Timber sash windows – U values.

There has recently been a lot of speculation on forums about the difficulty with achieving good U-values in timber sash windows. Many Irish people have lived in a house with draughty single glazed windows at one time or another. However, with all the scientific advancements in window technology, a modern timber sash window will achieve competitive U-values and low air leakage.

What is the U-value in windows?

The U-value measures the heat retention through a window or a door. It is a complex calculation as it considers the material and construction of the frame, the glazing bars, the glass and the seals. The result is the Uw value, measured in watts per square metre Kelvin. A lower U-value means more heat is kept inside a house on a cold day.

What is the U-value difference between single glazing and double glazing?

The U-value of a 4mm single pane of glass is 5.7. The U-value of a 12mm double glazed Slimline unit is 1.9.

A-rated sash windows

It is also worth noting that standard factory produced windows can achieve an A-rating but this will be exclusively for the exact size that has been tested. Any alterations will require individual testing in the UK. All our products are bespoke, and each has its own U-value. Our products are traditional designs in keeping with Victorian and Georgian styles and the U-values are dependent on the window chosen. Typically, the U-value for an energy rated sash window is 1.4.

At Timeless Sash Windows we have also developed an A-RATED sliding sash window. Each window we make is A-Rated by the BFRC which is the UK’s national system for rating the energy efficiency of windows and is recognized with current building regulations.

What are Energy rated windows

Windows have an energy rating that is like the colour coded rating on your electrical appliances graded from A to G. An A-rated window is the best. This energy rating includes the U-value, but also considers the heat gain (solar factor or g-value) and air leakage (L-factor).

• U value measures the insulation of the whole window (glazing and frame) or door, or how well it keeps the heat in in the cold weather. The lower the value, the better the insulation and the greater efficiency of the window.

• The solar factormeasures the transmittance of sunlight through the window. Sunlight is split into the visible and non-visible spectrums. In the non-visible spectrum UV light entering causes fabrics to fade while infra-red light is the heat energy.

The visible light is what we can see. Glazing technology is so advanced that there are special coatings that allow less UV and IR light in, yet do not impede the visible light from entering. Low-e glass, or low emissivity, lets the sun in but reduces how much escapes. This is called solar gain.

Various films can be applied to glass to alter the amount of UV or IR light entering the room, as well as specific glass coatings.

• Air Leakagemeasures the tightness of the seals. A well-made window will have little or no air leakage. Again here, the lower the number is an indicator of better performance.

What is an airtight window?

Air leakage is measured on a scale class 1 – 4. A class 4 air leakage rating is the highest performing. So, if you have a window that has not been assembled to be really well sealed, it will have the same U-value but the energy rating will have decreased due to the resulting draughts.

At Timeless Sash Windows, our windows have been independently tested in a wind tunnel in the UK by Wintech and have been awarded a Class 4 air tightness, which is the best available.

How do we achieve this? Through our Timeless double draught sealing technique.

We can also use air tightness tape in some situations where it is recommended.

Check Your Quote

When you get a quote for your windows, double check that the U-value included is the Uw value – this is the U-value for the whole window including the frame. Some suppliers quote the Ug value – this is the U-value just for the glass and this will always be a better U-value than the whole window. In other words, ensure that you are comparing like with like.

In all cases, beware, as the U-value of a window or door means nothing if it is not correctly fitted. Ensure that you hire a reputable joiner to fit them correctly to avoid any issues in the future. Most companies that manufacture their own windows also have a team that can install them to the highest standards of quality.

Do you have any questions? Comment below or call us on 046-9023323 for some advice.

Author: Jennie Ritchie, 12th April 2019