I've posted this before, but it's worth reposting. The US uses Imperial units and tends to use R values rather than U values. There are simple conversion equations to go between the different forms.

See http://en.wikipedia.org/wiki/R-value_(insulation)

Square feet and degrees F definitely do not magically tally with square metres and degrees C!

Paul in Montreal.

This was the web-page that confused me, written for US market, it says U value is BTU/hr.ftsquared.degrees F and the U values they quoted were tiny. So I guess the moral is don't compare U values unless you know what U is standing for? At least they kindly quoted their method for calculating the U value.

http://www.durisol.net/documents/ThermalPerformance.pdf

Seems R values also open to differnet calculation methods to! thanks Paul and CWatter for helpful comments

In the 1970s we, in the UK, also used U values based on BthUs/sq. ft/ deg F. Its a shame they did not call the metric U, Um (no thats the name!).
Frank

jerseyman - the confusion is here:- the scientific measure of conductivity of the material is in W/m2 per unit of temperature gradient in degrees across one whole metre, or K/m. One of the 'm's cancels out, leaving you with W/mK.

The u-value is a practical measure which takes account of the thickness of the material being used to give the loss per m2 per degK of temperature difference across that actual thickness of wall or whatever.

That works out to the u-value being the conductivity divided by the thickness in metres. Not surprising people get confused. Me too.

thanks guys! It is as confusing as I thought but much clearer now.

if a canadian window has a u value of 0.24 what would that be in the uk is there any easy way to convert

ok paul lets smoke the peace pipe lol
Well thats a turn up inline windows quote 0.24 u value wich i thought was great but i can get windows in uk at 0.7 u value so that knocks the socks off the canadian ones or am i missing something

When it comes to windows, Uwindow is not same on both sides of the Atlantic. It is very hard to accurately compare window U's for North American Windows and those for European Windows. I'm not even sure it's the right question, but i'll drift into that later.

Being Canadian i'm more familiar with North American approaches, but am learning more and more about how Europe calculates Uwindow.

In North America Uwindow is calculated at a -20C outdoor temperature. As I understand it, in Europe Uwindow is calculated at a 0C outdoor temperature. This has no effect on frame components. Their conductivity does not vary with small changes in temperature.

It does have an effect on gas properties and therefore affects Uglass. The result is the optimum spacing in North America is 12.7 mm (1/2”) , while it's 16mm in Europe. On a related front 3mm glass is permitted in North America, while 4mm glass is the minimum in Europe. So bringing a typical triple glazed North American window to Europe means the airspace is likely 11mm – even more sub-optimal. The work around is to use krypton instead of argon. When you do this North American windows would have equivalent Uglass's to their more widely spaced European counterparts – providing they both had the same type of low-e.

Another challenge is the different sizes used for calculating Uwindow. The sizes for calculating Uwindow in North American are as follows:
Outswing Casement: 600mmx1500mm
Inswing Tilt & Turn: 900mmx1500mm
Fixed: 1200mmx1500mm
Europe uses 1290mm x1470mm. As I understand it, BFRC divides this into a 645-645x1470 Casement-Fixed an the PH people simulate the whole thing as 1 Tilt & Turn. The differences in sizes are important because good glass insulates better than good frames. Even if temperature corrected, accurate comparisons require like sizes.

In the end the best way to compare windows is to do some sort of whole building simulation. The advantage of the PHPP is that it asks for Uframe and Uglass and calculates Uwindow for each window based on its size. The Canadian HOT2000 program has a detailed window mode that does the same thing.

The advantage of this more detailed approach is that besides accounting for the effect of Uwindow, it also accounts for the effect of G (SHGC in North America). This is where I start to sound like a broken record, but ….. windows are not walls they are not only a source of heat loss but also a source of heat gain. Even in relatively cloudy UK a superinsulated house will cost less to heat with good windows than with no windows at all.

When i've looked at this, the very best North American windows produced slightly lower space heating loads than a typical PH windows, despite the fact the typical PH window insulates better. Where North American windows shine, so to speak, is that their slimmer frames permit higher solar gains, and that matters, even in the cloudy UK.

Hope that helps