Gas Fill Longevity: For Loss and Diffusion, It’s About Recognizing the Issue(s) at Hand
By Dave Cooper
I’ve become involved in several activities involving gas-filled insulating glass units (IGUs) recently; a client’s customer has indicated he might purchase a spark emission spectroscopy (SES) handheld device to check inbound IGUs for percent gas-fill. Another client wants to know how to verify performance of the SES device, so they can reliably check purchased IGUs.
Clearly there is a need to understand the actual percentage of gas filling (normally Argon) within an IGU, but does it consistently meet the fabricator’s spec?
There is a certain leakage rate using Argon, or any other type of gas filling. Argon diffuses through the sealant system of an IGU. Why? Because there is a very high concentration of molecules within the IGU (say 95%) and a very low concentration in the atmosphere (under 1%), which causes a significant concentration difference. The resultant movement of gas molecules through the sealant (diffusion) continues until equilibrium is reached. Note that the opposite is true as well, as air, including nitrogen and oxygen, also diffuses into the IGU over time.
This points out an industry-wide problem with few answers. What is the acceptable leak rate for gas filling in an IGU? Also, what is the allowable thermal modeling gas-fill percentage, and what is that based on? Let’s explore both questions.
Finding a Reference Point
First, there is no definition of an acceptable leak rate for North America. Specifically, the ASTM E 2190 Standard Specification for IGUs does not specify an allowable leak rate. In Europe they have a standard (EN 1279-3) that measures the actual loss after a weathering test, with an allowable loss that’s equivalent to less than 1% per year. Therefore, we can assume that less than 1% per year is an acceptable value.
Second, the gas-fill rate is dictated by the filling apparatus itself. The higher the concentration of gas fill, the better the thermal performance. Based on a long history of filling IGUs, it is well established that lance filling versus gas press filling, versus evacuated (vacuum) gas press filling results in different levels of percent fill. NFRC-100 recognizes this for thermal simulation calculations:
• Evacuated chamber filling allows 97% for any gas type (this is the best value);
• Two-probe filling with a concentration sensor allows 95% for Argon, 90% for any other gas filling; and
• Single-probe timed filling allows 90% for any gas type.
When modeling a window’s thermal performance using Lawrence Berkeley National Laboratory’s Window/Therm package, the simulator needs to know the filling method. The window is verified to this performance by means of a physical test. If the product does not meet the target fill amount, the window may not pass the verification test.
Most windows are rated by the National Fenestration Rating Council (NFRC) and many are Energy Star-certified. Neither is possible if the verification test fails.
Fabricators need to recognize these gas fill issues and put robust quality control measures into place to ensure that their IGUs meet the intended specification.
Dave Cooper is a consultant and president of Fenestration Consulting Services LLC.
dcooper@fenconsultant.com
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