Aluminium Windows: What’s the Difference Between Uf and Ug Values?

When you begin to research the windows you require for your project you will often come across lots of info about the U Value of Aluminium Windows.

It is widely known that the lower the U Value the better the thermal performance of the window and the more insulation in provides. But it is easy to be misled about the various U Value measurements shown across the Internet.

Some unscrupulous companies will only highlight the lower U Value of the glass (“Ug Value”) rather than the more accurate “Uw Value” which is the combined measurement of all of the components of the window.

 

What does the U Value represent?

First let me give you a brief reminder about what U value is and how it is measured:

Protection from weather is a central purpose of any building, and protection from wind and cold is of special importance here in the UK. Heat is lost (although sometimes gained) through the building as a whole, and heat flow (also cited as heat transfer or thermal transmittance) happens, to varying degrees, in any building.

Heat flow may be quantified as “U‐value” being the heat flow through one square metre of a building when the temperature on either side of the building differs by one degree Celsius.

So the U Value is dependent on the insulation performance of the individual building materials.

 

What’s the difference between Uf and Ug?

Below are the commonly used terms in relation to U Values for glazing components:

Uf – refers to the U Value of the window frame itself without glass.
Ug – refers to the U Value of the glass itself.
Uw – refers to the combined U Value of the frame and the glass combined.

Usually the glass will be the lowest and best performing of these values (Uw).

And the Aluminium Frame will usually be the highest of these values (worst performing) because metal is a good conductor (Uf).

There is another measure of heat insulation performance used in particular for comparing the performance of IG Units (Insulated Glass Units) – that is the “Psi Value” (symbol: Ψ, pronounced “si”).

 

Psi Values

The Psi Value measures the heat transmitted across the join between the glass, and it has quite an effect on the resulting Uw Value. Here are some common glazing scenarios together with their typical Psi Values (as usual, a lower value indicates better thermal performance and insulation):

 

Frame material Spacer material Psi Value (Ψ)
Aluminium Aluminium Ψ 1.111
Aluminium Steel Ψ 0.068
Aluminium SWISSPACER V Ψ 0.039

 

This then has a tangible impact on the overall U Value:

ECO, 1.1 standard unit with Ψ 1.111 = 1.76W/m²k U Value
ECO, 1.1 standard unit with Ψ 0.039 = 1.68W/m²k U Value

 

What is SWISSPACER, and why does it reduce the Psi Value so much?

I’ll let SWISSPACER themselves answer that:

SWISSPACERSpacer bars insulate the edges of a sealed unit and keep the panes of glass apart. Traditionally spacer bars were aluminium, which is highly conductive to heat allowing it to pass through the window.

 

When it’s cold outside and warm inside, heat escapes through the spacer leaving the inside edge of the window colder than the rest of it. This heat loss means more heating is needed to maintain a comfortable indoor temperature. Under certain conditions, especially in colder temperatures, the moisture we breathe and produce from cooking and everyday living condenses on the coldest surface, usually along the edges on the inside of the window. Over time persistent condensation becomes a breeding ground for mould and bacteria. Mould spores get into the air we breathe and can aggravate breathing conditions and other health problems.

 

Warm edge spacer bars like SWISSPACER are made of an insulating plastic composite material that becomes a barrier to heat loss. Warm edge spacers reduce the amount of heat lost through the sealed unit. They keep the edge of the sealed unit warm, hence the name ‘warm edge’. As a result less heat is lost through the windows and heating bills are lower.

 

(Source: SWISSPACER – http://www.swisspacer.com/en/knowledge-centre/warm-edge)

So when comparing aluminium window systems to find the most energy efficient, make sure you are comparing Uw Values (rather than the more impressive looking Ug Values) to get the most accurate picture of how your windows will perform in your home.

If you have any questions about this, please do leave a comment below – or Call Us on 0203 00 22 088.

 

This article is part of our Where Do I Start? series. To read the next article, please click here.

 

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