Last 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 by clicking the link below for a more in-depth explanation.
Basically, there are 3 ways heat moves from one place to another.
- 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.
- Convection – This is hot air moving in and out of all the gaps (large and small) in your house. Convection requires air exchange for the heat to move.
- 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?
I set an Original Speedheater Infrared Heat Gun 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.
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 would likely yield worse.
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.
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.
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.
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.
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.
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.
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I love old houses, working with my hands, and teaching others the excitment of doing it yourself! Everything is teachable if you only give it the chance.