THE RIGHT
SERVICE FOR YOU
PUTTING FABRIC FIRST

THE RIGHT
SERVICE FOR YOU
PUTTING FABRIC FIRST

THE RIGHT
SERVICE FOR YOU
PUTTING FABRIC FIRST

Windows & External Doors

When you begin to research the windows you require for your project you will often come across lots of information regarding the U Value of 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 suppliers. Some 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 which is what you need to work to when comparing products.Triple Glazed Window

What does the U Value represent?

First let us give you a brief reminder about what a 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 happens, to varying degrees, in any building.

Heat flow may be quantified as “U‐value” being the heat flow through a 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 without glass. “Ug” refers to the U Value of the glass only. “Uw”refers to the combined U Value of the frame and the glass combined.

Usually the glass (Uw) will be the best performing of these U values, with the window frame (Uf) usually be the highest of these values (worst performing) because wood or metal frame is a good conductor.

There is also another measurement sometimes used that relates to the insulation performance, particularly when designing to Passivhaus standards of windows, that is the “Psi Value” (symbol: Ψ, pronounced “si”). Psi Values measures the heat transmitted  between the glass space (Spacer Bar), and it has quite an effect on the resulting Uw Value.

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 Insulated-glassleaving 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 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.

So when comparing 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.

  • We are now able to offer a complete building system using our with monolithic clay block & insulated raft system,