Solar Heat Gain Coefficient: European vs. North American Ratings
So, I’m aware that window U-values are measured under different conditions for European products compared to North American (NFRC) ratings, and that U-values can be presented in either American or SI units. I thought Solar Heat Gain Coefficient (SHGC) was effectively just a percentage of solar energy that’s transmitted thru the glass, and that there wouldn’t be either unit conversions or measurement differences to account for. However, I’m getting some interesting information from different manufacturers and it’s making me wonder if I’m missing something.
– From one American manufacturer, their standard SHGC offering is 0.24 with an available low of 0.17 and an available high of 0.38 ( with differences depending on the frame style — a clue??)
– From a Canadian manufacturer, the standard SHGC offering was 0.52, with available lows of 0.32 and 0.36, and high of 0.55, all at a slight price increase.
– From another Canadian manufacturer, SHGC offerings were .16 – .24
– From a European manufacturer, the standard SHGC offering was 0.5, with about a 10% increase to go down to 0.4 and “even more expensive” to go lower.
All of these are for triple pane windows with “low” U-values. My understanding was that it was generally easier to make low-U, low-SHGC windows and that getting the combination of low-U and high-SHGC was the more expensive / rare option, but from the European and one of the Canadian manufacturers, it was the opposite — lower U values were more expensive (and also couldn’t get as low as some of the other companies’ “standard” offerings). This is making me wonder if I’m really comparing apples to apples here.
Why I care: I have a large patio door / window (like 7′ x 10′) on a western exposure with shading to the south — so in the winter, it gets exactly zero direct sunlight, but in the summer it gets direct afternoon sun for several hours, contributing substantially to cooling load. So, I actually want both low U and low SHGC in this specific location (if Suntuitive was still in operation, I’d think about that after reading about it here). Aside: Has anyone seen this tool (https://susdesign.com/windowheatgain/) or have any idea of its accuracy?
The potential clue: Some of the manufacturers quote an “NFRC full-frame SHGC” which often varies between frame types. The ones with higher SHGC values just quote an SHGC value. Are some manufacturers reporting just the SHGC of the IGU (like a “center of glass” U-value) while others are considering what percentage of the rough opening is glass vs. what percentage is frame? Considering a hypothetical 48″ window with a 3″ wide frame, only roughly 75% of the area is glass which could take a “glass only” SHGC of 0.5 down to an “full frame” SHGC of 0.375. Is this the source of the difference I’m seeing? Or are there really drastic difference between the IGUs used on different continents?
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Replies
Jonny,
NFRC ratings -- the ratings shown on the NFRC paper labels on American windows -- require SHGC numbers to include the area of the frame as well as the area of the glass. Most European window manufacturers report SHGC as a glazing-only number. So these numbers aren't directly comparable.
Here's what I wrote on the topic back in 2010: "A window’s U-factor and SHGC numbers appear on the familiar NFRC label found on most new windows. Note that the NFRC reporting requirements require U-factors and SHGCs to be whole-window measurements, not glazing-only measurements. You can’t compare a whole-window U-factor with a glazing-only U-factor. If a window salesperson brags about a certain U-factor or SHGC, be sure to ask whether the advertised number measures the performance of the whole window or only the glazing."
These sentences come from an article you may want to read: "All About Glazing Options."
Hi Martin, Thanks for the reply! You've confirmed what I was beginning to suspect. To provide additional confirmation, I got glass data from a couple manufacturers who had provided NFRC numbers -- together with frame drawings and NFRC standard unit sizes, I was able to reconcile the difference between the glass and NFRC numbers, and the comparisons look more sane now.
I suppose the NFRC rating system is the way it is for a reason, but is it really "fair" to include the area reduction of the frame in the calculation of SHGC? This ends up providing different SHGC values for different window operation styles, using the same glass, from the same manufacturer, muddying the waters when trying to compare products. And, none of it will be directly useful for energy calculations on anything other than "standard size" windows because the frame:glass ratio will be different. I feel like I like the European approach of reporting glass and frame values separately to be a bit better. (Another example: Under NFRC, a mediocre frame and great glass can look the same as a more "balanced" frame and glass package, but might have other implications with respect to comfort or condensation, but I digress)
Also, it's still interesting that lower SHGC is an upcharge for the more European manufacturers -- I guess it's reflective of climate differences and what's the more common product geographically.
Consider two different windows that are identical in size and glass package, but window X has a wider sash/frame while window Y has a narrower sash/frame resulting in more glass area for window Y even though the windows themselves are exactly the same size.
In a glass-only scenario, SHGC as listed is going to be the same for both windows, but if SHGC is based on the entire window and not just the glass alone, then window X is going to have a lower SHGC than window Y.
As a consumer who is concerned about blocking solar heat gain (or alternatively wants more direct solar heat gain), what would make more sense when considering real world window SHGC performance, whole window or glass-only?
And as an aside, IMO Suntuitive glass was ultimately more about hype (or h0pe?) than actual performance.