My Low-E Story

Window Reflectivity

A couple years ago I was inspecting a home in Edgerton for a younger couple who were first time home buyers. It was my second inspection for them because they got out of the contract on the first one due to many items needing repairs. As was customary at that time, the clients were not coming until the end of the inspection to go over the findings, I hadn’t ever worked with their agent before either. After about 20 minutes their agent came and followed along with me, asking questions, and making small talk.

I inspect the same way every time, Lots and grounds, the exterior, roof, etc… (Check out the home maintenance series from last year) On this particular home, there was a


streak of melted siding on the north side facing the neighbors driveway. This was an afternoon inspection on a nice sunny spring day. As we made our way around the home I had noticed the streak and made the comment to him that it was likely caused by the neighbors windows. He said: “What? You’re joking right?” “No, watch the shadow on the wall as the sun moves, I’ll bet it lines right up with that melted area.” “We will be here for 3 hours, so we can check in on it periodically.” I honestly forgot all about that comment until the end of the inspection.

Low E Glass

In an effort to increase energy efficiency, window manufacturers have developed glass windows to incorporate technology that allows less energy loss and heat gain during the cooling season and less heat loss during the heating season. To understand how the glass and glass coatings make your windows more efficient, one must know a little about the solar energy spectrum. The solar spectrum includes, Ultraviolet light, Visible light, and Infrared Light.

  • Ultraviolet light is what causes items to fade such as window coverings or flooring
  • Visible light is what provides natural lighting to the living area
  • And, Infrared light is what causes indoor items to increase in temperature, otherwise known as solar gain.

Low E glass coatings have been developed to limit the amount of ultraviolet and infrared light that can pass through the window while not compromising the amount of visible light that passes through the glass. That is where low emissivity (or Low-E) coatings come in. Low-E coatings look to reflect ultraviolet and infrared waves to the exterior in the summer to keep the interior of your home cool. The same coating works on the interior of your home to do the same thing. So it works the same when conditions are opposite. Basically, it operates similar to a thermos, which keeps your drink either cool or warm, depending on the contents.

Effects on Vinyl Siding

At around 165 degrees, vinyl siding will melt. So keeping your grill away from your exterior wall is a no brainier. When a Low-E window reflects the ultraviolet and infrared light it gets magnified and can melt siding. There have been many documented cases of melted siding since the late 2000’s. Some lawsuits have also been filed between neighbors over siding damage. There are ways to limit this condition through additional coatings or awnings.


Back to the inspection…

I guess I didn’t notice but as I went through that home, that agent did disappear a few times. I assumed he had a call to make or something. As it turns out, he was checking that shadow the whole time. He came in the house as I was finishing up the kitchen and proclaimed, ” You’re a genius!” I said, “I hardly think that’s the case, but what on earth are you talking about?” “The shadow, it follows the melted siding just like you thought it would.” It was my first case of this condition but I had heard of this happening regularly in warmer climates. Conditions must have been just perfect at this home.

I honestly felt pretty good that day, most of the time I think home inspectors deal with unhappy people due to the nature of the job. I even got to go home and tell my wife what a “genius” I was, even if it was only for this one day.

As always if you have home related questions, please send me an email at, I will help in any way that I can.



Aaron M. Zuehlke is the owner and inspector at Zuehlke Inspection Service, LLC, a full-service home inspection company serving Southern Wisconsin. Specializing in Home inspection, Radon Testing, Mold Testing/inspection and Residential Thermal Imaging. He can be reached by email at or by calling the office at 608-931-7485.

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