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Community and Q&A

Room-Side Low-E – Worth It?

mculik5 | Posted in General Questions on

My wife and I have a 3200 square foot, two-story, colonial house in NJ (Zone 5), built in 1987. We will be replacing all windows this fall, and are leaning towards Marvin Integrity new construction windows. Current windows are Andersen 200 Series Narroline, circa 1986, with dual-pane clear glass. The house gets very little shade, and the back of the house faces south.

We’re thinking about speccing the following glass package for the new windows:

Surface 2 – Cardinal LoE-180 (except in our kitchen and bonus room, which are east and west facing and get really hot; these will get LoE-272)
Surface 4 – Cardinal LoE-i89

The idea here is to take advantage of solar heat gain in the winter to offset oil heating costs, understanding that we’ll take a slight energy hit in the summer vs. LoE-272, which seems to be the standard in our area. 


1. What do you think of this approach? Does it make sense, or should we go with simple/inexpensive LoE-272 only, which seems to be the standard in our area?

2. Is the LoE-i89 a worthwhile upgrade? Will we notice the difference, both in terms of comfort and heating costs? The upcharge is around $300 per window because it’s a custom request. Our goal is to be in this house for a long time, so we want to “do it right,” but obviously don’t want to waste money on things that will make little difference. 


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  1. Jon_R | | #1

    Low-E on surface 4 will increase drafts, which is the more likely cause of window discomfort.

    1. mculik5 | | #2

      Jon R - How does low-E on surface #4 increase drafts???

      1. Expert Member
        Dana Dorsett | | #3

        The short answer is that surface #4 is cooler than a single low-E (on surface #3 or #2) when it's reflecting rather than absorbing the radiated heat with i89. The lower temp increasing the potential for drafts.

        But in zone 5A that would only be a problem with tall / very tall windows.

        If you don't have draft problems with your clear-glass double-panes, it won't be an issue with similar sized windows with i89 on surface #4:

        A typical clear-glass double-pane will have a surface #4 temp in the low to mid 40s F when it's 0F outside.

        An argon filled single low-E would have a surface #4 temp in the low to mid 50s when it's 0F outside.

        Adding a hard coat low-E to surface #4 on the same argon filled low-E window drops the surface temp by a few degrees (maybe even edging 50F), but not all the way down to the low to mid 40s F, where your existing windows are.

        Whether it's worth fully $300 per window as a custom request seems questionable though. If it adds up to ten windows you could probably save more on heating costs with that money investing it in a 3/4 ton cold climate mini-split heat pump.

        1. mculik5 | | #4

          Thanks. I know about the condensation potential with interior low-e coatings, but hadn't heard of the draft issue before.

          We don't have draft problems now. We have window sealing problems, due to the age of the windows, but that's a different story...

          We will be installing a heat pump this fall, too. Current furnace has a cracked heat exchanger. The idea is that the heat pump will cover our heating load in the shoulder seasons, and we'll only need to run the expensive oil furnace in the dead of winter.

          As it relates to the windows, I guess I'm really just wondering how noticeable the interior low-e will be...? I could spend the $300/window on air sealing and insulation and almost certainly get a bigger ROI. But...we plan to be here for a long time, and I don't want to be kicking myself five years from now, after the HVAC and air sealing/insulation are done, for "not getting the good windows when I had the chance."

          So, is the interior low-e coating something that's going to make me go "Man, I should have done that years ago," or "I'll never live in a house without interior low-e again..."

          Am I making sense? It's not so much a budget issue, or some quest for lowest possible's more about long-term comfort...

          Also, do you think going LoE-180 for high SHGC and taking the summer AC hit is a good approach? Or should I just stick with LoE-272 as an all-around compromise?


  2. Expert Member
    Dana Dorsett | | #5

    If you download a copy of BeOpt and simulate your house you can answer LoE-180 for the south facing windows vs LoE-272 everywhere question.

    "We will be installing a heat pump this fall, too. Current furnace has a cracked heat exchanger."

    Have you run a full Manual-J on the house yet? Does the heat load number have a pretty good match with a fuel use heat load calculation?

    Since you're replacing the equipment NOW is the opportunity moment for right-sizing it all. This runs a much higher "Man, I should have done that..." risk than any answer to the "...i89 or not i89..." question.

    For details on how to run a fuel-use based load calc see:

    If you're on a regular fill-up service that stamps a "K-factor", that and your ZIP code (for the 99% outside design temp) is enough to get a good handle on it.

    It's highly likely that a modulating cold climate heat pump such as the "HA" series Mitsubishis with a modulating PVA series air handler could handle a 3200' house in northern NJ without heat strips or backup. The larger of the series, the HA42 (3.5 tonner) is good for 48,000 BTU/hr @ +5F (which is approximately your 99% outside design temp) without engaging heat strips:

    If that's oversized, they make a 2.5 and 3 ton version as well.

    The auxiliary heat strip options are cheap, and would cover the shortfall during Polar Vortex lows. With this approach you can get rid of the fossil burner entirely. Even though a modulating heat pump is more expensive than a 1 or 2 stager, it's also more efficient, and you won't have to mess around with crossover points on the fuel switching, or the higher air handler power it takes for both a furnace heat exchanger plus heat pump coil. I suspect overall the modulating heat pump would be more comfortable too.

    The Carrier Infinity with Greenspeed technology or Trane's modulating heat pumps can get you there too, but since it doesn't use a cold-climate type vapor-injection compressor it's capacity falls off faster, and it would likely need to be a ton or more bigger (in rated cooling capacity) than an HA-series Mitsubishi to hit the same output at +5F.

  3. walta100 | | #6

    Before you replace your windows please be sure to read this article.

    I did pull this quote from the article.

    “Your new replacement windows will save you so little money on your energy bills that the payback period for this investment may be more than 100 years — far longer than the new windows are likely to last.”


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