Converting Window U-values between Metric and IP: Quick Tip

Energy modelers and HVAC designers have to input SOMETHING into their software to simulate window performance, and it’s better to be in the neighborhood at least. Here’s the quick tip, along with a following disclaimer from an awesome follower who put all the problems with this simple conversion very clearly before us:

IMPERIAL U-FACTOR (BTU / HR X FT2 X DEG F) = METRIC U-FACTOR (W / M2 X DEG K) / 5.678

Now the problem with this equation:

‘Perhaps I've misunderstood the intent of the blog post, but it seems to suggest that European and American window U values are directly comparable, which is questionable. For window performance standards, EU uses ISO 10077 and US uses NFRC 100/200 (Canada CSA A440.2‐09 incorporates NRFC by reference).

One of the big differences between the two standards is that NFRC ratings are calculated at 0°F and ISO at 0°C. Very cold temperatures (large temperature differences) drive stronger convection currents between panes. Of course, window manufacturers will design to optimize published rating over real world performance. If you are designing for very cold temperatures, the tendency is to favor convection reduction at the expense of conduction reduction. Smaller gaps suppress convection currents at the expense of higher conduction, so window gaps tend to be smaller for US windows (rated at 0°F) than for EU windows (rated at 0°C). For my area, Massachusetts, NFRC is good input to Manual J since the reference temp is around 0°F, but ISO is better as a measure of thermal performance, since half the heating degree days are above/below 32°F. I think ISO would also be a better standard for a cooling dominated climate.

In case you are unfamiliar, I found that International Window Standards - Final Report April 2014 to be a great resource on this subject. The report's bottom line is that there is no good general correlation between ISO and NFRC standards. However, fig 3.10 in the report shows that ISO and NFRC calculate about the same U values for uPVC triple-pane windows over 4 different profile (frame) designs and 2 different window gappings. When building my home a couple of years ago, I was considering an American window and a design from Poland (btw Poland encouraged the window industry as a matter of national policy, so lots of good cheap windows are made there). Since both were triple-paned PVC, I felt justified in comparing the U values directly, despite the different standards.

I've seen your friend Matt Risinger (and Steve Baczek) comparing EU and US window ratings as if it is an apples-to-apples comparison, and this seems to be not quite accurate. That said, I think it is true that EU windows have better thermal performance for the same profile material and glazing, since the ISO standard is better aligned to real world performance conditions.

Hope you find this helpful,

Craig S.’

THANKS SO MUCH CRAIG, everybody be warned, but better this than just using default library items in the software and getting it completely wrong.