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What is a "U Value"?

What is a "U Value"?

U Values are used throughout construction to gauge how thermally efficient a component is.

In technical terms a U Value refers to the ratio at which energy (or more specifically; heat energy) is lost through a specific component per specific area. This is therefore represented as a number followed by the SI symbols: W/m2K (Watts per meter squared per Kelvin).


The lower the U Value the more thermally efficient the component.


Doors and windows always have an overall U Value, which can be written as either “U value”, “Ud”, or “Uw”. Each component that makes up a door and / or window has its own individual U Value. This can get confusing as some manufactures / suppliers like to give the glass U Values (Ug) which is almost always lower than the overall U Value which is actually the only important value to be considering.


Always look out for the denomination next to the U where U Values are quoted, if it is the glass U value (Ug) then it is important to find out what the OVERALL U Value is, as the Ug value will almost certainly be lower and can be misleading.


There are several factors that can alter a door / windows U Value. For example, the type of spacer bar used in the glass units, the gas in the glass units, the door or windows frame material, the number and type of seals an opening window uses, the frames thickness, the amount of glass panes (double or triple glazed) and even the opening type of the door / window may affect the U Value.


Wherever possible Olsen Doors & Windows will always quote the overall U Value of a door / window system but will sometimes quote the glass U Value to demonstrate the difference between specific options within a system.



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