Everything IG January/February 2023

January 20th, 2023 by Nathan Hobbs

Half Full or Half Empty? Every Gas Filling Comes With Its Own Formula for Performance

By Dave Cooper

One of the key factors that determines thermal performance of insulating glass units (IGUs) is the type of gas that fills the space between glass lites. Gases such as Argon, Krypton and Xenon are denser than air and have a lower thermal conductivity, making them effective at reducing heat transfer. Combined with low-emissivity (low-E) coatings, these two features are largely how insulating glass (IG) makes its mark on building performance.

A Look at the Choices

When it comes to the gas that’s trapped between lites, choices are relatively simple.

Air: Some IGUs do not include a special gas for filling. In those cases, air is dried to remove moisture when sealing and from any surface of the interior IGU cavity to ensure it does not condense. Air is a mixture of gases that is approximately 78% nitrogen, 21% oxygen, and trace amounts of other gases, along with water, but the moisture is removed by use of a desiccant material within the IGU spacer. The usual performance is at least -40°F dew point. Incidentally, this is also the minimum requirement for insulating glass (IG) certified to the ASTM IG durability standards. Dry air has a thermal conductivity of about 0.024 W/m*K at room temperature.

Argon: A colorless, odorless, and tasteless noble gas, Argon is commonly used as a gas filling in IG. It’s relatively cheap, at around 20 cents per cubic foot. Argon offers a thermal conductivity of 0.018 W/m*K at room temperature, which is about 40% lower than that of air.

Krypton: Another noble gas that is used as a filling in IG, Krypton has a slightly lower thermal conductivity than Argon, coming in at 0.009 W/m*K at room temperature, but is far more expensive at roughly $60 per cubic foot. Krypton is also the proposed gas to be used in thin-triple IGUs, since it achieves optimum performance at thinner cavity widths, compared to air
or argon.

Mixes or Blends: Blends of gasses may also be used, which are made up of any of the above gas components. Obviously, an IG with 50% Krypton would cost less to fill than one with 100% Krypton, with a (center of glass) thermal performance about midway between Argon and Krypton alone.

Xenon has a thermal conductivity of 0.005 W/m*K at room temperature, but is almost never used as a gas filling in IG due to its prohibitive cost. Carbon dioxide has a thermal conductivity of 0.010 W/m*K at room temperature, which is even better than Argon, but is never used. Why not? Well, according to my friend, Randi Ernst, a world-renowned expert on IG gas filling, there are good reasons, mainly to do with kinematic viscosity (resistance to movement and flow) and the ability of CO2 to “stay put.” Suffice it to say, this has been pondered and tested numerous times in
the past. Still, maybe there is a way for CO2 to be used for IG gas fill, thereby capturing this greenhouse gas for sustainability models and credits?

Overall, vacuum insulating glass (VIG) or traditional IG are an effective way to improve the energy efficiency of buildings, while creating more comfortable living and work environments. In this way, the material is a blessing for fenestration, no matter which gas is used.

Dave Cooper is a consultant and president of Fenestration Consulting Services LLC.
dcooper@fenconsultant.com

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