anyway.
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  • Ken Marsh
    Ken Marsh Solar Expert Posts: 114 ✭✭✭✭
    #4
    Re: Low-E vs regular windows in a south-facing room?

    Another thing you can do is glaze a third pane over the exiting two panes.
    I have done this in several houses.
    The result is dramatic. In some cases halving the heat needed to keep the house up to temp.
    Put the third pane over the whole sash.
    Most window types leak air considerably.
    The third pane not only halves conduction but can completely stop convection loss.

    You may not want to completely block window operation in bedroom windows.
    Codes want an operable window in each bedroom.
    But you can still usually get the third pane in such that it can still be operated.

    Optimum pane spacing is 1/2 inch.
    Use 100% silicon for caulking.
    On most surfaces it is stripable in the event you want to remove the third pane.
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #5
    Re: Low-E vs regular windows in a south-facing room?

    I like that idea.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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  • soylentgreen
    soylentgreen Solar Expert Posts: 111 ✭✭
    #6
    Re: Low-E vs regular windows in a south-facing room?

    Hmm, not sure how we got on the topic of Prii but thanks to all for the suggestions.

    After more thought, decided that since I typically have the windows open for more than 6 months of the year, worrying about Low-E windows was probably overkill. Decided to go with plain old glass.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,634 admin
    #7
    Re: Low-E vs regular windows in a south-facing room?

    Moved Prius as a Generator Conversations over here:

    http://forum.solar-electric.com/showthread.php?12962-Using-a-Prius-as-a-generator

    Thank you "TecknTrek".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • cupcake
    cupcake Solar Expert Posts: 254 ✭✭✭
    #8
    Re: Low-E vs regular windows in a south-facing room?

    Low-e is for cutting high UV...like in the desert...

    I'm in the desert... low e is a must have...

    In louisiana it will be 100 degrees on a cloudy day...so no real point in getting low e...

    Low e works best in dry heat areas....
    ~1.5Kw PV in parallel
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #9
    Re: Low-E vs regular windows in a south-facing room?
    cupcake wrote: »
    Low-e is for cutting high UV...like in the desert...

    I'm in the desert... low e is a must have...

    In louisiana it will be 100 degrees on a cloudy day...so no real point in getting low e...

    Low e works best in dry heat areas....
    As if everyday is a cloudy day. Low works everywhere you have sun intrusion that is a problem. On my house is is the south side where rooms are at least 5-6 degrees cooler with Low-E, let your comfort and wallet be your guide. I saved more in A/C bills in one year to pay the difference in cost.
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  • Lee Dodge
    Lee Dodge Solar Expert Posts: 112 ✭✭
    #10
    Re: Low-E vs regular windows in a south-facing room?
    cupcake wrote: »
    Low-e is for cutting high UV...like in the desert...

    I'm in the desert... low e is a must have...

    In louisiana it will be 100 degrees on a cloudy day...so no real point in getting low e...

    Low e works best in dry heat areas....

    It would be best to clarify the difference between "Low-e" and "solar heat gain coefficient," as they are different quantities with different performance characteristics. "Low-e" stands for low-emissivity, and it is intended to reduce heat losses from a warm house to a cold outside environment by reducing radiative heat losses from the windows. It means that a window surface has been coated with a thin film that has low emissivity in the long-wavelength infrared portion of the electromagnetic spectrum. At room temperature the thermal emissivity peaks at a wavelength of about 10 micrometers compared to the visible spectrum which covers the range of 0.4 to 0.7 micrometers.

    Low-e windows can have a low solar heat gain coefficient (SHGC) or a high SHGC, depending on the details of the coating. The low SHGC windows are desirable in the southern U.S., while the high SHGC windows are desirable in the northern U.S. and Canada. The low SHGC coatings only transmit light in the visible spectrum, while the high SHGC coatings allow a much broader range of wavelengths of sunlight through the windows, and specifically the short wavelength infrared radiation that contains a significant amount of the sunlight's energy. See http://www.fsec.ucf.edu/en/consumer/buildings/basics/windows/how/solar_gain.htm for some graphs showing the different transmission characteristics.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #11
    Re: Low-E vs regular windows in a south-facing room?
    Lee Dodge wrote: »
    while the high SHGC coatings allow a much broader range of wavelengths of sunlight through the windows, and specifically the short wavelength infrared radiation that contains a significant amount of the sunlight's energy.

    That requires some thought... usually I associate 'infrared' with the longer wavelengths of near visible light... I guess you're saying that the high SHGC glass transmits the higher frequencies in the infrared range of the spectrum better than the low SHGC glass.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Lee Dodge
    Lee Dodge Solar Expert Posts: 112 ✭✭
    #12
    Re: Low-E vs regular windows in a south-facing room?
    vtmaps wrote: »
    That requires some thought... usually I associate 'infrared' with the longer wavelengths of near visible light... I guess you're saying that the high SHGC glass transmits the higher frequencies in the infrared range of the spectrum better than the low SHGC glass.

    --vtMaps
    Yes, the near infrared radiation is transmitted through high SHGC glass, while that same radiation is reflected by low SHGC glass. But all low-e glass must reflect longer wavelength infrared radiation beyond about 2.5 micrometers in wavelength, because being a good reflector means that it is also a poor radiator. Rather than giving it too much thought, I would just look at the figure in this link.

    The infrared spectrum covers a large section of the electromagnetic spectrum, from wavelengths of 0.8 to 1000 micrometers (14000 to 10 cm-1). The difference in low and high SHGC glass occurs in the near infrared region of wavelengths of 0.8 to 2.5 micrometers (14000 to 4000 cm-1). All low-e glass, whether high or low SHGC, is reflective (i.e., poor radiator) in the mid-infrared region with wavelengths of 2.5 to 25 micrometers (4000 to 400 cm-1).
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