Windows: U value vs K value

[lfcjfc]

I am currently looking at windows for a new build. Depending on which company I look at, I sometimes see K values quoted and sometimes see U values quoted as measures of thermal insulation properties.
I have read material that says that the U value is the more appropriate measure for windows as it measures the actual rate of heat loss rather than the amount of heat loss. Can anybody tell me if there is a simple relationship between these two measures that will allow me to make a valid comparison between suppliers. Have seen info on the web for Niveau windows - look like quality products with a U value of about 1 for double glazed units and well below 1 for triple glazed - however I'm finding it hard to make products that specify K values.

 

[puffin]

Just remember to get U-value for complete window not just glazing, can make a big difference. We are looking at windows and have seen a variation of 1.6-2.2 for U value of complete window. I think 2.2 is recommended min.?

 

[Wicklow]

The K value is the thermal conductivity of a material where as the U value is a measure per sq meter of the structure / unit as a whole in preventing heat loss. U values are what you should focus on. there isnt a straight forward way of converting one to another.