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Code Q&A

NGA Advocacy and Technical team tackles questions on anisotropy, energy codes and more

Urmilla Sowell
Urmilla Sowell

Urmilla Sowell
NGA Technical and Advocacy director


Q: If anisotropy should not be considered a defect, how can I convince the building owner that the discoloration and non-uniform appearance is acceptable? Are there any standards that measure what is acceptable for anisotropy?

A: There are no existing standards for anisotropy, but ASTM Committee C14 on Glass and Glass Products is working on a new test method that will cover anisotropy. Meanwhile, you can refer to NGA’s Glass Technical Paper FB20-08 (2017) Iridescence in Heat-Treated Architectural Glass for an explanation of the phenomenon of strain pattern and guidelines for glass inspection on the construction site. 

Visit to access the technical paper, which is available for free download. 

Tom Culp
owner of Birch Point Consulting, NGA and GICC energy codes consultant

Tom Culp
Tom Culp

energy code changes

Q: What is the biggest change to energy codes in 2019? 

A: The code changes take the next step in stringency overall: mainly one step lower U-factor across all climate zones. [Solar heat gain coefficient] format is changing to show SHGC for fixed and operable products separately, which does not change the glass, but will encourage people to look at whole product numbers instead of center-of-glass numbers. There are more daylighting controls as well. I included specific details and numbers in my NGA Thirsty Thursday presentation Energy Code Evolutions 2020. 


Q: How do I know whether to follow ASHRAE or IECC for a building project?

A: The code allows the option to use either one, but you must choose all the requirements from either IECC or ASHRAE 90.1 rather than picking and choosing sections from each. The building owner will decide which code to follow… and their decision will be noted in the specification. While we worked hard to align the commercial fenestration requirements between IECC and ASHRAE 90.1, there are some differences in other parts of the code that could drive the decision.  

thermal bridging

Q: ASHRAE 90.1 is looking at thermal bridging requirements. Is that being looked at anywhere else?  

A: The idea of thermal bridging, that you cannot bypass the insulation, has been discussed in the building science community for many years. From a building codes perspective, Vancouver requires thermal bridging be incorporated in performance modeling, and that all buildings over a certain size must use performance modeling rather than the prescriptive path. California has indicated they want to consider it for their 2022 code update, but likely will delay to the next cycle. 

laminated glass

Q: On laminated glass, which surface location is best for low-emissivity coatings? 

A: It depends on the purpose. If you embed the low-E coating within the laminate, it reduces the SHGC but you don’t get the lower U-factor benefit of low-E coatings. To get both lower SHGC and U-factor from low-E, put it on the outside of the laminate facing the glazing cavity. 

View the archived presentation at

Nick Resetar
Roetzel & Andress, NGA and GICC Fire/Structural and Safety Glass Consultant

Nick Resetar
Nick Resetar

Renovations and additions

Q: In a remodeling project, what square footage of glass must be brought up to the code?

A: Renovations and additions must comply with the current energy code just like new construction for that portion of the building. New glass installed in hazardous locations must meet the current safety glazing standards.

Full window or sash replacement must meet the same prescriptive requirements as new windows, but ASHRAE 90.1 allows up to 25 percent of the window area to be replaced with the same or lower U-value and SHGC. Renovations that add interior panels or storm windows are exempt because of the improved efficiency gained, but ASHRAE 90.1 requires them to be low-E coated. 

Glass-only replacements are considered repairs and do not have to meet current energy code requirements, but the replacement must have equal or lower U-factor and SHGC if ASHRAE 90.1 is enforced.  

In hazardous locations where accidental human impact could occur, best practice dictates bringing replacement glass up to current safety code standards.

Bird-Safe glass

Q: What qualifies as “translucent glass” when referring to HR919 Bird-Safe Buildings Act?

A: The term translucent is not defined in the bill itself, which refers to “opaque, etched, stained, frosted, or translucent glass” as potential acceptable bird-safe solutions. Likely translucent refers to semi-transparent surfaces, in which light can pass through, but objects and shapes are not easily seen. Anything transparent or clear could exacerbate the bird strike problem. 

Q:  What are some types of glazing that are available for bird-friendly glazing design? 

A: Glass products are available with surface treatments such as etch, frit, film and ultraviolet patterns that create visual markers for birds. Muting reflections using angled glass, awnings and overhangs, sunshades and screens are also strategies for reducing bird collisions with buildings.