European superiority skylight edition!
peter2022
| Posted in Energy Efficiency and Durability on
If you’ve shopped around for the most efficient skylights in North America, you’ll find two things.
1.) the U-factors are much higher than windows with an equivalent glazing type (double pane, low-E, etc)
2.) European manufacturers advertise much lower U-factors for some of their skylights
#1 is simply due to the dependence of U-factor on the angle of the glazing. Vertical glazing has higher U-factor than angled glazing even with the same IGU, due to differences in convection.
There is a paper from Vitro that demonstrates the effect: https://www.vitroglazings.com/media/2mvdlt2g/vitro-td-130.pdf
What about the Europeans? Well we already know not to directly compare glazing U-factors for NA to EU windows:
But for skylights, it gets worse!
NFRC requires skylights to be rated at an angle of 20 degrees (slightly less than 5/12 roof pitch) to incorporate the effect of angled glazing.
However, European U-factors under CEN are modeled at 90 degrees! That’s right, essentially as windows!
This is a huge difference that greatly skews the results.
Here’s one example of a European skylight, the Fakro FNP with U5 glass.
It’s a triple pane unit with 10mm spacers, low-E on #2 and #5, and Kr fill.
The published U_glass is 0.09 to CEN standards (I put all U-factors here in Btu/h·ft2·F).
Modeling this in WINDOW confirms the U-factor under CEN conditions.
The same unit simulated at NFRC conditions has U_glass = 0.16
Just for curiosity, switching to Ar fill brings U_glass to 0.17 (NFRC).
For North American skylights, one of the better IGUs I’ve seen is a triple pane from Cardinal with 366 on #2 and 180 on #4, Ar fill with 10/11mm spacers.
At NFRC conditions, it rates to 0.18 U_glass according to WINDOW, which is just 0.01 off the European IGU with Ar fill.
At CEN conditions – U_glass is 0.14 which seems worse than the Fakro until you try with Kr fill, then you get matched U_glass = 0.09.
Again these CEN values are not very meaningful for a skylight, but it shows there’s nothing special about the Euro glazing.
Hope this helps someone.
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Replies
A 5/12 pitch would be extremely rare in Germany, probably a lot of Europe. A 12/12 is a shallow pitch on many homes. The house accross the street from my MIL's has a shed roof dormer with a 12/12 pitch into the main pitch that is maybe 16/12 or greater.
What I'm saying is that the testing is probably more relevant to real world usage in their respective areas. A 5/12 is a pretty standard assumption for the US.
But it is still good to know how big of a difference the pitch at which it is tested makes a good bit of difference. Is the testing temperature also different between them like it is for regular windows? I assume so. That is my biggest frustration with the NFRC values. It's a silly temperature to use for most of America.
Fair point on the roof pitch. There's one more wrinkle I forgot. Passive House testing requires rating skylights at a 45 degree angle which matches the common 12/12 roof pitches you're referencing. So you'll see some European skylights with a CEN U-factor that is low and then a PHI rating with a higher U-factor. The PHI rating temperature also varies by climate zone.
The EU rating is done at 0 C like windows. Better than the -18 C for NFRC, I agree. I'm in climate zone 4C and will never see -18 C. Knowing that if the U_glass under NFRC is 0.18 (R 5.5) the real performance at 0 C is actually U = 0.14 (R 7.1) makes you feel a little better about these thermal holes in the roof :)
0C is actually close to the minimum for U-value in skylights. As you go up above 10 C, the U value starts going up again.