What are the best ways to measure the efficiency or heat loss on window?  Applying masking tape to both sides is what I have been told is the way to do it. I know they are reflective to Longwave cameras and I often see them in Thermalgrams are appearing hot.   The reason I ask is a replacement window saleman is using an IR camera to show his prospects (marks?) they need replacement via the imagages on the camera.

Windows are about 20% reflective in long wave but also very specular so what we see seems overwhelming. The best way to compare windows is when they are side by side and with equal inside and outside temperatures. The attached shows single, double and high performance windows all on the same wall as viewed from inside in cold weather.

 

Of course, showing things accurately would not necessarily promote sales! It is easy to make a window look bad by moving to the point where it reflects a warmer or colder background. You could certainly use tape on the window to improve emissivity and reduce reflections but I'm not sure a single temperature measurement would give you what you need, unless you could compare other windows under the same conditions. I'd be interested in a website for this sales person if you have one!

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Thermally Yours,

John
ASNT NDT Level III #48166
The Snell Group
www.thesnellgroup.com
www.thermalsolutions.org
800-636-9820

 Dave,

 

I'd like to add to Johns comments.

 

I would not trust anyone using an infrared camera to compare window performance unless they first show an understanding of radiational physics as it applies to the infrared spectrum and competence in the use of an infrared imager.

 

Inspecting glass with an infrared imager presents a variety of potential pitfalls for the unaware (unconscious incompetent) thermographer. First, glass is spectral, meaning that its emissivity varies with wavelength. The emissivity in mid-wave is different than in longwave. Secondly glass is transmissive in wavelengths below 4.8 microns meaning that we are not only seeing radiation emitted from the surface but are seeing some radiation coming through the glass from the other side.

 

Most portable imagers being used today are long wave (8-14 microns). In long wave, uncoated window glass will have an emissivity somewhere from .8 to .9. Most of the samples I have tested are about .85. That means that glass is about 15 % to 20% reflective. Said another way, what you see is very dependent on what radiation is reflecting off the glass. As John mentioned above glass is also specular, allowing the thermographer to control what is reflecting off the surface by arranging the angle of view between the window, the background and the thermal imager. This allows a shyster (unscrupulous window salesperson) to play tricks with the IR camera. The old joke in IR thermography when playing with reflective materials is, “How hot would you like this to look?”

 

And as far as comparing one window to another to show relative thermal efficiency, that is job that can only be done in a controlled thermal environment. I'd stick to the reported R values on the window data sheet. 

 

As the cost of IR imagers decreases and the number of untrained, inexperienced people using thermal imagers increases we will see more and more of these disturbing practices.

 

Rob Spring

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Rob Spring, P.E.
ASNT NDT LIII #65375
The Snell Group
rspring@thesnellgroup.com
800-636-9820