Is There Such a Thing as Energy Efficient Glass?

By Scott Sidler October 24, 2016

energy efficient glassLast week I had a blast destroying a bunch of glass samples in my shop for Part 1 of this glass series. This week I’ll reveal the results of my thermal testing to see if there really is such a thing as energy efficient glass.

I’ll preface this with the fact that I am not a scientist and this test is not scientific. I’m just a historic restorationist and this anecdotal test gets me close enough to see how each piece of glass performs so we can make better decisions when it comes to upgrading the efficiency of your old windows and doors.

 

Is There Energy Efficient Glass?

Marketers talk about insulated glass or energy efficient glass, but in my opinion it’s all smoke and mirrors. Comparing the energy efficiency of one glass to another is like comparing gas mileage between mega yachts. They’re all bad! Glass is not designed to be energy efficient.

[Tweet “Glass is designed to allow light into a space not insulate it.”]

Walls, siding, plaster, insulation those are all intended to keep us warm and safe inside, but glass does not serve that purpose as its primary function so let’s not try to put a square peg in a round hole.

Do we want glass that excels at keeping heat in its place? Absolutely, but before you go comparing the R-value of glass to spray foam think about their very different purposes.

 

The Different Types of Heat

I’ve written about the 3 different types of heat before and you can read it here for a more in-depth explanation.

Understanding the 3 Types of Heat Transfer

Basically, there are 3 ways heat moves from one place to another.

  1. Radiation – This form of heat comes directly from the sun’s rays and moves through building materials with ease unless it is stopped by a radiant barrier like foil.
  2. Convection – This is hot air moving in and out all the gaps (large and small) in you house. Convection requires air exchange for the heat to move.
  3. Conduction – This what happens on your stove. Heat transfers via contact from the stove top to the pot to the water and the water boils.

The testing I did for this post reflects mostly conduction and some radiation. So let me stop yammering and get to the test results.

 

The Test: How Efficient is Glass?

speedheaterI set one of my Speedheater Infrared Heat Guns 10″ away from each piece of glass and left the glass in front of the heater for exactly 1 minute before measuring the temperature with a FLIR TG130 Spot Thermal Camera. I measured the temperature in the center of the glass on the inside (the side opposite the heat source). The glass all measured approximately 82°F prior to the test as they had been resting in my muggy shop for a number of hours.

The Speedheater runs at approximately 400ºF but I was unable to get a precise temperature reading beforehand because The FLIR camera peaks at 302ºF which seems like an odd temp to end at, but hey that’s what FLIR decided to do for this entry level camera.

 

The Results

 

how energy efficient is antique glass

6th Place – 3/32″ Antique 114ºF

It was no surprise to me that antique glass conducts a lot of heat. It’s thin and it serves to simply let light in and block the elements. Though I love it, it is the least energy efficient when it comes to conductive heat transfer. There are varying thicknesses of antique glass so thicker versions would likely yield better results and thinner worse.

 

 

how hot does glass get

5th Place – 1/8″ Annealed 105ºF

Predictably the modern double strength glass came in just ahead of the antique glass. The added thickness is what I would attribute this improvement to not the age. Again the thicker the glass option you choose, the slower the heat transfer you can expect.

 

 

laminated glass and heat

4th Place – 1/4″ Laminated 93.2ºF

I use 1/4″ laminated glass a lot and was very curious to see where it fell in the rankings. I’m not terribly surprised, but feel like in addition to the sound benefits it affords, this increased conductive heat blocking is a nice side benefit.

 

 

low-e glass heat transfer

3rd Place – 1/4″ Tempered Low-E 89.3ºF

I was really impressed with the performance of the Low-E coated tempered glass. The thicker 1/4″ glass helped I’m sure and I think the Low-E coating made an additional improvement that kept it below 90º. Well played Low-E.

 

 

IGU heat transfer

2nd Place – 3/4″ IGU Low-E 85.8ºF

Any of my readers know I am not a fan of IGU’s (aka disposable glass) because of their limited lifespan, but I cannot deny the fact that they do an excellent job (while they are still functioning) of stopping heat transfer. An air gap is always an excellent insulator and when combined with a Low-E coating this test proved it with a nearly 3ºF drop from the 3rd place finisher.

 

 

impact glass

1st Place – 5/16″ Laminated Impact 84.2ºF

This was actually a big surprise to me! Adding a mere 1/16″ of plastic between the 2 panes of glass brought the inside temperature down nearly 10ºF from the 1/4″ laminated glass! And it comes at a minimal additional expense since the 1/4″ laminated glass costs around $15 per SF and the impact glass is $18 per SF.

 

Final Thoughts

 

Glass is just one part of a window or door, and armed with the information in this post you can make better decisions on how to upgrade your existing windows.

Changing out glass in historic wood windows and steel windows and may be something to think about if you have already tackled the important task of weatherstripping. I hope this helps test helps!

4 thoughts on “Is There Such a Thing as Energy Efficient Glass?”

  1. Where Do you get Low-e glass to replace broken single pain glass. Or tempered glass for a bathroom. I have looked around and can not find a place that sells single sheets

  2. Scott, as you know I like testing the materials we use to develop basic understandings of the stuff. I like to call them “Stuff Tests.”
    Let me see if I understand your testing method and results. Here you are measuring the temperature of the glass, which is an indication of how much heat the glass captures and holds. Some of the heat is reflected off the surface of the glass, some may be refracted and pass out of either surface, and some may pass on through the glass. In deciding what glass to use in a particular situation I’m not too concerned with the temperature of the glass or how much heat the glass holds, but with what happens to that heat and the rest of the heat in the space on both sides of the glass. Is there a way to measure the amount of heat passing through the glass, or that does not get through the glass? That info would be more helpful to me as I decide what glass to use.

    1. John, I had another friend bring up this option and I planning to do a modified test to add to the results. This version basically tested just the surface temperature of the glass and how quickly or slowly the glass surface will transmit heat.
      The additional test I plan to do is to place the glass in a box and set the same heater up with a thermometer in the box to see how much the ambient temperature changes.
      The other variant I will test is to place a wooden object inside he box to see what temperature that object is (radiant heat). Looking forward to checking the results soon.

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