Don’t cook your client by confusing g-value and SHGC Understand glazing performance data when energy modelling high-performance homes

16 February 2024 by Jason Quinn

When it comes to specifying windows, do you know your g-value from the SHGC number? It’s important that Homestar and Passive House designers do. There are two different methods used internationally to calculate window performance, ISO EN410 (producing g-value) and NFRC (producing SHGC). The numbers are slightly different. Only recently did I understand that the difference matters. This is especially the case in cooling climates; using SHGC instead of the g-value could see your design miss out on Passive House certification.

New Zealand’s climates are all heating-dominated. This is true even in the so-called winterless north. (Even at the top of Northland, more energy is needed for heating than cooling—at least, this has been true for every design the Sustainable Engineering team has ever seen.) In heating climates, if a designer unwittingly entered the SHGC number instead of the g-value into PHPP, it would give the design more margin for heating; it’s conservative. However, it will under-predict the overheating level. And this summer, overheating is what everyone is talking about.

In northern Australia—cooling-dominated climates—using the SHGC number would under-predict cooling demand/load. Then it really is important to make sure the g-value is used in the model. 

Relevant ISO standards used in New Zealand use standard calculation methods to compare framing systems, spacers and glass separately. One essential piece of data is how much solar energy enters the building. This is the g-value. It is calculated for the centre of the glazing based on the glazing material properties. PHPP assumes you are entering a g-value. (This is just as relevant for Homestar designers because your software uses PHPP as a backend.)

The National Fenestration Research Council (NFRC) is the industry peak body in the US. Over there, window data methodologies take entire window assemblies and report these for standard size windows. The NFRC also uses slightly different boundary conditions and calculation methods and produces a number for the solar heat gain coefficient or SHGC, which can refer to either the centre of the glazing or the whole window, including the shading from the frame (which is radically different, up to 50% different!). Confusing, right?

The g-value is not exactly the same as the SHGC value, even at the centre of the glazing. The NFRC’s method can produce a very different number—up to 23% different compared to the ISO result for the same glass. The better the glass at protecting from solar heat gain, the more this difference matters.

Everywhere in the EU uses ISO standards and New Zealand is changing its building code to follow ISO standards too. You should expect windows out of Europe to report g-values. Unfortunately Australia is following the US and using the NFRC values. 

It’s also apparent that some suppliers have figured out their products look better if they cite NFRC values. If you don’t know any better, you could be easily misled when comparing glass data sheets. So the single most important takeaway is this: use g-value (Solar Factor to EN410), not the NFRC’s SHGC number. EN410 fits with the other ISO standards referenced in the NZBC, such as EN673 for glass U-value. 

If you are specifying windows and the supplier’s performance data cites a SHGC number, go back to them and ask for the g-value. Any company that supplies Europe will know it. If they don’t sell to the EU, the manufacturer or importer can obtain it from their glass supplier.

Let’s take a closer look at the numbers. SHGC uses a different set of boundary conditions and calculation methods. In these examples the difference is 0.02 to 0.03, but some technical references mention a difference of up to 8% (because if the number is 0.025, a 0.02 change equates to 8%)

Comparing g-value and SHGC (for centre of glass) for a New Zealand supplier’s products. © 2024 Sustainable Engineering Ltd.

The biggest difference I’ve seen in the (admittedly limited) literature is 0.04 difference between g-value (EN410) and SHGC (NFRC) for centre of glass. This was in Ebanks, Peta-Gaye, and Richman, Russell, “Towards Harmonizing the NFRC and CEN Window Performance Simulation Methods,” 2018.

Figure from Hanam, B., Jaugelis, A., and Finch, G., “Energy Performance of Windows: Navigating North American and European Window Standards,” 14th Canadian Conf. on Building Science and Tech., Toronto, Ontario 2014.