FYI I just found the g & U-value for Velux's various glazing options. It took me quite a while so I thought I would record it here:

 

In short, the common triple-glazing option '66' is 0.6/.44, Slightly, better options are '62' & '67' at 0.5 and average g-values. '70' is the standard double glazed at U-value of 1.0.

 

The 'heat protection' option '69' is an oddball with g-value (transmittance) of .27 but a not so great U-value of 0.7.

 

 

This is from: Velux_GGU_Technical_Data_Sheet.pdf

Don't get too hung up on it, if the are not massive. My Passivhaus certified Roof-Maker ones have a U_w of 1.0. I spent more effort to get a thermal bridge free install, thought that was way more important.

Uw varies by window size so I think there's a very good chance that Velux chart is based on a standard window size (1.2 X 1.4m) much larger than the average Velux and the Uw of smaller Velux windows will be worse, because a much higher percentage of the window will be frame

I’m using PHPP to calc Uw so I just needed the glazing number. Point taken though - the glazing may make inconsequential difference to whole house numbers.

On 28/10/2025 at 17:00, Alan Ambrose said:

It took me quite a while so I thought I would record it here:

I bet it did! Took me ages to find this data also. 

On 28/10/2025 at 19:04, JohnMo said:

I spent more effort to get a thermal bridge free install, thought that was way more important.

Good point John. If you have a deep (thick roof) then the reveals can form quite a large area. The closer you get to the top surface of the roof the less effective thickness of insulation.