Low-E Glass Explained

• ϵ = Emissivity
• A =  Surface area of the part radiating heat
• ΔT = The difference of the temperature from inside and outside (“delta T”)
• σ = Stefan-Boltzmann constant (Don’t worry about it)

The only thing you need to know about the above equation is the emissivity (ϵ) variable.  What this means in a nutshell is that the lower the ϵ the lower the heat loss due to radiation.

Three methods of heat loss (Sorry times 2)

1. radiant heat loss
2. convective heat loss
3. conductive heat loss

I’ve already talked about radiant heat loss by describing the equation above.  Convective heat loss is the heat lost (or gained) as a result of air circulation.  So for example, when you have a heat duct blowing air on a window or a cold wall, the wall absorbs heat from the air molecules that bounce off and contact the wall.  Conductive heat loss can best be described as a warm body contacting physically a cold body.  Kind’a like when your wife rubs her cold toes on your body.  The man losses heat and the wife gets all warm and cuddly. (It’s not fair, but that’s just the way heat transfer works—it’s physics and physical.)

It gets even more bazaar
Low-E glass is actually made with the chemical element Silver—that’s right the same silver used in the 1964 silver, President Kennedy $0.50 piece.  The technology, however, is pretty complex.  Over simplistically, the silver is applied to the glass via an emitter technology.  Silver bars are loaded above the glass and the glass passes below the silver bar. Somehow the glass carries an electrical charge and the silver carries the opposite charge.  Opposite charges attract, so the silver bar “emits” silver atoms that are deposited on the glass.

You can’t see it, but the silver is there at an atomic level. Depending on the series of glass you order, there can be as many as three coats of these “silver emissions” onto the glass. (The visible transmission of the glass is reduced by a minimum of 60% if the glass has three layers of silver–so the glass looks slightly tinted.) And since silver has a low emissivity (ϵ), the glass will reflect the radiant heat.

Here’s where it gets bazzar, the Sun’s radiant heat is high energy (short wavelength) UV–radiant heat. This energy will penetrate the glass and continue inside the house.  Sounds Star-trekish enough until I explain the next concept.  Your body is also a heat “radiator”, it will radiate heat to colder surfaces.  The heat that is radiated is “low energy, longer wavelength” heat (infra-red).  This longer wavelength, lower energy heat (low energy in a physic’s context) will bounce back or reflect from the glass. The same type of radiant heat is emited from all objects, your furniture, your cat, your carpet—everything!  This reflected “low energy radiant heat” is a significant amount of heat. In term’s of the home owners wallet, it is not low energy—it is high cost radiant heat that hits the check book.

Sidenote:  Have you ever noticed in the winter when the inside temperature is 72^oF, that you feel cold–and then in the summer, and the inside air temperature is about 72^oF, you feel OK–ever notice that …huh.  Well, if you have– your not crazy, the reason is that the outside walls of your house are cold in the winter.  You actually do “radiate” more heat to that cold wall.  It’s really a bazaar concept (and yes, we really did land on the moon.) You actually do lose more heat from your body–even with the air temperature being the same–in the winter because your body (“the radiator”) is radiating more heat to that cold wall. (As Cap’ian Kirk could’a/would’a  have said: “give me all the power that your thermostat’s got Scotty, I’m cold.”)

I hope this explains Low-E a little better.  Stay cool, ‘er warm—whatever.